# Looking for plans for Cox radial model airplane engine



## MickeyDD (Oct 3, 2019)

Hi all,
As my next model airplane project I would like to build a small 3 or 5 cylinder radial model airplane engine utilizing Cox .049 or .051 cylinder/piston assemblies. I have built 5 small engines so far (3 diesels and 2 glow; both single cylinder and twin). I am using a manual Sherline lathe and mill (no CNC). I don’t want to gear complete engines together but would prefer to make a common crankcase and machine the internals. I know that there have been many home made Cox radial engines built but I have been unable to find a complete set of plans for one. Does anyone have plans for a Cox radial engine as well as where to purchase a thread tap that matches the Cox cylinder thread. I understand it is a very unique thread and a tap was once available but no longer.
Thank you,
Michael


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## rlo1 (Jan 5, 2021)

also interested in a .049 radial.  Any luck finding plans?


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## jkimberln (Jan 5, 2021)

MickeyDD said:


> Hi all,
> As my next model airplane project I would like to build a small 3 or 5 cylinder radial model airplane engine utilizing Cox .049 or .051 cylinder/piston assemblies. I have built 5 small engines so far (3 diesels and 2 glow; both single cylinder and twin). I am using a manual Sherline lathe and mill (no CNC). I don’t want to gear complete engines together but would prefer to make a common crankcase and machine the internals. I know that there have been many home made Cox radial engines built but I have been unable to find a complete set of plans for one. Does anyone have plans for a Cox radial engine as well as where to purchase a thread tap that matches the Cox cylinder thread. I understand it is a very unique thread and a tap was once available but no longer.
> Thank you,
> Michael


I do not know the thread size but if you find out what it is, try Victornet.com for a tap and die source.  They have been my go-to source for strange threads for years.


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## Shopgeezer (Jan 6, 2021)

The Cox cylinder thread is 17/32-40. It is very hard to find. No supplier that I know of has any on hand. The only solution seems to be to have one made. Commercial suppliers will do it but very expensive for just one. It would help to get some model engineers together who want one and make up a dozen or so. The cost goes down the more you order.


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## Ed T (Jan 6, 2021)

A million years ago I made a 90 degree V-twin from two COX .049's. I grafted two crankcases together to address the threading problem and modified the con rods to fit onto the crank pin. It ran on both cylinders, but I failed to take the crankcase compression issue into account so it didn't run very well. For a two cycle radial with a common crankcase, having crankcase compression becomes problematic since there probably won't be any; so some kind of supercharger becomes necessary. That is probably a bigger problem than the rest of the engine since superchargers don't scale very well. Somebody probably figured out how to do it, but it's a lot more involved than just the mechanicals of the crank, master rod and having the pistons go up and down.


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## Mechanicboy (Jan 6, 2021)

The super charger can't work in the two stroke engine due the exhaustport and transferport is still open and mostly of the fuel mix is blown out of the exhaust port and  then the transferport is closed before exhaust port, no filling the pressure in the cylinder and fuel mix is low in the cylinder. Tuned pipe works best in the two stroke engine. Enough about this supercharger in the two stroke engine (Two stroke with exhaust valve who are timed and works best with supercharger, mostly used in large ship engines) . Use reed valve in each crankcase in the common part who has the center of propeller shaft who has connection with each Cox engine via gear wheel.


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## mu38&Bg# (Jan 6, 2021)

Supercharger, means a pump of some sort to provide scavenging, not necessarily as a means of boost pressure. There are countless examples of pump scavenged two stroke from Cox radials to ship diesels.

This video has been popular recently.



Jens, I read that electric cars outsold IC cars last year in Norway. And, they want to ban IC Cars by 2025?


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## Tim Wescott (Jan 6, 2021)

On a two stroke it's probably more accurate to call that pump on the intake "forced scavenging".  Basically, on a typical little two-stroke, there's a pump that uses the changing crankcase volume to generate primary compression and force crankcase air into the cylinder through the transfer ports.  On big two-strokes, and on multi-cylinder models where the total crankcase volume doesn't change (because there's a piston going in any time another one is going out) then you have to use a positive-displacement blower.

I _think_ a roots-style blower would work, but I don't know.

Does a Shurline lathe do threading?  I'm not the world's best machinist, and I've made holes that Cox cylinders will thread into, using a Smithy lathe and single-point threading.


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## SmithDoor (Jan 6, 2021)

The down side cox engines is they used odd thread sizes.

Dave 



MickeyDD said:


> Hi all,
> As my next model airplane project I would like to build a small 3 or 5 cylinder radial model airplane engine utilizing Cox .049 or .051 cylinder/piston assemblies. I have built 5 small engines so far (3 diesels and 2 glow; both single cylinder and twin). I am using a manual Sherline lathe and mill (no CNC). I don’t want to gear complete engines together but would prefer to make a common crankcase and machine the internals. I know that there have been many home made Cox radial engines built but I have been unable to find a complete set of plans for one. Does anyone have plans for a Cox radial engine as well as where to purchase a thread tap that matches the Cox cylinder thread. I understand it is a very unique thread and a tap was once available but no longer.
> Thank you,
> Michael


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## Mechanicboy (Jan 6, 2021)

dieselpilot said:


> Supercharger, means a pump of some sort to provide scavenging, not necessarily as a means of boost pressure. There are countless examples of pump scavenged two stroke from Cox radials to ship diesels.
> 
> This video has been popular recently.
> 
> ...




Yes, I saw at the engine in Youtube of this engine  with centifugalpump to deliver the fuelmix to all cylinders due lack of the crank case pressure greated by the pistons. 

In fact it's not so many electric cars when the diesel cars is most sold in Norway. 300.000 registered electric cars is sold. 1 295 134  dieselcars, 1 033 036 petrol cars in Norway


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## Canyonman (Jan 7, 2021)

Dieselpilot
I am extremely interested in getting or purchasing plans of your project!!!
Ken


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## olympic (Jan 7, 2021)

"Does a Shurline lathe do threading?"

The Sherline lathe does do threading with the right attachment, and I think it would work for this.


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## Canyonman (Jan 7, 2021)

Hi All
I contacted Victor on the 17/32-40 tap. They responded promptly with a nice note. All in all a single tap will be approx $120. Less the more we get.
Ken


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## mu38&Bg# (Jan 7, 2021)

Canyonman said:


> Dieselpilot
> I am extremely interested in getting or purchasing plans of your project!!!
> Ken




I didn't mean to imply that was mine, it isn't. It's not the first Cox based radial and won't be the last. It is a bit odd that there isn't a plan for one. I certainly could design one and have enough cylinders to build several, but I have far too many other things going right now.

Don't let the tap stop you. Lathe threading isn't difficult, but does require that the lathe has the ability.


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## Tim Wescott (Jan 7, 2021)

Learning how to thread on a lathe is a good way to build your skills.  The apprentice way is to just cut and try until it finally fits -- I still haven't gotten to the point where I can just cut a thread, say "it's done!" and have it fit nicely without being sloppy.  So I do a lot of itty bitty cuts until it fits -- but I'm getting better.


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## rlo1 (Jan 7, 2021)

I can thread mill any thread on my Tormach.  Just learned!  I am still looking for radial plans for the .049.


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## SmithDoor (Jan 7, 2021)

Canyonman said:


> Hi All
> I contacted Victor on the 17/32-40 tap. They responded promptly with a nice note. All in all a single tap will be approx $120. Less the more we get.
> Ken


That is why Cox engine used the odd tap. But using aluminum you can make a form tap for soft aluminum out of O1 or W1 tool steel or just make your own cylinder with a of shelf tap. That is very close to 9/16 UNEF thread tap. I have tap in my box. 

Dave


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## Canyonman (Jan 7, 2021)

MickeyDD,  I keep coming across a n .049 radial called the "Tarantula" but I can't seem to nail it down.
Ken
PS...OK, the Tarantula was a 9 cyl radial built of Cox .049's covered up by dummy engine parts for a Wasp radial look.  The engine was built from plans by John V Thompson in the 80's. Far as I got.
PSS... According to Model Engineering News (dot) org  John's plans are no longer available. Also his design might be problematic. He designed a way to have all 9 cyls on the compression stroke at the same time..???
Onward ;-)
Google is not always your friend ;-)


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## SmithDoor (Jan 7, 2021)

Canyonman said:


> Hi All
> I contacted Victor on the 17/32-40 tap. They responded promptly with a nice note. All in all a single tap will be approx $120. Less the more we get.
> Ken


Here form tap it has no teeth just press in to the metal. It only for soft metals like aluminum Odds are this type tap Cox used.





Dave


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## petertha (Jan 7, 2021)

The Tarantula was mentioned here


			Tarantula nine cylinder two-stroke radial.


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## mu38&Bg# (Jan 7, 2021)

SmithDoor said:


> Here form tap it has no teeth just press in to the metal. It only for soft metals like aluminum Odds are this type tap Cox used.



If you examine a Cox engine you'll see they used cut taps.

The "odd" (non standard very fine pitch) sizes were used out of necessity. The joints needed to be compact.


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## SmithDoor (Jan 7, 2021)

Cox used 17/32" 0.53125" [13.49375mm] that is real odd size. 
Now they could used 0.5625 witch only 0.035" difference in size. 
You look glow plug size they did the same way use a off the wall size. 

Cox engine did good job of keeping the parts there own by using custom taps and dies. 

This something that had work around most life. Machine Tool Manufacturers are well know for this pain, like using a 7/16" 10 acme when the standard is 1/2 10. This part not mush of problem if machining a new screw but big pain if just nut.  Other time manufacturers making gear 51 tooth a standard deviding head does not work for making a new gear.

Dave 



dieselpilot said:


> If you examine a Cox engine you'll see they used cut taps.
> 
> The "odd" (non standard very fine pitch) sizes were used out of necessity. The joints needed to be compact.


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## mu38&Bg# (Jan 7, 2021)

The odd thread didn't prevent Cox from being successful. Has it prevented the creation of countless custom engines using Cox cylinders? I guess we'll never know....


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## Tim Wescott (Jan 7, 2021)

I don't think it's such an odd thread, for that matter.  If you're making both parts, and they're never meant to interchange with anything but stuff you make, why does it have to be "normal"?


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## mu38&Bg# (Jan 7, 2021)

That's my point. They optimized the design. Availability of taps had nothing to do with the decisions.


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## SmithDoor (Jan 7, 2021)

ENGINE MODELENGINE
SIZEGLOW
HEADCYLINDER
NECKBACK
PLATEVENTURIFUEL
TANKPROP
SCREWOLYMPIC DRUM VALVE.15​¾-32​¾-32​13/16-32​3/8-32​N/A​#8-32 X 1.5​OLYMPIC.15​¾-32​¾-32​13/16-32​3/8-32​N/A​#8-32 X 1.5​SPORTSMAN.15​¾-32​¾-32​13/16-32​3/8-32​N/A​#8-32 X 1.5​QUEEN BEE .074.074​11/16-40​11/16-40​N/A​N/A​​#5-40 X 1.0​MEDALLION.15​¾-32​13/16-32​13/16-32​N-A​N/A​#8-32 X 1.5​MEDALLION.09​11/16-40​11/16-40​11/16-40​N/A​N/A​#6-32 X 1.25​MEDALLION.049​17/32-40​17/32-40​​N/A​N/A​​TD .09 R/C.09​11/16-40​11/16-40​11/16-40​5/16-32​N/A​#6-32 X 1.25​TD .09.09​11/16-40​11/16-40​11/16-40​5/16-32​N/A​#6-32 X 1.25​TD .050 R/C.050​​​​1/4-32​N/A​#5-40 X 1.0​SPECIAL MARK II.15​¾-32​​13/16-32​3/8-32​N/A​#8-32 X 1.5​TD SPECIAL MARK I.15​¾-32​​13/16-32​3/8-32​N/A​#8-32 X 1.5​TD .15.15​¾-32​¾-32​13/16-32​3/8-32​N/A​#8-32 X 1.5​TD .051.051​17/32-40​​​1/4-32​N/A​#5-40 X 1.0​TD .049.049​17/32-40​17/32-40​​1/4-32​N/A​#5-40 X 1.0​TD .020.020​25/64-48​25/64-48​#2-56 X 1/4​​#2-56 X 1/4​#3-48 X 3/4​TD .010.010​​​​​​#2-56 X 5/8​DRAGONFLY.049​17/32-40​17/32-40​N/A​N/A​​#5-40 X 1.0​RC BEE.049​17/32-40​17/32-40​N/A​N/A​​#5-40 X 1.0​VENOM.049​17/32-40​17/32-40​N/A​N/A​​#5-40​KILLER BEE.049​17/32-40​17/32-40​N/A​N/A​​#5-40​KILLER BEE.051​17/32-40​17/32-40​N/A​N/A​​#5-40​RR 1.049​17/32-40​17/32-40​N/A​N/A​​#5-40 X 1.0​TEXACO JR.049​17/32-40​17/32-40​N/A​N/A​​#5-40 X 1.0​TEXACO.049​17/32-40​17/32-40​N/A​N/A​​#5-40 X 1.0​BLACK WIDOW.049​17/32-40​17/32-40​N/A​N/A​​#5-40 X 1.0​GOLDEN BEE.049​17/32-40​17/32-40​N/A​N/A​​#5-40 X 1.0​QRC.049​17/32-40​17/32-40​N/A​N/A​​#5-40​QZ.049​17/32-40​17/32-40​N/A​N/A​​#5-40​BABY BEE.049​17/32-40​17/32-40​N/A​N/A​#2-56 X 15/16​#5-40 X 1/2​PEE WEE.020​25/64-48​25/64-48​N/A​N/A​#2-56 X 1/2​#3-48 X 1/2​SPACE HOPPER.049​17/32-40​17/32-40​N/A​​N/A​#5-40​STRATO BUG.049​17/32-40​​N/A​N/A​​#5-40​THERMAL HOPPER.049​17/32-40​17/32-40​N/A​​​#5-40​SPACE BUG JR.049​17/32-40​​N/A​N/A​​#5-40​SPACE BUG.049​17/32-40​​N/A​N/A​​#5-40​SPACE BUG EARLY.049​​​N/A​N/A​​#5-40​


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## SmithDoor (Jan 7, 2021)

dieselpilot said:


> That's my point. They optimized the design. Availability of taps had nothing to do with the decisions.


Here his list of Cox thread sizes 
On past post. 

Dave


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## ajoeiam (Jan 8, 2021)

SmithDoor said:


> Here form tap it has no teeth just press in to the metal. It only for soft metals like aluminum Odds are this type tap Cox used.



Form taps 'can' be used in other than soft metals. 
The material ductility is very important (forget in cast iron!!). 
Check with the form tap specifications - - - there should be plenty of information hidden in there. 
If not enough information - - - time to talk to the supplier and/or the manufacturer!


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## L98fiero (Jan 8, 2021)

SmithDoor said:


> Here form tap it has no teeth just press in to the metal. It only for soft metals like aluminum Odds are this type tap Cox used.


Form taps are for ductile metals, they won't work on cast iron but pretty much everything else a hobby machinist would use, there is a limit on how hard the tapped material is but one local shop is tapping duplex stainless 5/8 UNC, lots of torque on a horizontal machining center and exactly the right tapping compound or the coating comes off after one hole is tapped.
One of the attributes of those taps is that they produce no chips so they are really suitable for blind holes. It should be noted that the tap drill size is different and that the threads are generally stronger than cut threads.
Tap drill size for Inch taps is: OD - (0.0068x % of thread required)/TPI
and for Metric taps: OD - (% of thread x pitch)/147.06


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## mu38&Bg# (Jan 8, 2021)

SmithDoor said:


> Here his list of Cox thread sizes



Yes, I'm quite familair with this list. Very few are doing anything with this information other than replacing screws.


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## SmithDoor (Jan 8, 2021)

I do not think any would build a small engine out of cast iron.
Most use aluminum for crank case this where the form tap will very well. The form can made in most shops. It only 1 to 9 holes needing tap so it made without heat treatment. You buying the cylinder in this case.
If making everything from bar stock you pick your tap and die to use

Dave 



L98fiero said:


> Form taps are for ductile metals, they won't work on cast iron but pretty much everything else a hobby machinist would use, there is a limit on how hard the tapped material is but one local shop is tapping duplex stainless 5/8 UNC, lots of torque on a horizontal machining center and exactly the right tapping compound or the coating comes off after one hole is tapped.
> One of the attributes of those taps is that they produce no chips so they are really suitable for blind holes. It should be noted that the tap drill size is different and that the threads are generally stronger than cut threads.
> Tap drill size for Inch taps is: OD - (0.0068x % of thread required)/TPI
> and for Metric taps: OD - (% of thread x pitch)/147.06


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## stevehuckss396 (Jan 8, 2021)

Very crude drawings. They were never meant to be released to the public. Use at your own risk. I built one but never tried to start it.

View attachment 121951


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## SmithDoor (Jan 8, 2021)

stevehuckss396 said:


> Very crude drawings. They were never meant to be released to the public. Use at your own risk. I built one but never tried to start it.
> 
> View attachment 121951


It great 
Good job on building 

Dave


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## SmithDoor (Jan 8, 2021)

See how your project is doing.
The tap they used custom but. The cylinder Cox probley a die like Landis that you buy dies for pitch and adjust the die head. 
You can just make a form tap out of O1 or W1 tools steel with no heat treat as said in past posts. 

Hear is Cox web site for new cylinder and pistons.





__





						Cox .049 Tee Dee Cylinder and Piston > Top End > Cox International
					

Cox .049 Tee Dee Cylinder and Piston




					coxengines.ca
				




Have fun 
Dave



MickeyDD said:


> Hi all,
> As my next model airplane project I would like to build a small 3 or 5 cylinder radial model airplane engine utilizing Cox .049 or .051 cylinder/piston assemblies. I have built 5 small engines so far (3 diesels and 2 glow; both single cylinder and twin). I am using a manual Sherline lathe and mill (no CNC). I don’t want to gear complete engines together but would prefer to make a common crankcase and machine the internals. I know that there have been many home made Cox radial engines built but I have been unable to find a complete set of plans for one. Does anyone have plans for a Cox radial engine as well as where to purchase a thread tap that matches the Cox cylinder thread. I understand it is a very unique thread and a tap was once available but no longer.
> Thank you,
> Michael


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## Canyonman (Jan 8, 2021)

stevehuckss396. Hi, I'm having trouble opening your file types. Any options?  Thanks Ken


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## KenC (Jan 9, 2021)

I have the write-up and drawings for a 9 cylinder radial looking like a Pratt and Whitney Wasp based upon Cox 049 engines. It uses complete engines so it is really just 9 engines geared to drive one shaft. Any interest please pm me for more details.


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## stevehuckss396 (Jan 9, 2021)

My apoligies. I meant to upload these. Those were the cad files. These are the same thing in PDF.

Sorry!


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## SmithDoor (Jan 9, 2021)

Great job.
Just think of a two cycle full size engine

Dave



stevehuckss396 said:


> My apoligies. I meant to upload these. Those were the cad files. These are the same thing in PDF.
> 
> Sorry!


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## Canyonman (Jan 9, 2021)

Great Steve!!!  Much Thanks!!!!!    Ken


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## Tim Wescott (Jan 9, 2021)

@stevehuckss396 : those look like good drawings if you already know what's going on (you did say they were crude).

I think the only thing missing for most people is how the compressor works -- if anyone's wondering, take apart some air tools and look at how they're done.  

Having maintained air tools in my youth, I'd probably make the vanes out of phenolic (the vanes are the rectangular things 1/32" thick, that fit into the compressor rotor, which is the round thing with the four matching slots).  They should slide easily in the slots in the compressor rotor, without binding -- making sure that the interior of those slots is as smooth as possible, with just the barest (and smooth!) chamfer should extend the life of the compressor considerably.


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## Canyonman (Jan 11, 2021)

Hey Guys!!! I just ordered a 17/32-40 Tap for 54.95 from--





						EX Model Engines
					

Cox Engines at EX Model Engines



					www.exmodelengines.com
				



Ken


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## Shopgeezer (Jan 11, 2021)

Excellent. That is the best price I have seen. I’ll get an order in too.


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## SmithDoor (Jan 11, 2021)

That is a good price for a special tap.

Dave



Canyonman said:


> Hey Guys!!! I just ordered a 17/32-40 Tap for 54.95 from--
> 
> 
> 
> ...


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## SmithDoor (Jan 11, 2021)

Here is way I plane to make a small engine cylinder.
Using all standard thread size. It also a standard glow plug. 







Dave


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## SmithDoor (Jan 12, 2021)

Web sites



			Ageless Engines
		






						Ron's Model Engineering and Model IC Engines Index
					

Rare and Home Made Model Engines



					www.modelenginenews.org


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## lohring (Jan 12, 2021)

I would like to correct the misconception about supercharging two strokes.  By raising the exhaust bachpressure along with the intake pressure they can be highly supercharged.  Snow mobiles are commercially available with turbocharged engines.  The Rolls Royce Crecy and the Napier Nomad were a supercharged two strokes.  The engine pictured below developed around 800 hp with nitrous oxide injection.

Lohring Miller


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## Shopgeezer (Jan 12, 2021)

Rats.  I tried to order the 17/32-40 tap from EX model engines but they are out of stock. I sent them a message asking if they were getting more in. Did anyone actually get an order in? I know other Cox suppliers still list the tap but have no stock and don't intend to get any more.


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## SmithDoor (Jan 12, 2021)

You can make a insert using 3/4" NF and single point the 40 TPI most lathes this easy to do. You use Aluminum or Brass. Note if use Aluminum and Aluminum body the insert may not come out 

Dave 



Shopgeezer said:


> Rats.  I tried to order the 17/32-40 tap from EX model engines but they are out of stock. I sent them a message asking if they were getting more in. Did anyone actually get an order in? I know other Cox suppliers still list the tap but have no stock and don't intend to get any more.


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## Canyonman (Jan 12, 2021)

Shopgeezer.  I managed one. At least they acknowledged my purchase and payment.
Maybe we started a run on the bank ;-)
Ken


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