# Offenhauser Mighty Midget Racing engine



## Eccentric (Dec 23, 2021)

I am going to try my hand at developing plans for an Offenhauser Mighty Midget Racing Engine. There were many variations manufactured through the decades and my version will likely end up being an amalgamation of many of those.  I will also be taking liberties for ease of manufacture/assembly and to increase the chance of it being "a good runner"--or at least a runner.  Inspiration came from Terry Mayhugh's build of Ron Colona's Offy and I will be taking advantage of his engineering insights shared in his (Terry's) build log.  Why don't I just build that engine?  Well, it is a very complex model and I don't feel ready to tackle that level of sophisticated craftsmanship.  Second, I want to have control of the plans and have the ability to freely distribute them if they ever get to that point.  The variant of the 97 Cubic Inch Midget Offy I will build will have two valves per cylinder instead of the large Offy's four valves per cylinder, the crank will be supported in three places instead of the large Offy's five and the 97 cu in Mighty Midget Offy is smaller overall.  I will design in 1/4 scale so the model engine will be about  5.375" long, 5.5" tall and 4 inches wide.






Source: Fred Offenhauser Photos and Premium High Res Pictures - Getty Images





Source: https://www.amazon.com/Offenhauser-Legendary-Racing-Engine-Built/dp/1626540411





Source: Amazon.com







Source: Amazon.com



I will use the line drawings available in the book noted as a source for the attached pictures, then create 3d solid models of the major components, and I literary mean solid as there will be no internals initially.  I will layout the moving components, timing gear train, cam shaft, crankshaft etc as simple stick models to define their geometry.  Then I will add increasing detail to the engine, realizing there is a high degree of interdependence between all the components; a small tweak in one place will ripple through the whole engine. Once I am happy with the 3D CAD model I will begin developing the plans themselves.  Most plans I have seen are manufacturing method agnostic, that is, they can be used for manual machining methods as well as CNC machining.  My plans will be developed specifically with some limited CNC machining in mind.  Home grown CNC routers are more common now and mine has given me greater flexibility in producing a complex part in a reasonable amount of time.  What this really means is that I will not only produce a set of dimensioned prints, but some IGES files designed for specific machining operations.  If someone chooses to build to my plans, they will not need to create their own 3D CAD model from a set of dimensioned prints, they can use the supplied IGES files.

I have scoured the internet and downloaded lots of pictures, enough I think to guide me in producing a realistic replica.





I scale a 2D drawing and import it into my CAD program.





Then I extrude the major components.





And create an assembly.





Then begin dimensioning the major components.

I will start nailing down the models specification's, for example I think it will have a bore of .75"


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## stevehuckss396 (Dec 23, 2021)

Great project. I'll be keeping an eye on this one.


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## mayhugh1 (Dec 23, 2021)

Very ambitious. Am looking forward to following your progress. - Terry


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## kuhncw (Dec 23, 2021)

Which CAD software are you using?

Chuck


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## a41capt (Dec 24, 2021)

Looking forward to following your build. Ambitious project, but one that’ll be rewarding!

John W


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## Majormallock (Dec 24, 2021)

Count me in please for a set of plans, looks a great engine and your approach works for me


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## propclock (Dec 24, 2021)

Fantastic I will be following along. I have a 3/4 axis cnc mill
and also would like to know what software you are using. 
Beautiful engine there.


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## dnalot (Dec 24, 2021)

I will be following along as well

Mark T


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## Eccentric (Dec 24, 2021)

Gear Train

I need to define the gear train that drives the camshafts and magneto (distributor) from the crankshaft.  My goal is to define the pitch and tooth count of all the gears.

Below is a picture of the Midget Offy's  gear train.





Source: 

The constraints for the gear train include:


I would like to use a "standard" gear pitch.  The smallest that can be considered standard  is either a 48DP or a .5 Module (which is about 50DP).
The camshaft has to turn 1/2 the speed of the crankshaft.
Would like to have a single plane gear train for simplicity.  The Mighty Midget has a dual plane gear train as can be seen in the photo above.  The dual gear in the gentleman's hand is the idler between the crankshaft pinion and the gear train in the gear tower.
The camshaft gear, as well as all of the others, has to fit in the gear tower housing
The crankshaft should have as many teeth as possible.
There should be as few bearing sets as possible.  This is an advantage of a two plane gear arrangement, multiple gears can use a single bearing set. In a single plane gear train this means the number of gears between the crankshaft gear and the camshaft gear should be minimized.
A plus would be to have as few gear types as possible, also "standard" tooth counts would be preferred.
There are other mechanical constraints such as the 88 degree inclusive angle between the two valve banks and the distances between the camshaft and crankshaft.
Also the magneto needs to turn at the same speed as the crankshaft.
If a single plane gear train is used, the camshaft gear has to have twice the number of teeth as the crankshaft gear.  It does not matter how many gears are in between or how many teeth they have.  I made an excel spreadsheet to prove this to myself.  I think I will settle on 48DP, .5 Module does not give me a measurable advantage in tooth count. The largest camshaft gear that fits has 32 teeth; this gives me a 16 tooth crankshaft pinion.  I would prefer to have more teeth, but this is acceptable.  I might design a dual plane gear train just to see if it would be worth it.

I will be using the gear pitch circle in my diagrams, this diameter is calculated as the number of teeth divided by the diametrical pitch.  If I am using 48 DP then it is easy to translate back and forth between pitch diameter and number of teeth.  My go to book for info on gears is Ivan Law's "Gears and Gear Cutting".





The above example uses a 16 tooth crank pinion, 32 tooth camshaft gear used in three places and two 50 teeth gears.  Below is another option, but it does not fit in the gear tower as the idler gear interferes with the highlighted bolts.






Would like to see if I can fit an 18 tooth pinion and 36 tooth camshaft gear.  Below I try this with the gear tower housing.






Opps, the gear driving the magneto needs to have the same number of teeth as the crankshaft;  the magneto needs to turn at the same speed.  This is because the magneto will be used as our distributor and this is a four stroke.  I add this to the list of constraints.

Also I have been using the gear's pitch circle, when we look at the gear outside diameter,  as below, we find that the 36 tooth gear does not really fit.  The distance between the gear and the outside wall is only 1/16" of an inch, too small. Back to the 32 tooth cam gear and 16 tooth crank pinion.










With this configuration I have 4 gears between the crank pinion and the camshaft spur gear, I think this is the minimum.  I have four different tooth counts, 16, 32, 40, and 56 with a diametrical pitch of 48.  I will  move forward with this configuration for now, I believe it meets all of the criteria I laid out. I may find more issues as I make more progress elsewhere.






This is where I ended up on this Christmas Eve.


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## aonemarine (Dec 25, 2021)

Following...


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## Jan Dressler (Dec 25, 2021)

Hello... Nice, and very interesting project!

It kind of reminds me very much of my own, the type 35 Bugatti engine... Only that I work on mine since 2013 (not continuously, of course, but there are a couple of 1000 hours into research (most of them) and CAD work).

Based on my experiences, a few comments:


Eccentric said:


> I will use the line drawings available in the book noted as a source for the attached pictures,


That is basically exactly where I started. The CAD pictures remind me very much of my own. 



Eccentric said:


> then create 3d solid models of the major components, and I literary mean solid as there will be no internals initially.  I will layout the moving components, timing gear train, cam shaft, crankshaft etc as simple stick models to define their geometry.  Then I will add increasing detail to the engine, realizing there is a high degree of interdependence between all the components; a small tweak in one place will ripple through the whole engine.


Exactly. With this approach, you need to get all the confinements that define the parametrics of the CAD model really, really right. 

"Add increasing detail" means doing more research finding more drawings, other books, pictures, doing dozends of screenshots of youtube videos, and so on... And you will, on one point, stumble upon a thing that basically throws everything down again. One major dimension that needs to be changed a tiny little bit to be "just right", one moment of "Oh... THAT is for what this feature is for" that means that a whole lot of things have to change to be "accurate"... 
You will forever know if something is just not "right" 

And this means the parametrics of your model have to be very well thought out, and "robust". Ask me from where I know that...  



Eccentric said:


> I have scoured the internet and downloaded lots of pictures, enough I think to guide me in producing a realistic replica.


Trust me, there will never be "enough"  If you want it to be realistic, you will end like me, visiting museums, meetings where cars with this engine are supposed to turn up, and so on and so on... 



Eccentric said:


> I will start nailing down the models specification's, for example I think it will have a bore of .75"


My scale was actually dictated by the crankshaft roller bearings... The exact same type is available in 1:3 scale, so 1:3 it was.

Good luck, I will follow this with interest!


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## Eccentric (Dec 25, 2021)

*Inside Timing Gear Tower*

I import pictures and drawings into my CAD program (SolidWorks) and scale them to my working scale, this includes rotating them to establish some horizontal and vertical reference points.  Most photos and drawings are not very good representations of a part as there is parallax and vanishing points and focal distances that distort the resulting 2D image.  But I am not interested so much in just capturing an outline, but understanding the relationship of surfaces.  For example the outside edges of the gear tower that contains the gears to the cam shaft have a negative inclusive angle, that is the sides are not parallel and get closer together towards the top; I have this angle shown as 3 degrees below.  I'll take many photos and drawings and attempt to ascertain this angle.  There were variations between engines and drawings so I am a able to take a little liberty in establishing an angle that works for my design, looks representative to the eye and is true to the information I have at hand.  But I don't agonize over getting it "perfect".   






Source: Offenhauser 110 Engine


Serial number 214 Built early 1947

I have a spline estimating the curve shown in the highlighted red circle above.  I don't like splines, I try to reduce complex curves to a series of tangent arcs, but sometimes splines provide the best fit.  When using splines I try to reduce the number of points to the minimum, maintain tangent entry and exit points and have some mathematical relationship between the points.  Not that engineers design this way, but the eye is very sensitive to minor visual imperfections.  Below is another try at estimating this curve using just two tangent arcs (highlighted in red).  This is the simplest and I think the best.





In the picture below the included angle of the gear tower is about 6 degrees.  Also the two arcs represent the side curve pretty well.




Dated 1935 - Source Gordon Eliot White's "Offenhauser the Legendary Racing Engine and the Men who Built it" (as all following diagrams)

In the real Midget Offy, the shape of the gear tower was dictated by the size of the gears it was housing and these were as varied as were customers for the Offy. 






Drawing from 1953






1965 Drake Engineering Drawing

So what do you think?  I am leaning toward going with 6 degrees.  OK, that is one dimension, now on to the next.   I hope this provides a little insight into my process for developing a CAD model without having the factory drawing set (wouldn't that be great).






I smell turkey cooking in the kitchen and my wife is called for me to get the extra leaves for the dining room table as company is due soon.  So this is going to be it for today, Merry Christmas/Happy Holidays everyone.


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## stewart drummond (Dec 26, 2021)

cam gears vary according to the customer ? cam rotation is 50 % crankshaft rotation no matter the cam/s or the customer for that matter


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## Eccentric (Dec 26, 2021)

You are correct, the gear ratios did not change, but the displacement/bore/stroke frequently were custom, so the height of the gear tower, thus the size of the gears was changed.   That was a lot of work.  The ease of changing the bolck and gear tower height is one advantage of using a chain drive as opposed to a geared drive.


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## Eccentric (Dec 26, 2021)

Front Timing Gear Tower and Bearings

The front timing cover at first glance looks like a mirror copy of the rear timing cover, but on closer inspection one can identify some features we would like to include in our model.  The front timing cover is thinner than the rear timing cover as well.






Features of note on the front Timing Gear Tower:


A - a raised lip around the edge of the front cover that extends out past the edge of the rear cover.
B - Sizable radius on edges of the front cover, much smaller on the rear cover.
C - Countersunk mounting hardware joining the two timing cover halves. Also there is a boss that extends above the edge radius noted by B.
D -  raised round boss and support web to house main idler bearing.  This is a double gear so the bearing housing extends further forward.
E - Raised Boss for larger mounting hardware holding timing covers to the block
F - There is a raised boss that the camshaft gear cover mounts to.
This is also a good time to define the stack up of the timing gears, their bearings and the bearing supports machined into the timing covers.  I am in the US so I am most familiar with imperial dimensions and parts, but metric components are just as easy to obtain here.  In some instances metric parts are cheaper and more readily available, bearings are a good example.  I am thinking of using a 1/4", 3/16" or 5mm shaft size for my timing gears.  Since the gears are 48DP (imperial) I will start with imperial dimensioned shaft and bearings.  A quick search on the internet and 3/16" X 3/8" X 1/8" bearings and 3/16" shafts should be a good place to start.  Gears 7/32" wide with small shoulders should fit well.


















I know what cosmetic features I want to add to the Timing Gear Tower, but this is good enough for now.  No sense putting in that detailed work if I need to reconfigure everything later. I am also missing the bevel gear housing for the magneto, but that will not impact the general layout, so that will have to wait.


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## Eccentric (Dec 27, 2021)

*Front Cover*

Today I will be working on the front cover, this is indicated by the arrow in the picture below:








The early Midget Offy's like the one above hard mounted the front engine mounting plate to the engine as can be seen above indicated by the red circles.  The flat plat with the wings on it mounted the engine to the frame.  A similar plate is on the rear of the engine.  This was a problem because the race cars had flexible frames and the twisting of the frame imparted a twisting load to the engine block and caused them to crack. 

The picture below represents a later version of the Midget Offy with the front engine mount removed. It it can be seen that it is no longer bolted to the front of the engine, but is allowed to pivot on the crankshaft cover.  This allowed the race car chassis to twist without imparting that twisting load to the engine.




Source: The Miller/Offenhauser For Sale Page

However, we are modeling an earlier vintage Midget Offy so we will have the larger boss for the motor mount rigidly mounted to the front of the engine.

The crankcase needs a little work to blend into the front cover.  Below is a wonderful picture of the crankcase without the block and we can get a good view of the shape of the crankcase where it meets the front cover.





Source: In the Shop: 1934 Offenhauser Engine - Update



I create a cocktail napkin sketch of the front cover, more to capture the features I want to create than to get the dimension correct. 




Then I start working on the 3D model.

This front cover and crankcase will require some more work. As I add more parts into the assembly model, there are more interactions among them and more time is spent making them all work together.





Here is where I ended up today.  It is going to be raining the rest of the week, so I hope to be spending a little time each day continuing the work on the Midget Offy.


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## Eccentric (Dec 28, 2021)

*Side Covers*

The side covers are very distinctive feature of the Off and will be fun to model.  The crankcase breathers are afixed to the top. I start by importing a picture with a side view, scale it and overlaying the outline.








Here is a side view showing how the middle is raised above the edges and the fin detail. The side cover and breathers are one of my favorite features of the Offy.




Then take a shot at the swept vent feature.






Then begin to lay in the cooling fins. There are a total of 9 on the Crankcase cover.





And the Block cover plate in yellow.   I also worked a little on the timing gear tower and added the housing for the bevel gears that drive the magneto.  That is the extension on the very left middle. I also cleaned up the front cover.  The magneto is mounted atop the small shelf at the top of the front cover.


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## petertha (Dec 28, 2021)

I was looking at your 88-deg included angle (44-deg per side bank). It also corresponds with the real world photo. Wondering out loud if it was 90 & 45 respectively. Have you been able to find any documentation that states the angle, or an independent method of verifying? It may well be 'odd' like that. I don't know Offy's well but I recall other models are noticeably shallower angles so there must be an underlying design reason. 

Reason I mention this is I went through a similar exercise on a few radial engines: imported a digital image into CAD, adjusted axis & aspect ratio as best as possible, did overlay drawing on a single cylinder, typically vertical #1, then rotate copy. Result was many of the image cylinder axis did not line up to the CAD model which was even multiples of 360-deg/number of cylinders. Similarly, if I extend the centerline down to the oil pan, it doesn't align there either.  

I suspect between back in the day hand drawn drafting, multiple copying & state of printing technology, there are many opportunities for distortion so one shouldn't expect perfection. It's actually impressive its this close. I guess in the absence of hard data, you as the designer are in charge. Just wondering out loud if 90-deg included angle would make your life easier from machining standpoint? Looks like lots of other design features cascade off of that initial angle decision - cam gear train, valves, seat, head... 

(not a critique, just food for thought)


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## petertha (Dec 28, 2021)

What are you contemplating for fuel & ignition system?


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## Eccentric (Dec 28, 2021)

Peter.


You make an excellent point. I "thought" I read that the midget had an 88 degree included angle between the banks of valves, but just now I have not been able to put my finger on it. I measured several drawings and I "confirmed" the 88 degrees. But now I am wondering if it was confirmation bias that lead me to measure the 88 degrees; the tendency to have new information conform to an existing belief. But I really like your point with respect to the manufacturability of 45 degrees vs 44 degrees. Even if there is clear documentation showing the included angle is 88 degrees, it is in my best interest to round that off to 90 degrees. If I used 44, I am sure one would end up 44 and the other 46. I have tooling for 45 degrees but, 44 degrees would be a nightmare.

I do have several drawings calling out the valve bank offset in the larger 255 and 270 cu in Offys being 36 degrees.

I never take input as a negative critique, I appreciate your comments. Keep 'em coming.

As far as ignition and fuel I had not given it much thought other than to use hall sensors triggering the spark and a functioning distributor to distribute the spark to each of the four spark plugs. As far as fuel goes, I am not going to make this a high compression engine, that will not be to scale. I like Coleman fuel with a splash of WD40 (shaken, not stirred). I am planning on two carburetors.


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## Eccentric (Dec 29, 2021)

*Rear Motor Mount and Bell housing*

Today I turn my attention to the rear of the engine.  I started to work on the bell housing/clutch assembly, but then realized the bell housing attaches through the rear motor mount and extends beyond the sides of the crankcase. So I start with the motor mount.  It is relatively simple as it is a single plate of aluminum that bolts to the rear of the engine and to the frame of the race car.










I import the image of the rear of the engine, scale it and import it into my assembly.  Then I lay reference datum lines and take measurements.  I build the rear motor mount model and check it in the assembly.






Then I do the same thing with the bell housing /clutch.









This is where the rear of the engine is for now.

As per some earlier discussion with Peter, I changed the angle of the valves from 44 degrees to 45 degrees. This change only impacted two components, the green head and the magenta timing gear tower. I also worked on the purple front cover (below), I found I had made it too thin.  I added some detail to the magenta timing gear tower to blend it into the front cover.






I now have the basic structure of the engine and can start thinking about the internal components.  I will also start normalizing dimensions and thinking about where the critical dimensions are and what tolerances need to be. Up to this point in time I have only been focusing on putting the lines in the correct spot and have not worried what the dimensions ended up being. For example, if a dimension is now .12734" , I will determine if I can change it to .125" or even .12".  this process has to be done systematically as these dimensions will ripple through the entire model.


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## petertha (Dec 29, 2021)

Hmm now I'm even confused myself. Turns out I have the book you referenced, although my cover photo is different & its been a while since I thumbed through it. I assume that's what you are referencing?





						Offenhauser: The Legendary Racing Engine and the Men Who Built It: White, Gordon Eliot: 9781626540415: Amazon.com: Books
					

Offenhauser: The Legendary Racing Engine and the Men Who Built It [White, Gordon Eliot] on Amazon.com. *FREE* shipping on qualifying offers. Offenhauser: The Legendary Racing Engine and the Men Who Built It



					www.amazon.com
				




On page 54 it says t_he 255 used a slightly larger block and case than the 270 but the same 72 degree valve angle, early 220 and 255 engines are difficult to tell apart.... _If 72-deg valve angle means the same as what we have been wondering about (88 vs 90) that's quite different again. Does the name 'Midget' correspond to a specific model number (like 255) or more of a generation series they produced? 

I noticed quite a bit of variation in piston crown shape amongst the various engine models, although the conical combustion chamber seems to be consistent hallmark (closely aligning to valve seat plane?). I'm guessing maybe different aspiration or maybe even fuels used at the time? It would be nice if a detailed matrix of engine specs/features were provided, but unfortunately not present in this particular book. Once you start introducing real world dimensions & components like valves, cages, ignition plugs & target CR.. I'm sure the design iteration will take many twists & turns. All part of the fun. One thing that stands out in my mind when mucking around with radials is how thin castings would scale. Real world cast cooling fins of 0.050" thickness x 2" deep turn into ten thou feeler gauges at 1/5 scale LOL


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## portlandron (Dec 30, 2021)

If you don't have a set of, or want to make the gear cutters, you might consider going with the 0.5 Module gears rather then the 48DP.
I needed to make gears for Rudy's Steam Tractor and tried to find 48DP gear cutters. Ended up going with 0.5 Module instead. Got a complete set for the price of one 48DP cutter. Granted the are Chinese but work fine for the brass gears.


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## Eccentric (Dec 30, 2021)

*Revisit Timing Gear Train*

I spent today revisiting my choice of 48DP gears for the timing gear train.  48 DP means that a 48 tooth gear will be 1 inch in diameter (really the theoretical pitch circle is 1 inch, the tangent circles where the gears mesh). The alternative is to use Module .5 gears. .5 module means that a 48 tooth gear will have a pitch circle of 24mm(or .9449 inches).  That is, 48 teeth * .5 Module = 24 mm pitch circle.

Ron brought up a good point that made me question my original decision.  In my earlier post about the gear train, I laid out nine criteria for the design.  Ron is suggesting another one: Use gear cutters that are readily available and inexpensive.  I did an internet search and I also found that Module .5 gear cutter sets are more available and less expensive (if you are willing to buy from China or Новосибирск in the, Russian Federation) than 48DP gear cutters.  I make my own gear cutters, but if another builder wants to buy gear cutters or gears, metric gears may be a better choice, even in the US.

It was actually quite straight forward to pull up my old 48DP gear study and overlay a couple alternative Module .5 analysis. 

My previous solution was to use 5 gear types, 16, 30, 32, 40, and 56 with a diametrical pitch of 48.  By using Module .5 gears I can still get away with 5 gear types, 18, 28, 36, 40, and 64. teeth.  I like this solution for the following reasons:


.5 Module Involute gear cutters are less expensive than 48 DP gear cutters.
Still have the same number of gear types.
The crankshaft Pinion will have more gear teeth, 18 instead of 16 and will be ever so slightly larger.
The basic geometry is not impacted, for example there are still 4 gears between the crankshaft pinion and the camshaft spur gear.
The down side is the magneto drive train is a little close to the edge of the gear tower and the engine is 1/16" shorter.

The other change this will drive is the gear shaft diameter and metric bearings.  I had settled on 3/16", but metric gears in this size typically use 5mm shafts, so instead of a 3/16" X 3/8" X 1/8" bearing. Bearings with a 5mm ID commonly have a 2.5, 4 or 5mm thickness.  The timing gear tower was designed around the 1/8" bearing thickness, so this also needs to be adjusted.  Running some numbers, a gear with a width of 5mm and two bearings with a width of 4mm each will fit nicely in the timing gear tower. So MR115-2RS Bearings with 5mm ID, 11mm OD and 4mm thickness should work.






Original 48 DP option above






.5 Module solution with 6 gear types above, clean magneto gear mesh






.5 Module solution with 5 gear types, not optimal magneto drive gear mesh.



I have three solution:

 the 48DP option that has:


2 ea 16 tooth gear
1 ea 30 tooth
4 ea 32 tooth
2 ea 40 tooth
2 ea 60 tooth
A .5 Module options has:


2 ea 18 tooth gear
1 ea 32 tooth gear
5 ea 36 tooth gear
1 ea 48 tooth gear
1 ea 56 tooth gear
1 ea 64 tooth gear
A second .5 module solution with fewer gear types:


2 ea 18 tooth gear
1 ea 30 tooth gear
5 ea 36 tooth gear
1 ea 40 tooth gear
2 ea 64 tooth gear
I spent way too  much time on this today, I think I will noodle on it a bit more before committing one way or the other.


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## CFLBob (Dec 30, 2021)

Eccentric said:


> The down side is the magneto drive train is a little close to the edge of the gear tower and the engine is 1/16" shorter.



I'm way out of areas I have any experience in, but I'm not sure I see where this comes from in your drawings.  If this results from the number of gears with the smaller pitch diameter can't the physical size of the engine remain the same?   It would change the positions of features, the shafts attached to those gears, but that 0.9449 vs. 1.0000" diameter doesn't need to impact every other dimension, does it?


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## Eccentric (Dec 31, 2021)

Bob.

I am attempting to maintain the distance between the crankshaft, water pump, magneto and the camshafts.  When ever I change the diameter of the gears these dimensions are impacted.    I am also fighting the fact that the camshaft gear has to be twice the diameter of the crankshaft pinion, right now I think the crankshaft pinion is too small and the camshaft gear is too big. Also as can be seen in the photo below, the gears driving the magneto should be in a stright line.




The original Offy used gears in two planes to solve the issue with the size of the crankshaft pinion and the camshaft gear.  The gear pair in the gentlemans hands helped reduce the ratio of the camshaft/cranksaft.

You are right, the difference between 1" and .9449" is very small and in the end I could ignore it and my diminsions would be off a bit.  But there is a stackup where all 7 gears are off by a bit and the error adds up.

I am taking a break from this timing gear train design and will noodle on it in my sleep.

Happy New Year Everyone!


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## Eccentric (Dec 31, 2021)

*Crankshaft*

I need a break from timing gears, so I am going to turn to the Crankshaft.

I import a scaled drawing and build a crankshaft on top. Notice in the drawing that the two center cylinders are pushed towards the two ends of the bock to make room for the center crankshaft bushing.  Also the conrods are offset from the center of the crankshaft journal as well, again to make space for the crankshaft bearings.






As shown below, In my Wallaby engine I used two ball bearings in housings mounted to the end of the crankcase and a bronze bushing mounted in the center of the crankshaft screwed to the upper crankcase half.  The ball bearings do not need oil delivered to them, but the center bushing does.  This scheme worked well and I am going to use it again and see if I can fit everything in. 




*My Wallaby engine and its crankshaft and bearing arrangement*​







I need to add some features to the crankcase for the crankshaft.  These are shown in the picture below:






Then I create a sketch to define the dimensions of the crankshaft.  Not sure if this diagram is going to make sense to anyone else but me. 






I am offsetting the connecting rod journals on the crankshaft 1/16" to allow for more room for the crankshaft bearings.  That is, the center of the connecting rod journals are offset 1/16" with respect to the center of the cylinders.  If I can get rid of this later, I will.






I draw up the crankshaft according to my napkin sketch and drop it in the crankcase.  I have to clear some material inside the crankcase to clear the crankshaft webs.  The bearing at the back, flywheel end, looks good.  The bearing at the front of the engine is going to be a challenge because I also have to deal with the timing gears at the front of the engine.



I am designing a model of the 97 cu In Midget Offenhauser.  It was the larger 255 and 270 cu inch Offys that were used in the Indy cars.  The MIdget Offy was used in the Midget Racers of the 1930s and 1940s.  I guess they still race them today, but that was the heyday.


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## Eccentric (Jan 1, 2022)

*Gear Train Final(?)*

It is important to nail down the gear train as it dictates the engine geometry.  The gear mesh tolerance has to be perfect and the distance between the gears defines the position of everything else.  I think I finally have a solution I am happy with.

These are the Constraints I laid out earlier for the gear train:


I would like to use a "standard" gear pitch.  The smallest that can be considered standard  is either a 48DP or a .5 Module (which is about 50DP).   Use .5 Module
The camshaft has to turn 1/2 the speed of the crankshaft.  Yes
Would like to have a single plane gear train for simplicity.  The Mighty Midget has a dual plane gear train as can be seen in the photo above.  Use Dual plane gearing, that is, use a cluster gear to allow the use of a larger pinion on the crankshaft, 32 tooth instead of 16.
The camshaft gear, as well as all of the others, has to fit in the gear tower housing Yes
The crankshaft pinion should have as many teeth as possible. 32 is a good number
There should be as few bearing sets as possible.  There are a lot of bearings, but I think this is the minimum possible.
A plus would be to have as few gear types as possible, also "standard" tooth counts would be preferred. Yes, 4 gear types and they are standard: 28, 32, 36 and 56 teeth.
There are other mechanical constraints such as the 88 degree inclusive angle between the two valve banks and the distances between the camshaft and crankshaft. All met, went with 90 degree inclusive angle on the valves
Also the magneto needs to turn at the same speed as the crankshaft. Yes
Use an inexpensive involute gear cutter set.  Yes, used .5 Module.  these are inexpensive and readily available.  so are the gears if one want to go that route.
Below is the final(?) solution:










The Camshaft gear can be no larger than 36 teeth, otherwise it would be too big for the gear tower.  If I used a single plane gear train the crankshaft pinion would have to be 1/2 of that, 18 teeth.  Not the end of the world, but the crankshaft would be pretty small and I had a hard time defining a gear arrangement that met all of the dimensional requirements.  By using the cluster gear set as shown above, I am able to keep the camshaft gear to 36 teeth and have a crankshaft pinion of 32 teeth.  In the picture above you can see I can have a larger crankshaft diameter at the pinion.  The engine geometry worked out spot on with the .5 Module gear pitch.


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## petertha (Jan 1, 2022)

You lucky USA folks have McMaster Carr & similar domestic suppliers. But if you are shifting focus to smaller module gears or maybe even bevel gears or belts for driven accessories, I had good experience with Maedler (Germany). Also, the RC (radio control) car/heli/etc mechanical models make extensive use of metric gears & accessories so you might find some stock from those arenas. 






						MÄDLER - your expert for power transmission elements
					

Stuttgart Düsseldorf Hamburg. Einer der führenden Produzenten und Lieferanten von Antriebselementen und Normteilen. Über 31.000 Artikel in bester Qualität. Hohe Verfügbarkeit, schnelle Lieferung. Webshop mit Preisen, Bestandsanzeige, vielen technischen Informationen. Maschinenbau Konstruktion...




					www.maedler.de


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## Eccentric (Jan 2, 2022)

*Crankshaft Continued*

I continued to work on the crankshaft.  I have already established the bore to be .75" and now I want to confirm the stroke.  A stroke of .75" is a good place to start.  I develop a model representing the volume inside the cylinder with the piston at the top of the stroke and at the bottom.   The valves are at 45 degrees with respect to the top of the piston so the roof of the combustion chamber is a cone.  The flat spot at the top is where the spark plug will be mounted.

The numbers circled in red in the photos below are the volumes of the two models.








Dividing them gives the compression ratio.  .39 cubic inches : .06 cubic inches or a compression ratio of 6.5:1.  This is a good compression ratio for a model engine, I personally don't want to go any higher.  I think that model engines with a higher compression ratio don't start as well, don't idle as well and have a larger tendency to blow out head gaskets.

I create a simple model of a piston and connecting rod, knowing that these are just to give me something to work out the crankshaft dimensions and clearances.  I will revisit both later.






Above is a cut away looking  at where the piston ends up at the bottom of the stroke and to see if the cylinder sleeve hits the crankcase.






Here the piston is at the top of its stroke.

I started out with a simple internal space inside the crankcase, but the pictures below show that I have interference and need to open up the space inside the crankcase.




Above the connecting rod hits the top of the crankcase space.





Above the connecting rod cap screw again hits the side of the crankcase.





Here I open up the internal crankcase space and shorten the cylinder sleeve.  When I finalize the piston and connecting rod later, I will need to revisit these interference areas.





This is my solution for the front ball bearing holder for the crankshaft is shown above.  The green part marked by the arrow will house the bearing and then screw into the front of the crankcase.  I have made sure the bearing holder and its retaining screws clear the gears.  I should mention that my plan is to split the crankcase at the crankshaft.






The above photo is a cutaway showing how the front crankshaft bearing holder is mounted to the front of the crankcase.  






The rear crankshaft bearing holder is much simpler as it does not interfere with anything like the timing gears in the front.



To be continued....

.


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## Eccentric (Jan 3, 2022)

*Upper Crankcase Half*

Today I worked on the upper crankcase, the crankshaft and its bearings.  In the photo below a cut away exposes the detail of the upper crankcase and illustrates how the crankshaft is mounted.  The center crankshaft bearing is still missing.








I have highlighted  detail of interest in the bottom diagram.  The front and rear ball bearings are mounted in aluminum bearing holders (colored green).  These are produced on the lathe in a single operation so that all of the features are concentric.  The crankcase will be line bored so the mounting points of these crankcase bearing holders and the center bearing will be concentric.  I have decided to use metric bearings throughout the engine.  The crankshaft big end journals are not centered on the cylinder center axes.  By offsetting them as in the real engine, I can get more material at the ends and middle of the crankshaft to make room for larger crank bearing surfaces.  In this small model I need as much meat as I can get in the crankcase for the screws that secure the bearings and the two crankcase halves.  The real engine has cutouts in the side of the crankcase covered by the crankcase breather covers.  Since these will not be seen I will not make these large cutouts, just a single hole for the breather.  If I were to put these cutout in, I would lose surface area and screws to secure the two crankcase halves.

Oil will be delivered under pressure  to the center bearing through and oil gallery in the crankcase.  Internal oil holes in the crankshaft will then deliver oil to the big end journals.






Below is a rear view of the engine showing the rear motor mount and the rear crankcase bearing holder.





Below is the beginning of the crankshaft drawing.  I want to drill out the connecting rod crank journals, but the outer most crank web is in the way, I may have to eliminate it as I need these holes to deliver oil to the conrod big ends.





Below is a picture of the 97 cubic inch midget Offy crankcase with the side holes highlighted.  I will eliminate these as I will split my crankcase as I have shown in the above pictures.





Next I will turn to the lower crankcase half.  I think I will add an oil splash tray in the bottom of the crankcase.


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## petertha (Jan 3, 2022)

Looking good! You probably already know this but depending on how you orient your valves, their size, & where the ignition plug bottoms out in the combustion chamber, collectively the CR can get altered a bit of your head looks like sketch. On my engine I was kind of surprised by how much, but its a function of the engines own geometry. Yours may well be different or not be worth fussing about on gasoline/spark. On my (methanol) engine I erred on high CR side with default geometry. I can more readily add head shim to reduce CR without too many consequences, but more difficult to alter things to increase CR after the fact. Food for thought.


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## Eccentric (Jan 4, 2022)

Peter,

You are right, I need to pay attention to the compession ratio design with a more complete model.  I have not thought about the head yet, but when I do I need to insure I am satisfied with the compression ratio.  I cannot add a head shim as it would raise the valve box and my timing gear would no longer mesh.


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## Eccentric (Jan 4, 2022)

*Crankcase Bottom*

Today I turned my attention to the crankcase bottom.  Since the crankcase is split I need some method of  screwing the two halves together and I would prefer if I could hide the screws, the original engine has a one piece crankcase.






One possible idea is to hid them behind the side crankcase breather covers.  I need to make sure: 1. I can insert the screw,  2. There is access for the tool to tighten it. 3. the slots don't interfere with the side cover mounting screws.






I worked on the internal features.  I want to have a baffle under the crankshaft, I need to think about adding features for it to mount to.  I am thinking two pieces of thin aluminum sheet mounted in a "V" with the slot open between them.  Simple.  The baffle is intended to help keep the oil from frothing up so it can be pumped out of the dry sump.






I added some features to the front timing gear cover.  I am thinking the housing for the magneto bevel gears will be a separate piece.

Spent the day helping the wife un-decorating the house, so not much time was spent on the engine model.


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## Eccentric (Jan 5, 2022)

*Top End*

I have started looking at the top of the engine, the head, cambox, and camshaft.  But I got sidetracked working some of the subtle shapes of the crankcase that bothered me.  I did not like the way that SolidWorks generated the fillets circled below.  I spent a fair amount of time adjusting the control points and did not like this trial and error approach.






So I decided to use surfaces instead of solid bodies.  I have not used surfaces for a couple of years so I spent some time researching and practicing. Below the surfaces are being laid in:





And once the surfaces are created, they are knitted together to create a solid body.  I am much happier with the result. Below is the result of my test crankcase using surfaces instead of fillets.




Below is the beginning of my study for the head and the cambox.  These are complicated parts.  Both coolant and oil travel through the head.




Below is the underside of the head.







I have spent a fair amount of time today reading through Terry Mayhugh's Offy build log; what an absolute mother lode of information.  I have a lot of studying to do before I can begin to add the next level of detail the Might Midget 97 cu in. model engine.  The model engine will be about 24cc.


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## Eccentric (Jan 6, 2022)

Cylinder Head - Part2

Today I am looking at the head combustion chamber, valve, and valve cage.  I start by pulling in a model of a valve I have used before and tweak it to fit.  This is used in the Wallaby, a 1" stroke and 1" bore engine.






The first thing I find is that there is not much room for the lip on the valve guide in the Offy cone shaped combustion chamber.  In the past I have used a light press fit with Loctite to retain the valve cage in the cylinder head.  Terry used a bit looser fit and then relied on a steel pin to retain the valve cage in his Offy.






The second thing I see is that due to the large angle of the where the manifolds mount, and the angle of the combustion chamber, there is not a straight shot  for the intake charge or exhaust gasses in and out of the combustion chamber through the valve cage.






The way the side hole is drilled in the valve cage, the edge disrupts the gas flow in and out of the combustion chamber.  OK, I thought, I will install the valve cage in the head without the side hole drilled, then drill the hole through the head and the valve cage at the same time.  This approach is shown below:





And then the resulting hole through the side of the valve cage would look like the following:




I don't like the shape of the resulting valve cage above.  The lip is too thin and could deform over time.  Also it is a risky machining operation where the valve seat could be damaged.  I rejected this idea.

Another idea I considered for a moment was eliminating the valve cage and fabricating the seats directly in the cylinder head.  I rejected this idea as it would be too risky, I would hate to reject a head because I messed up one of the valve seats.  I like to test the sealing of my valves and valve cages before I install them in the head.

I also looked at altering the angle of the hole in the head that interfaces to the hole in the side of the valve cage, I tipped it up, tipped it down, but did not find an orientation that significantly improved the air flow.





As can be seen above highlighted by the circle, the valve will hit the top of the piston.  I can rectify this by raising up the combustion chamber or by having a more complex shape at the top of my piston.  The original had a tent top shaped piston.  Whatever I do, my compression ratio will be affected and I need to take this into account.





The head is quite riddled with holes and there is very little area to run coolant.  The two red circles above highlight the only area that is really available.

I might be able to get away with a smaller valve and valve cage.  The one I started with is from an engine with a 1" bore and stroke, where as the Midget Offy model has a cylinder volume about 56% of that.  The diameter of the cylinder is 75% (.75"/.1") so theoretically I could get by with a smaller valve and valve cage.  Below is a valve cage from Westbury's seagull that has a .75" bore.  This is from the Model Engineer Magazine dated September 14,1950.





I love Westbury's drawings, everything is done in fractions, nothing in thousandths.  

 I know bigger throat and valves are better, but it is good to have another data point.

If I increase the size of the combustion chamber I need to understand the ramifications on my compression ratio.  So I repeat my modeling of the space inside the cylinder at top and bottom of the piston stroke.

When using a flat top piston I get a compression ratio of 4:1, a bit too low. So I model a piston with a tented top.  I also add the space at the top where the spark plug is and the area the valves extend into the combustion chamber.






By adding material to the top of the piston I can increase the compression ratio from 4:1 to 9.4:1.  What this tells me is that I can increase the size of the combustion chamber to give additional clearance to the valves and adjust the compression ratio within a reasonable range just by changing the shape of the piston top.

So, where did I end up today?






The above cut away sums up where I am with the combustion chamber, valve and valve guide.


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## Eccentric (Jan 7, 2022)

*Head Design - Part 3*

Continued work on the cylinder head by adding the features shown below.  Oil is pumped through the camshaft to the cam lobes.  This oil drips down through the cam boxes and collects on the heads where the valve cages are.  There needs to be a trough machined to collect this oil, and then drain holes to allow it to drain back to the crankcase.






Also added the threaded mounting holes for the cam box and the manifolds, intake and exhaust.  These holes may need to be moved when those other assemblies are worked.






I also added raised bosses for the coolant tubing flange. As shown above  I do not see this on the original engine but it is nice to have the additional material for the flange mounting screw to bite into.  The head coolant passage is just below these holes.






Bottom View of the head with the deeper combustion chamber.  There will be many additional holes on the bottom of the head, but these will be defined as part of the block, then their positions transferred to the head.  If needed they may be relocated.

Due to my work with the new combustion chamber and recalculating the compression ratio, I have revisited the Crankshaft design.  By shifting some of the dimensions around I was able to increase the stroke from .75" to .875".  This additional stroke with the larger combustion chamber and a flat top piston give me a reasonable 6.3:1 compression ratio.  There will be repercussions to the inside of the crankcase as there are now interferences, but they can be easily addressed.





Latest Crankshaft design - This can be compared to the initial crankshaft drawing to be found in an earlier post.  I was able to increase the stroke by going to a smaller connecting rod journal,  this also reduced the thickness of the crank stock needed from .625" to .5".  I also added the crank pin lightning holes and oil galleries.  The ends of the crank pin lighting holes will be plugged to allow oil to flow from the main crank bearing out to the connecting rod journals.

*Block Design*
OK, now I am turning my attention to the block.




Here is the block with a single cylinder sleeve installed.  I need to add some holes to:


mount the block to the crankcase
mount the head to the block
Water passages from the head
Oil passages from the head to the crankcase
mount the side plates
I think the best approach is to lay the holes in by eye and then move them around to allow for the most amount of material around them and to insure they do not run into each other. It will be very important to maximize the amount of surface area around all of the holes at the head to block interface.  There is a lot of combustion chamber pressure attempting to blow out our head gasket.





Above and below are pictures of my Wallaby block showing the holes that need to be added.




The screws that secure the block to the head will be mounted from the bottom, through clearance holes in the block and threading into holes in the head.  I am wrestling with what way the screws should go to secure the block to the crankcase.  The outside holes and the center holes are easier to mount from the top, down from the block, because there are webs in the crankcase supporting the crankshaft bearings in the way.  The holes between cylinders  1 and 2, then again between 3 and 4, are behind the oil return tube and would be easier to install from the bottom, from the crankcase side.  These are identified below as the screws that I may not be able to access behind the oil drain tube.




If I relocate the oil return tube, I can insure that I get a full 60 degree swing on the allen wrench.





I had to redo the holes securing the block cover, these will not match the original, but this combination minimizes interference between holes.





This is the current state of the block, a dizzying array of holes


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## Eccentric (Jan 8, 2022)

*Top End*

Plan for the day is to define the Cam box end plate, cambox cover and the cambox top features.  These include the bearing surface for the cam and a way to collect oil used to lubricate the cam and pass it down to the cylinder head.  The cambox end plate may seem like an odd place to start, but this plate will blend with the cambox and the cambox cover and define the shape of the cover and how it blends into the cambox.





Looking at the picture above and below, it can be seen that the cam box cover blends very nicely (tangent entry) with the cambox.





Below is my first attempt at the cover and end plate.  I did not understand the relationship of the shapes yet and the cover and cambox do not blend well.





The issue seemed to be whether area between the cam box side and the curvature of the cover was vertical, or parallel with the cambox.





So I loaded a picture of the cam box end plate into my cad software and overlaid a couple of circles and a tangent line between them.  Then I dimensioned the drawing and generated ratios I could use to define the shapes. Using this I create a new cam box end plate 3d model.




Below is the resulting 3d model, I am much happier with this look than my first attempt.





Below is a cut away of the cambox and cover showing the way they are joined.






Next I simply transfer the features on the head where it mounts to the cambox over to the cambox bottom as shown below.  These features include the large holes that support the cam followers and the small mounting holes.  FYI, the cam follower is a part that rides on top of the valve and provides a larger surface area for the cam to work against.  It is really just and inverted cup with the valve stem  sticking up into the end and the cam riding against the outside.






Then I add the features to the top of the cambox:  the oil troughs to collect the oil used to lubricate the cam and the holes to let the oil drain down to the head.





The troughs are cut with a ball end mill, giving them a rounded, funnel shape.






This is where I finished today, I still have some work to do at the interface between the gear tower and the cambox.  This area is shown by the red circle and you can see the current interference area.



Next I'll work on the camshaft, then refine the interface between the cambox and the gear tower.


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## Eccentric (Jan 9, 2022)

Camshaft

My design criteria for the camshaft includes the following:


Turns on two ball bearings (all shafts turn on ball bearings, crank, gears and camshaft)
A center bush in addition to the two out board ball bearings
Oil delivered to the four cam lobes and the center bearing
Cam Lobe definition - covered later (lift 3/32")
Firing Order - (1-3-4-2)
Direction the cam turns - (counterclockwise when viewed from the front)
Center bearing diameter - (1/4")
Ball bearing selection - (same as the timing gears 5mm X 10mm X 4mm)
Method to deliver oil to the camshaft- 
Method to retain cam timing gear - key and nut
Method to allow cam timing adjustment - the timing gear can have multiple key slots, each with a slightly different timing.
My tentative solutions are above in parenthesis.

Also my current thinking is not consistent with the cambox design and will require changes to other major components to deliver oil to the camshaft.




I need to get oil down the middle of the camshaft and there are three ways to do this: Through the back end, through the middle bushing and through the front end.  Getting oil delivered to the middle bushing does not look possible as there is not enough material in the cam box to drill an oil gallery.  Sending oil down the cambox cover likewise does not seem feasible.  Bringing oil in through the front of the camshaft is complicated with the presence of the cam timing gear, so.. First I look at brining in oil through the back of the camshaft




Below is an example of a technique I have used in the past to deliver oil to the center of the camshaft.  Oil is delivered to the bearing holder, then to the center of the camshaft through the back.




Pros of this method are it will be easier to get oil through the backend, but the cons include the extra distance the oil has to travel from the front of the crankcase to the rear.  Also I am a little concerned using the 1/16" thick rear motor mount as a cover for a very long oil channel up the back of the engine.

The other alternative is to bring oil up directly from the oil pump and into the camshaft through a bushing.  Does it replace the ball bearing or work in conjunction with it? Do I just use the aluminum cambox as the bushing?  Then do I abandon using ball bearings on the camshaft?




That front camshaft bearing wants to be mounted in the rear gear tower to maintain predictable gear meshing.  But on the other hand the camshaft needs to be securely mounted in the cambox.  Now I am thinking  that using ball bearings on the camshaft is causing more problems than it is solving.

I am going to think about the bearing and oil delivery system a bit more so I am going to move on to the cam lobe design.  The design parameters are:


Lift - 3/32"
Major diameter - 7/16"
Base Circle - 1/4"
Bearing diameter - 7/32"
Exhaust Lobe duration - 110 degrees
Intake Lobe Duration - 125 degrees
Exhaust Lobe nose radius - .025"
Intake Lobe nose radius - .050"
These data give the exhaust cam lobe shape shown below




I would like a way to adjust the camshaft timing after the engine is built.  Becasue there are 36 teeth on the camshaft gear, I can only get 10 degree resolution by moving fromone tooth to another.







If the camshaft timing gear is held on with a nut and its registration to the camshaft is achieved with a key, there could be 5 keyways cut in the gear.  This would give us 2 degree resolution in our adjustment of the camshaft timing.  36 teeth times 5 keyways divided by 360 degrees.  This can be seen by looking at where the lines emanating from the center intersect the gear teeth.

I have deleted the camshaft ball bearings and am having the camshaft ride directly on the cambox, at least for now. 






This is where I ended up for the day, an exhaust camshaft sitting in the cambox.  Alignment between the cam box and gear tower needs to be perfect to get correct gear mesh. Does not look much different than where I started.


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## Eccentric (Jan 10, 2022)

Top End Assemblies Finalized

Today I have gone back and attempted to "finalize" the assemblies that have no known design issues. I put "finalize" in quotes because nothing is ever final.  I have decided to machine some coolant cavities into  the head to allow for more coolant volume and better coolant flow through the head.  The cavities will be machined and then sealed with an aluminum plate.  I will used high temperature structural adhesive to bond in the cover plate.






Some of the final details incorporated into the head assembly are the plugs sealing the ends of the lengthwise coolant passages and the dowel pins securing the valve cages.  The oil passage was added from the timing gear front cover through the head to the cam boxes.  I borrowed liberally from Terry Mayhugh's Offy build.




I added all of the holes to the bottom of the head to match the block, the coolant passages, the oil drain and the head bolt holes.





The block assembly is complete at this point.





As is the cambox assembly.




I think I can make the cambox cover from 5/8" aluminum tube with a .049" wall thickness.  I will have to slit it, remove some material, then mount it in a mandrel for the bottom machining.


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## Eccentric (Jan 12, 2022)

*Cambox Caps*

To make the cambox cap, start by imitating the shape of the top of the gear tower. 





There is a radius on the front and back of the caps, but they are different.  The rear of the cap mounts flush with the cambox cover.




The front however, has a raised lip and the edge has a larger radius.




There is a flange at the base of the cap and the stud mounting features that align with the holes in the gear tower.  bolts mount through these holes securing the caps into place.








I worked on finishing up the gear tower sub assembly with the bearings, shafts and gears in place.












Above are a couple of images of the engine in its current state.


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## Eccentric (Jan 13, 2022)

*Oil Delivery System*

Today I worked on the oil delivery system, including the oil pump and the oil galleries required to deliver oil to top end (Camshafts) and to the crankshaft middle bearing.  The engine will have a wet sump oil system, that is oil will be stored in the sump of the engine and then pumped from there to where it is needed.  The original engine was of a dry sump design with an external oil reservoir.  A dry sump in a model engine is more complicated to implement; it would require two oil pumps, one to return oil to the reservoir and another to deliver oil to the engine bearing surfaces that need it.

Since I have chosen to use a wet sump I will provide an oil splash guard in the sump that separates the rotating crankshaft from the oil sump.  The engine has two breathers on the side of the crankcase, I will use one of the breather top caps as a dip stick/oil-fill and the other as an operational breather.

The image below shows the flow of the oil from the sump, through the oil pump, up through a gallery in the crankcase bottom, through a gallery in the crankcase top. At this point the oil can travel one of two ways, continue straight up through the gear tower to supply oil to the camshafts or back through the crankcase to the main crankshaft bearing.






Below the image shows the channel on the back of the timing gear tower that routes oil up from the crankcase to the head.  The block forms the other side of this oil gallery, an o-ring surrounds the channel in an attempt to minimize oil leaks as the oil is under a fair amount of pressure.





Below is a diagram showing how the oil makes its way from the crankcase to the internal camshaft gallery. 










Today I figured out how get oil distributed throughout the engine.  However, I still have a few design issues I need to resolve.  Above I show a couple of items that still need to be worked out.  Due to the direction of the engine rotation, the oil pump wants to deliver oil to the left side when viewed from the front, but it is difficult to bring oil to the top end on this side because of the gears that drive the distributor in the gear tower are in the way.  So the oil pump needs to deliver pressurized oil to the right side of the engine when viewed from the front.  There is a loopy path that creates an interference between the oil pump and the front crankshaft bearing holder mounting screws.

Below is a cutaway showing some of the issues still to be resolved.  Again, as always, I would like to thank Terry Mayhugh as I have derived many of these solutions from his Offy build.


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## Eccentric (Jan 14, 2022)

Magneto

The Offy used a Bendix Aircraft Engine Magneto to fire the spark plugs.  The engine did not require a battery as the magneto generated its own power.  Today I research these and collected  lots of good images.  My current thinking is to make the magneto a project all by itself.  It deserves good treatment, it sits right in the front of the engine for all to see.



























I also cleaned up the design of the oil pump and resolved some design issues resulting from component interference.  This is shown below






I started a Bill of Material spreadsheet capturing all of the hardware size and lengths for the engine.  This will be a living document as I refine things moving forward.


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## johnmcc69 (Jan 14, 2022)

Nice work!


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## petertha (Jan 14, 2022)

Re magneto, you mean you want to explore making a functional one?


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## Eccentric (Jan 15, 2022)

Thanks for the kind words @johnmcc69.  

Peter,
I am planning to have the magneto house the hall effect sensors to trigger ignition and the distributor for the spark plugs.  I had not thought to make it an actual functioning magneto and have it power the ignition system.  That is an interesting idea.

When I said the magneto deserved to be a project by itself, I was thinking that due to its complex shape, internal bevel gears and bearings, positioning of the hall effect sensors and the desire to make the timing adjustable, that it could be a standalone project with its own 3D model, plans, and build log.

The Offy engine build is a bit overwhelming and it would be nice to spin off the magneto and discuss it seperately.

Just a thought


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## CFLBob (Jan 15, 2022)

Eccentric said:


> The Offy engine build is a bit overwhelming and it would be nice to spin off the magneto and discuss it seperately.



Earlier in the week, I was thinking that maybe you shouldn't call this an Offenhauser because of the enormous amount of design you're doing.  It's in the range of saying things like, "inspired by the Mighty Midget" or something like that, but it looks to me like it's your design.  

Which reminds me I was going to say in the last design picture you posted, where you removed the cooling fins in the center of the oil pump area, you could leave the fins there but cut them to some fraction (like a third or a quarter) of the depth of the ones farther away from the gear.  With the engine fully built, someone would have to look really closely to see they're not full depth.  Even more so if they're only the reduced depth in the area around the gear you're trying to clear.


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## Eccentric (Jan 15, 2022)

Bob,

I really like your suggestion for the fins on the bottom.  I reduced the depth of the fins in the critical area, leaving them full depth where it does not matter.  I am sure no one will know the difference, except maybe us.





Regarding your comment on calling the engine an "Offenhauser Mighty Midget", I am not sure of the semantics.  Outwardly I want the model engine to resemble as much as possible the famous Offy, but I have take lots of liberty on the internals to make it an accessible design for model engine machinists like ourselves.  I would like the name plate to say Offenhauser, and people who are familiar with it say, "Oh yeah, that's an Offy." 

A purest might say, "that is not an Offy model because the Offy crank is nothing like the one you use."  Ron Colona designed and built a quarter scale 270 cu in Offy that is incredibly true to the original.  To install the crankshaft in the original engine, the crankcase had to be heated with a blow torch to expand it so the crankshaft could be installed through the end.  Then these intricate bearing holder plates were installed and the bolts tightened through small openings cut in the side of the crankcase.  This was very difficult for the original engine builders.  Ron duplicated the way the crankshaft is secured in his model (he did not have to use a blow torch  ).  He is an amazing craftsman and I know I do not have the skill to pull off building a model so detailed.






*Heating an Offy crankcase to install the crankshaft*
Source: Assembling A 270ci Offenhauser IndyCar Engine: Step By Step

As far as it being my design, I have to say that I am standing on might tall shoulders, Ron Colona for one and Terry Mayhugh for another.  I am drawing heavily from Terry's build of Ron's Offy.  I have learned so  much reading and following his builds.  I cannot express how much I appreciate the time these gentlemen have spent documenting their work. I am also using techniques that I have used successfully in the past and learned from other engine designers such as Westbury, Britnell, Howell and Hucks, to name a few.  But in the end, I do want to have plans that I can freely modify and distribute.

Thanks for the help with the fins.


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## CFLBob (Jan 15, 2022)

Eccentric said:


> As far as it being my design, I have to say that I am standing on might tall shoulders, Ron Colona for one and Terry Mayhugh for another. I am drawing heavily from Terry's build of Ron's Offy. I have learned so much reading and following his builds. I cannot express how much I appreciate the time these gentlemen have spent documenting their work. I am also using techniques that I have used successfully in the past and learned from other engine designers such as Westbury, Britnell, Howell and Hucks, to name a few. But in the end, I do want to have plans that I can freely modify and distribute.



Well, add yourself to that list as far as I'm concerned.  I'm overwhelmed reading this build, Foketry's Porsche 917, Mayhugh's Ford 300 Inline Six, and more. 

Glad that idea on the fins worked out for you.


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## Eccentric (Jan 16, 2022)

Oil Pump Assembly

Today I worked on the oil pump assembly.  I am using O-rings to seal the drive shaft and the oil passages in and out of the pump.  For pressure regulation there are two grub screws that can be used to limit the passage width to both the top end and the bottom end.  I do not have any form of relief valve in the oil system, just a way to divide the oil flow between the top and bottom end.






There are still a lot of details to be worked out, but I need to start working on the actual mechanical drawings.  The desired end result is a set of plans, after all.  I have not given any thought to part numbering or descriptions standardization of the parts.  I have a rough idea of how I want to maintain configuration control.  I am thinking that once I start fabrication I will reset the version numbers to version 1, then increase from there as I find issues.





Below is a cut away view of the oil pump in the lower crankcase half showing how it is tucked into the engine.






When I kicked this project off on December 23rd last year, I gave myself one month to complete the design phase of engine.  This means I only have one more week to wrap it up.  I am getting antsy to get back out into the workshop and make some parts.  I have learned the hard way not to start machining parts before the design is completely finalized.  I have found many design errors after parts are made, that if I had been more thorough in the design phase, could have been prevented.


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## Eccentric (Jan 17, 2022)

*Finalizing the Crankcase*

Today I worked on finalizing the crankcase which consisted of insuring the crankcase properly interfaces with all of the mating components.  What this boils down to is making sure all the attachment holes match the mating components and that all the holes are accounted for.  Also that the holes don't run into each other inside the part.





The breathers in the side cover will function, they are connected to large holes to the top of the inside crankcase and there are small holes that act as oil drains near the bottom of the breathers.  Reviewing the above image, I think I can eliminate the block hold down screws behind the camshaft oil return holes.





Most of the above is self explanatory.  The timing gear bearing relief insures that the bearing can be fully seated in the bearing pocket and that the inner race does not interfere with the inside of this pocket.











Above shows the crankcase side breather panel in place and below shows the detail under the cover.  I like the way I was able to hide the crews that join the two crankcase halves behind the side panel.  A ball end Allen wrench will be able to tighten these screws.






I also worked on some of the smaller parts highlighted in the next two images: The breathers, bell housing, fly wheel, front motor mount, right side block and crankcase covers.









And then on to the Honey Do list, sand blast and repaint the patio furniture.  With a little help from Harvey.


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## mayhugh1 (Jan 17, 2022)

Don't forget to allow for the thickness of the gasket or sealer between the two crankcase halves. It's non-zero (I used .004" vinyl) and will affect your gear meshes, oil passage transfers, and drilled hole locations on the front face. Nice work so far. - Terry


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## Eccentric (Jan 20, 2022)

Thanks Terry,

I will incorporate gasket thickness into my design.  I see that you use a wide array of gasket types and thicknesses.  Vinyl .003" ish,  Teflon in.004",  .010" and .020".  It also seems that some flexibility during assembly is required as tolerance variation may required different gasket thickness to resolve stackup issues.

I am researching the viability of fabricating a drag knife for my little CNC router.  I am terrible at cutting out gaskets by hand, I would really like to automate the process.  I'm thinking I should be able to adapt one of these:



			https://www.amazon.com/YF-Tungsten-Lettering-Cutting-Plotter/dp/B07P8DMC2J/ref=sr_1_2?crid=B9I0DYIGZ89M&keywords=drag+knife&qid=1642724378&s=office-products&sprefix=drag+knife%2Coffice-products%2C124&sr=1-2
		






Post Script:  I found many designs for drag Knifes on Thingiverse.


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## Eccentric (Jan 21, 2022)

*Crankcase Plans*

I am starting the process of creating the plans for each of the parts.  The first few are going to be the most challenging.  I am an engineer, not a draftsman, so I am learning as I go.  On complex parts like the crankcase, the drawing is broken up into multiple pages and I am not sure how they should be organized. I think like a machinist and organize the prints in terms of order of operations, not sure if that is a good way to go.  Should I have prints of the top and bottom crankcase halves individually, or the whole crankcase?  As you can see, I am winging it here.  Ultimately it may not matter as long as all of the needed information is present.

 Eventually I will put these plans on my website for free download:  

GregsMachineShop.com













The other consideration I am pondering is: should I include instructions? how detailed should they be? What level of machinisht should I target with the instructions?

Thanks, Greg
GregsMachineShop.com


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## johnmcc69 (Jan 21, 2022)

Hello Greg, I think you're doing a pretty good job with all this.
 I know from my experience, I liked to see only one view of the part as it was presented to the machine spindle, (I.E.=how many operations can be completed in the least amount of set-up's). Make as many pages as you think it takes to keep things CLEAR in the drawings. I've seen too many drawings that were excessively cluttered & very hard to read. Good machinists can read these cluttered drawing like a book & have no problems doing it, but for the hobbyist, try to keep them uncluttered & simple. 

 I think you're better off just doing good, clear, concise drawings. Dimension & tolerance features as required, for pistons & cylinders I've just added a note.."running/sliding/" fit to item "#XX. For everything else, "General" tolerances apply. Add a short note to items of importance "Lap valves item #X to valve cages item #X at assembly". 

 Leave any instructions for the end when you get it running, setting the timing, carb settings, ETC.
 But, definitely keep notes on what you do. You may find that you have an opportunity to have this engine published & will be glad you scratched those notes on napkins...

 I hope this helps in some way & look forward to other reply's from the builders here that do this all the time.

 John


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## mayhugh1 (Jan 21, 2022)

I wouldn't spend a lot of time on machining instructions. Assembly drawings can be really helpful. Nice Work. - Terry


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## Foketry (Jan 23, 2022)

This forum is a valuable source of technical information, experiences, and knowledge from around the world that are fundamental to improving our projects, our hobby.  I will take some ideas from your designs for my future model engines.  I may have lost some information but I have not seen what system you use to push the valves, directly with the cams on the valve stem or will you use tappets?
You are doing a great job, thanks


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## Eccentric (Jan 28, 2022)

Fokety,

You already know this, but it is worth repeating. The beauty of the overhead cam engine is that there are no lifters, push rods and rockers to rob the engine of power; fewer parts to expand and contract with the heat, to wear, and to mess up the vavle lash.

In my model engine, there is a simple cam follower that rides in the pocket of the cam boxes between the cam lobes and the valve.  It is an upside down cup, with the valve riding inside the cup and the cam lobe pressing agaisnt the bottom of the cup.  In the real engine the valve stems were manufactured over length and the engine builder filed down the top of the valve to adjust the valve lash.  In my case the cam followers will be made after the valves and the thickness of the bottom of the cup will be varied to adjust the desired valve lash.  A picture is worth thousands of my words:






Oh, it is a Valve Spring retainer, not Sprint.


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## Bentwings (Jan 30, 2022)

Eccentric said:


> I am going to try my hand at developing plans for an Offenhauser Mighty Midget Racing Engine. There were many variations manufactured through the decades and my version will likely end up being an amalgamation of many of those.  I will also be taking liberties for ease of manufacture/assembly and to increase the chance of it being "a good runner"--or at least a runner.  Inspiration came from Terry Mayhugh's build of Ron Colona's Offy and I will be taking advantage of his engineering insights shared in his (Terry's) build log.  Why don't I just build that engine?  Well, it is a very complex model and I don't feel ready to tackle that level of sophisticated craftsmanship.  Second, I want to have control of the plans and have the ability to freely distribute them if they ever get to that point.  The variant of the 97 Cubic Inch Midget Offy I will build will have two valves per cylinder instead of the large Offy's four valves per cylinder, the crank will be supported in three places instead of the large Offy's five and the 97 cu in Mighty Midget Offy is smaller overall.  I will design in 1/4 scale so the model engine will be about  5.375" long, 5.5" tall and 4 inches wide.
> 
> View attachment 132136
> 
> ...


how are you planing on finishing valve seats?


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## Eccentric (Jan 30, 2022)

Bentwings said:


> how are you planing on finishing valve seats?




The way I currently plan to finish the valve seats and the best way to do it are probably not the same.  In the past I have had good luck with a simple technique.  I make the valve cage in one setup on the lathe, the last operation before parting it off is to set the cross slide to 45 degrees and take a small cut on the valve seats, less than .010".  After parting off the valve cage, I use the finest polishing compound I have, put some liberally on the seat, insert the valve.  Then by hand I rotate the valve with a little pressue into the seat, in random twists, lifting and reseating the vavle, then perform more random twists.  I do this just a little bit to get a polish on the valve seat, I don't think I am removing material at all.  Finally I perform a simple vacumm test through the valve steam of the valve cage.  I pull the vacuum through the rear of the valve cage, past the stem of the valve. I seal the side hole and then take time measurements to see how long it takes to bleed down in different configurations. With the valve held slightly open it takes 3 seconds to bleed down--this is my baseline. This measures the leakage between the cage and the valve stem. Lightly holding the valve closed with my thumb I consider a pass to take at least 30 seconds to bleed down, this represents the valve spring holding the valve closed. The only time I had a failure with the vacumm test was when I was cleaning up the side port burrs inside the valve cage and accidently put a scratch in the seat with the X-acto knife.  I didn't attempt to repair the valve guide as I made spares.


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## Eccentric (Feb 9, 2022)

*3D Model Complete*

I feel that the 3D model of the Offenhauser 97 cu in. Mighty Midget is complete enough to take it to the next stage and start building the prototype. Including the hardware, there are 86 different part types in the model and over 300 individual parts.







I have 3D printed a few of the major components to get a sense of scale.





The gear tower is pretty small and has very intricate details.






I am going to machine the crankcase top first. Working on the mill, I square up the work piece and drill 4 holes as entry points for the 1/4" flat end mill that will rough out most of the inside material.






I create the tool paths in Fusion 360 then output G-code for the LinuxCNC router.





Then I start making chips.





I believe that this will wrap up this thread in the "Plans" section. I will start a new thread in the "Work in Progress" area to document the machining of the prototype. As I build the prototype, I will be feeding back what I learn into the 3D model and the drawings.

Thanks for tagging along,

Greg
GregsMachineShop.com


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## propclock (Feb 9, 2022)

Fantastic I will follow .


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## stevehuckss396 (Feb 10, 2022)

See you there!


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## CFLBob (Feb 10, 2022)

I don't see the building thread this morning, so when you get started, post something here about it.  Could be just the name of the thread, or a link.  Anything so that those of us watching this thread will know to move over there.


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## Eccentric (Feb 11, 2022)

The build log for the Offy can be found here:
Offy Build Log


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## petertha (Feb 11, 2022)

Will be following the build! 
Maybe I missed it or maybe my brain has been devoted to my own after-the-fact gear issues, but what is the plan to key the timing gears to the shafts once clocked in?


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## Bentwings (Feb 15, 2022)

petertha said:


> Will be following the build!
> Maybe I missed it or maybe my brain has been devoted to my own after-the-fact gear issues, but what is the plan to key the timing gears to the shafts once clocked in?


I saw that too. What’s the plan?


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## propclock (Feb 15, 2022)

Man that is small!! it is one thing to look at your cad drawings but with a real life  block next to 
a caliper. It really brings it in to reality.  Too much like watch making for me.  Perhaps 2x would be 
big enough for these old eyes.


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