# Valve timing 4 stroke glow



## petertha (Mar 11, 2012)

I put together a little comparison of valve timing figures among some engine specs I had access to (all 4-stroke, glow plug ignition, methanol fueled). I was interested in how they lined up more as a guide to something Im drawing up now, so excluded gasoline/ignition. 

The commercial engines were referenced in Clarence Lees book: The R/C 4-stroke Engine. He mentions the factory specs & also his own measured timing, sometimes they don't correspond. The other 'shop construction' engines I had to try & figure out from SICtype drawings. (Hmm.. I think I got it close, but Im still scratching my head on some). Im still getting my head around cam math so hopefully its accurate. 

What stands out is a surprising variation when they are lined up despite what I'd think to be similar requirements: easy starting, good idle, nothing really racey or high performance? Id appreciate any comments on this. Maybe it just doesnt matter that much? Or maybe any home-shop simplification trick trumps ideal settings?

Also, does anyone have comparable 'Intake Open BTDC' type numbers on methanol radials like the Edwards 5? I havent evolved from the primordial ooze to understanding bumpy cam ring planetary gear type engines yet. ;D For example, the Rob Sigler pdf plans show a tabular cam profile chart: Intake Lobe 1: Lift = 0.0 at 355 deg, Lift(max) = 0.0750 at 25 deg, Lift = 0.0 at 55 deg. How do I turn that into comparable numbers?


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## gbritnell (Mar 11, 2012)

I can't say I know nor have I studied 4 stroke, glow plug methanol fueled engines but I have studied 4 stroke, spark fired, gasoline fueled engines. I do know that glow engines have a higher compression ratio (not compression pressure) than spark engines. That being said by looking at the cam specs you posted I see where generally the intake is open is open much later in it's cycle, ABDC, and the exhaust opens much earlier in it's cycle, BBDC. When you leave the intake open later on an engine you reduce the compression pressure (not ratio, that's fixed) so with a generally higher ratio you would have to limit the amount of cylinder fill otherwise the compression pressure would become extremely high. Probably the reason for opening the exhaust earlier is to make sure that by the end of cycle that the exhaust gases are completely evacuated. I don't know for sure but I would think if you look at the specs for 2 cycle, glow plug engines you might see similar numbers. As has been discussed in other threads about cam timing specs the raw numbers can't necessarily be applied to every engine. A cam manufacturer will want to know what type of compression ratio your engine has as well as rpm range, exhaust manifolds/headers etc. All of these figure into getting the best performance out of a specific cam profile. If a cam is designed for certain features and your induction system won't flow what it needs to then the cam won't operate optimally. 
With higher rpm engines like small glow plug engines the scavenge/fill characteristics would be quite a bit different from larger slower spinning engines. 
gbritnell


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## mu38&Bg# (Mar 13, 2012)

Peter, what is your goal?


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## petertha (Mar 13, 2012)

Hi Greg. The (very lofty!) goal is to make this Kinner B-5... and hopefully witness it run reasonably well... one day! So far, its a still a W.I.P dream in my mind, but slowly coming along. I have no misguided notions that constructing it will be a personal challenge, but I'm commited starting the journey. I get plenty of inspiration from the awesome projects & accomplishments on this website. And useful answers to my never-ending stream of questions! ;D


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## Swede (Mar 14, 2012)

I have a wonderful old book from the 1930's called "aircraft engine maintenance", back when almost all airplane engines were round. It has an appendix with all of the timing numbers for the various engines of the day. If you want, I can look up a particular engine.

I've found with models that it is simply not all that critical unless you are building a race engine. The common goal is easy starting and reliable running, and if the valves open roughly where they are supposed to, then the engine will run.

It is very easy to get all worked up over cam profiles. Again, the reality is that if the cam lifts the valves open anywhere close to theoretical, the engine will run fine. Hand-filed cams work, as do milled cams. One need not pursue cosmic cam grinders and the like.


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## mu38&Bg# (Mar 14, 2012)

Model aircraft glow four stroke engines are designed to be powerful and there are several manufacturers competing for profit. So, compared to some of what you see in model engineering, valve timing looks a bit wild. Enya in particular have long duration, but really don't run appreciably different than any other brand. Many factors come into play and cam timing is just one. I can say that all of these engine use "real" cams. That is to say they are not four arc, or flat flank. None of them have ever updated their cam profile, either. 20 year old cam part numbers appear in recently released engines. However, the rest of the system has changed, valve and induction tract sizes, possibly rocker ratios. Small diameter flat followers do limit how aggressive the cams can be.

If you're looking at building something of comparable power to weight ratio as a model airplane engine, some serious research is required, or just borrow the cam profiles. Complicated cam profiles are not easy to design or produce. The profile (actual valve time-area) is what matters not really the timing figures themselves. Those that can design good engine cams get paid well for it and demand they get paid.

Differences in cam profile are not very obvious in smaller displacements, so I wouldn't worry about it all that much. I see Swede posted before I did. For an easy to start engine I'd go with timing of the OS engines before the surpass series.

Greg


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## petertha (Mar 14, 2012)

Swede  said:
			
		

> I have a wonderful old book from the 1930's called "aircraft engine maintenance"...., . If you want, I can look up a particular engine.



Yes please, that would be interesting. Because the 1:5 model will be methanol/glow vs aviation gasoline on the 1:1 banger, probably different timing but I'm curious anyway. I was able to order the the 1942 overhaul instructions which has some good info. There are some pics of the cams & gears, rebuild specs, parts details etc. but unless I missed it, didnt see timing specs. 

This Kinner had some model variations but of anything pops up in your book under Kinner 'B-5', 'R-440-1' or R-440-3' thats the one.


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## petertha (Mar 14, 2012)

dieselpilot  said:
			
		

> ... or just borrow the cam profiles....Differences in cam profile are not very obvious in smaller displacements, so I wouldn't worry about it all that much. ...For an easy to start engine I'd go with timing of the OS engines before the surpass series.



Yeah I was able to take apart my dad's old rusty OS FS 61 for a looksee. It is comparable cylinder displacement, shorter stroke though. Visually the cam has a very slight curve, but when I sketched it out with it's 0.315" base circle, 0.070" lift, timing tangent line from the quoted specs & nose radius, there just isnt a lot of difference between a gentle curve & straight line connecting the tanget lines to the nose to satisfy the geometry. For now I have decided to just clone the timing, adopt a straight line for machining simplicity & move on with the drawing. I'm familiar with the RC stuff & how they run & seems like many constructed engine have similar flats over hyper-watcha-ma-call-it profiles. 

It just struck me that the timing was quite different among the constructed engines, meaning how the cams were phased relative to each other & TDC, BDC etc. But who knows, maybe they had a different idea or less nitro or...


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## Swede (Mar 15, 2012)

petertha  said:
			
		

> Yes please, that would be interesting. Because the 1:5 model will be methanol/glow vs aviation gasoline on the 1:1 banger, probably different timing but I'm curious anyway. I was able to order the the 1942 overhaul instructions which has some good info. There are some pics of the cams & gears, rebuild specs, parts details etc. but unless I missed it, didnt see timing specs.
> 
> This Kinner had some model variations but of anything pops up in your book under Kinner 'B-5', 'R-440-1' or R-440-3' thats the one.



Kinner B-5 specifications:
125 HP, 1925 TO RPM, 295 lb, 5.25:1 compression, dual Scintilla magnetos
Ignition timing, BTC 26 degrees
Intake opens 25 BTC, closes 82 ABC
Exhaust opens 65 BBC, closes 42 ATC


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## mu38&Bg# (Mar 15, 2012)

The main issue you'll see with a flat flank is wear. The edge of the follower will ride on the cam. With good materials and low run time it will still last forever. Occasionally these cams in model engines still ear away completely when they blow the heat treat on the cam. Some modelers put a lot of time on engines, so they do have to last.

Greg


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## petertha (Mar 16, 2012)

dieselpilot  said:
			
		

> The main issue you'll see with a flat flank is wear. The edge of the follower will ride on the cam.Greg


Thats an interesting point you bring up, I was wondering about this too. Here are some pics of the typical OS valve assembly. I think Im seeing ever so slight curved cam profile between base circle & nose radius. But a perfectly flat bottomed tappet. Its not obvious on this pic because its showing the pushrod ball depression end, but Im holding one now from this gutted engine & its flat. Actually I think its even free to rotate around.

On this OS engine cam, its the opposite wear pattern. The flatish part is stock looking like the base circle but the noses are shiny wear. (Not the picture, thats from a parts catalog, mine looks like it was resting with the Titanic). Might have something to do with my dad's lack of valve gap setting maintenance!  Anyway, it has a lot of hard miles on it. If I could get that I'd be happy.

Im also showing my drawing idea. Right now the cam has a flat profile, along the lines as how Ive seen others make them, presumably because thats easier. It has a 0.080" lift (probably more than I need, 0.070 is what I want the valves to move but the I have to figure out the pushrod angles still). The cam radius is 0.100" which leaves a tiny witness flat on top. With this geometry I measured the flat segment to be 0.136" long. But I have a curved bottom on the tappet. So I wonder, would a [curve tappet bottom + flat segment cam] combo wear about the same as the OS [slightly curved cam, but flat bottom tappet] combo?


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## petertha (Mar 16, 2012)

Swede  said:
			
		

> I have a wonderful old book from the 1930's called "aircraft engine maintenance",



Thanks for the specs Swede. Is this the book you have by chance? Or maybe you can provide the author etc

http://www.amazon.com/dp/B000NQ539E/?tag=skimlinks_replacement-20


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## petertha (Mar 16, 2012)

I stumbled on a review article for a YS 140 FZ. I used to run those engines back in the day, but I had no appreciation for their cam specs. These were the psuedo-supercharged type, but look at how different the timing is vs the other commercial 4S's. And YS's weren't really revy, they were propped best at being grunters.

IO = 62 deg BTDC
IC = 68 deg ABDC
EO = 100 deg BDC
EC = 32 ATDC
overlap = 94 deg!
intake valve dia = 15mm (0.590"), cam lift = 0.160" (=27% of cam dia). Exhaust valve = 13mm (0.512)

I think the OS lift works out to about 21% of valve dia by comparison, thats what I assumed for mine.


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## gbritnell (Mar 16, 2012)

Peter,
The one thing you have to realize with the rounded base lifters is that they will need some type of appliance to keep them lined up with the cam. With roller lifters there is a link between pairs of lifters to keep them from rotating. With flat tappets this is a non issue.
gbritnell


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## Swede (Mar 16, 2012)

petertha  said:
			
		

> Thanks for the specs Swede. Is this the book you have by chance? Or maybe you can provide the author etc
> 
> http://www.amazon.com/dp/B000NQ539E/?tag=skimlinks_replacement-20



Yeah that's it exactly. At $5 for a used copy, I'd jump on it. It's got a lot of excellent information.


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## mu38&Bg# (Mar 16, 2012)

Peter, that first pic of the OS cam is one of mine ;D. I posted it in a forum when discussion came up of OS cams shedding their nickel plating. Tha I once found a site or software online somewhere that did some cam math. I came to the conclusion that OS followers are just barely large enough to avoid riding the edge. As soon as I get my 4th axis running in the mill I'm going to plot the profile for the cams that I have for some deeper research.

If you look at the OS cam and note the narrow contact area, that's where you'd have the follower edge dig in if you aren't careful about the cam design. With flat flanks you'd want to design so the follower is large enough that the nose radius is under the follower when it reaches that point. Think OHC with buckets on the valves. That will be a hard jerk when running. I've spent some time reading about cam design a couple years back and decided that I had much to learn if I wanted to produce RC engines. BTW those followers in your pic are Saito and they are smaller in diameter than OS, some are "top hat" with a larger diameter head that contacts the cam. If space in an issue I would choose that route first in your design.

The YS cams do look quite a bit different than OS. Other interesting engines that are harder to find are the car series OS four strokes, FS26S-C and FS-40S-C. These had larger valves, heavier springs and much different cams. A tuned FS-26S-C could run up to 25kRPM, making peak power near 22k or so! The 40 was a bit slower.

Greg


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## petertha (Mar 16, 2012)

gbritnell  said:
			
		

> ...with the rounded base lifters is that they will need some type of appliance to keep them lined up with the cam.



Yes, I know exactly what you mean. My original thinking here was to have flat faces between the tappet 'shoes' & positioned with a very close slip fit between them. So the idea was the faces allowed them to slip up & down, always sharing some contact area with one another, but not be able to rotate (much) because the stem part is constrained concentric to their holes. 

But I can already see that any necessary running gap, even a couple thou, means they will rotate to some degree. I was aimig to see how much. I think I've already moved off this idea in favour of a cylindrical, free-to-rotate tappet....which ideally favours a more profiled cam shape. Seems like its a chicken & egg thing!


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## petertha (Mar 16, 2012)

Swede  said:
			
		

> Yeah that's it exactly. At $5 for a used copy, I'd jump on it. It's got a lot of excellent information.



All domestic (USA) shipping. Grrr... you guys get ALL the good stuff!


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## petertha (Mar 16, 2012)

dieselpilot  said:
			
		

> ...Peter, that first pic of the OS cam is one of mine....
> ... found a site or software online somewhere that did some cam math.



Uh-oh! What are the odds. Hopefully no copyright lawyers in your family. I'm an honest guy & usually always quote references... I swear! ;D

Ive seen references made to Camcalc? Java? program. I think it generates those hypo-?? (curvy) cam profiles & a table of cut depths at degree increments once you input valve timing & some geometry. I rememebr it worked for me once when I was just poking around, but then not again. Might have been a short term glitch, I'll earch again.

Seems like model designers go down one of 3 paths: simple tappet & more involved cam, simpler cam & more complicated tappet, simple tappet AND cam & live with any consequences!


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## mu38&Bg# (Mar 16, 2012)

I mess around a lot with OS four strokes. I have an FS-40S on the bench right now that will soon be diesel.

Camcalc from www.modelenginenews.org is a modelers type of design. I haven't checked it in a while, but a lot of people here and elsewhere have used it with good success and long lasting results. I don't remember the one I found, I only know that it was some kind of demo. It let you design and analyze all kinds of wonderful cams, but you couldn't get the data out to build one.

Because cams and followers could easily be replaced if you find the means to create better parts I'd just go with what you can build now. if you get to the point where you can do better, it will be easy to make another set.

Greg


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## Swede (Mar 16, 2012)

petertha  said:
			
		

> All domestic (USA) shipping. Grrr... you guys get ALL the good stuff!



Seller FREDSUSA has a copy for $7 + $13 shipping, although it'll take a few weeks. Believe me, anyone who loves radial engines will also love this book. It was printed in 1939, at the outbreak of WW2, and discusses the maintenance of basically all of the major Western-made radial engines, like the Wright series, the Wasp, Wasp Jr, Continental, Kinner, etc. 

Lots of superb information. I'm surprised there are so many copies available, so cheap.

Good luck with your project. FWIW, my deHavilland Cirrus uses tool steel cams and tappets, very basic and plain, and there is very little wear. My larger engine has a 4340 steel ring cam hardened to RC46, and the tappets are very hard A2 steel, with a hemispherical surface, no rollers. The cam surface has nothing more than a shiny band, NO wear or defacement otherwise, and the engine has been run for well over 100 hours on a stand.


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## petertha (Mar 18, 2012)

Swede  said:
			
		

> FWIW, my deHavilland Cirrus uses tool steel cams and tappets, very basic and plain, and there is very little wear. My larger engine has a 4340 steel ring cam hardened to RC46, and the tappets are very hard A2 steel, with a hemispherical surface, no rollers.



I've always wondered about this. Your larger engine, I assume the Hodgson? From what I am able figure out of that one, the tappet end is hemi-shaped as you say & rides over a similar lift profile of 0.078"? So after extended running one would expect either 1) a flat facet wearing on the end of the tappet hemi profile 2) a groove wearing on the cam profile... based on their relative hardness. You say no wear, thats awesome.

So on simpler cam profiles with (base circle > flat > nose radius) profiles, I believe like the Cirrus, flat bottom tappets would seem to have a few disadvantages: the potential for the edge digging into the cam as its rising, almost like a lathe tool. Potential for timing jumps as its transitions from flat to edge vs rounded tappets that kind of ramp a bit smoother into the cam profile. It obviously 'works' as you say. Maybe rounding those little buggers is not as easy as simply drawing them & thts more the issue? ;D How did you do yours BTW?

Also including the missing pic of the housing method I was considering to retain my curve bottom tappets running in aliggnment to the cam, loosely modeled after the real engine (sans rollers). It looks like a lot of extra fussy work that could hopefully be simplified with hemi-end tappets?


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## Swede (Mar 19, 2012)

> So on simpler cam profiles with (base circle > flat > nose radius) profiles, I believe like the Cirrus, flat bottom tappets would seem to have a few disadvantages: the potential for the edge digging into the cam as its rising, almost like a lathe tool. Potential for timing jumps as its transitions from flat to edge vs rounded tappets that kind of ramp a bit smoother into the cam profile. It obviously 'works' as you say. Maybe rounding those little buggers is not as easy as simply drawing them & thts more the issue?  How did you do yours BTW?



When I say "no wear" obviously the correct answer is actually very slight "acceptable" wear. Less wear than I anticipated.

On the Cirrus, the diameter of the tappet bases is quite large relative to the cam. Here are the pictures. Note the cam shows some shine on portions of the nose, but as this cam was cut with an end mill and profiled by hand, there were bound to be high sections which wear first, and as it wears, surface contact improves. I think with any model IC engine, there's going to be some run-in time where the valve clearance will need to be adjusted more than once, to take up this initial wear.

The tappet bases don't even show much of a shine.











The Hodgson - the tappets had a full hemisphere made of A2 steel. Each of these now has a polished, flattened profile on the tip that measures perhaps 0.020" across. A better choice than a hemisphere profile might be having the tappets chamfered first, say 45 degrees, and then simply smoothing the remaining small diameter at the tip somewhat, so a larger surface rides the cam rather than a theoretical point.

Also, I think it's important to make the cam harder than the tappets, causing more wear on the tappets vs. the cam. Tappets are a lot easier to replace in the future than cams.

In retrospect, I wish I had engineered roller tappets using tiny instrument ball races, with the tappets keyed to their bushings. 1/8" square stock would do the trick, with each set of tappet bushings being one piece and pressed into the crankcase.

I freely admit to having only a basic knowledge of cams, cam profiles, and the performance changes that can result. I was far more into materials, how they wear, and getting basic, reliable performance was the goal from day one. After digesting a half-dozen cam articles on how to wring the very last joule of energy out of an IC engine, I was a bit discouraged, until I followed the deHavilland Cirrus 1/6th scale build in Strictly IC; saw that he milled the cams, polished by hand, and I understood that if all I wanted was a runner, not a race engine, it's not all that critical.


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## gbritnell (Mar 19, 2012)

If the cam and tappets are both hardened and the tappets have a hemispherical shape then the wear is going to be much, much less than if one or the other is unhardened. I have always made my flat tappet cams from unhardened drill rod (W-1) and hardened the tappets/lifters. On some engines the oiling is by splash while others are pressure fed. I have checked the cams after many hours of running and the cams are polished on the lift part of the lobe but not on the flank diameter (valve clearance). On my V-twin and now on the radial I have opted for roller lifters and use a similar setup for both engines. The lifters and the rollers are unhardened drill rod and the cam is likewise. 
If you design a lifter with a hemispherical shape then the valve timing specs are going to change relative to a flat type lifter. (tangent points). I wouldn't recommend a 45 degree chamfer as that would introduce a sharp edge (albeit small) where the chamfer meets the flat and here again it would change the valve timing because of the tangent points.
Attached are several pictures of the radial I'm building with the lifters, crankcase and cam ring. The lifters have a small flat milled on them and the lifter boss is extended outward to accept a small set screw that has the tip ground flat. Upon final assembly the set screws will be adjusted for minimal clearance and Loctited in place. I will use a low setting Loctite for future removal if necessary to work on the lifter.
gbritnell


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## Swede (Mar 19, 2012)

Excellent information!



> I wouldn't recommend a 45 degree chamfer as that would introduce a sharp edge (albeit small) where the chamfer meets the flat and here again it would change the valve timing because of the tangent points.



In my mind, I'm picturing a tappet chamfer of 45 degrees, leaving a round tip diameter of say 0.032" to 0.050"... now, I'm going to mount the tappet in a drill press or lathe, and with a cratex abrasive in a dremel, apply the cratex to the revolving tappet tip. This will smooth and round off the circumference of that end circle.

Basically, a full hemisphere is a point that rides on the cam ring. On the base circle portions, there is little or no wear, no real pressure... they are essentially floating. As the cam lobe approaches, there'll be initial contact slightly off tappet axis, switching to the other side after cam peak. As the tappets rotate in their bushings, the overall wear will create the small, shiny circular spot that I've seen on my own hemispherical tappets. 

But in the end, a hardened hemisphere works fine. It's just a thought.


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## petertha (Mar 19, 2012)

gbritnell  said:
			
		

> ... now on the radial I have opted for roller lifters ...
> ...lifters have a small flat milled on them and the lifter boss is extended outward to accept a small set screw that has the tip ground flat.



Very nice George. The FS Kinner roller lifters are actually very similar to what you have made there. One key difference is they are orientated as a sliding pair, clustered right adjacent to each other. Your very neat set screw idea riding on the flats works perfect on the cam ring style engine because exhaust & inlet each have their own holes in the crank case & their own respective set screw sets. The FS Kinner 'houses' the roller lifter pair assembly in yet another part that retains them by their outer flats. And they slide on one another by inner flats. My cad picture 211544 shows a crude equivalent. I'll try & scan it from the manual just for interest, but I'm sure you already know. Replicating that is where its starting to look 'newbie-challenging'. But never say never, right? I just realized I've come full circle without a single metal shaving to show for it!

Can you provide a few dimensions for reference (lifter body dia, roller ball dia, axle dia/material). Another thing Im coming to realize is model cam assemblies, at least mine, is quite big by comparison. If you scaled the FS down it would be watch jewelry so that in iteslef requires some compromises.


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## petertha (Mar 19, 2012)

Swede  said:
			
		

> On the Cirrus, the diameter of the tappet bases is quite large relative to the cam. Here are the pictures.



Thanks for the pics. For my engine, that typical inline engine cam spacing is part of the problem. The wider the cams + longer the intermediate segment + blank shaft allowance for 3 widths of timing gears.. means the the back half of the crankcase starts to get quite long & look a bit weird. Its all about compromises so Im doing what I can.

But maybe I missed something important. Is there some sort of cross pin in the tappet that keeps them retained for rotating freely in their hole?


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## petertha (Mar 20, 2012)

Kinner roller lifter assembly.


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## gbritnell (Mar 20, 2012)

Hi Peter,
Here's a PDF of the drawing sheet with my roller lifters. 
George 

View attachment ENGINE RADIAL SHT 4e.pdf


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## Swede (Mar 21, 2012)

petertha  said:
			
		

> Thanks for the pics. For my engine, that typical inline engine cam spacing is part of the problem. The wider the cams + longer the intermediate segment + blank shaft allowance for 3 widths of timing gears.. means the the back half of the crankcase starts to get quite long & look a bit weird. Its all about compromises so Im doing what I can.
> 
> But maybe I missed something important. Is there some sort of cross pin in the tappet that keeps them retained for rotating freely in their hole?



On my Cirrus, the tappets are free to rotate. As to how much they actually do, I have never checked.

The pin in the cam is a cheat... the cam is darned small, and rather than mill a keyway, which would potentially enter (and mar) the rear journal area, I simply drilled and pressed in a hardened pin. The cam gear had its keyway slot created by the common technique of using the lathe, and grinding a tiny bit with a hook - the bit is run in, fed, and retracted, like a shaper. Commercial keyway broaches aren't common in that small size.

I've had zero problems using this pin rather than a true key. If this is done, I think it helps quite a bit if the gear slot is shallow, so that when the gear is fitted, you get a decent pressing action of the top of the pin with the bottom of the slot. Tight is good.

Another cheat to key a gear to a shaft is to score the shaft lightly in two opposite sides, fit the gear, then drill undersized for a hardened pin. The drill follows the scoring, and you end up with two holes, each occupying 1/2 of the gear and the shaft. This is light duty stuff, but works fine for tiny setups. Any forces strong enough to strip these round keys is going to be also strong enough to cause other damage like stripped gear teeth.


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## petertha (Mar 21, 2012)

Swede  said:
			
		

> ...the tappets are free to rotate....
> ...the pin in the cam is a cheat...



I'm glad I pursued this because I was obviously completely discombobulated. I thought that picture was showing the tappet resting on the end of the crankshaft just to prop it up for photography because its tiny. the color of that end segment looked different than the adjacent cam area. You mentioned the tappets were free to rotate which made me wonder why then the cross pin. Nooooww I get it!

Well that conversational detour saved me yet another question as to good ways to keep timing gears retained! Double bonus. Thanks for that.


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