rcuser004

My Feedback: (287)

Anyone have a source for head gaskets/shims for a Pee Wee .020. I am trying to lower the compression to run a larger prop. Anyone have success making them yourself and if so, how did you do it?

ajcoholic

My Feedback: (10)

I have some I could sapre - email me your address and I will send you three in an envelope.

I have tried with punches to make these shims with some success, but its tricky.

Andrew Coholic

[email protected]

dhansen

RB Mike.

What size prop are you looking to swing and what are you using it on?

Thanks, Dennis in Maine.

jboy381-RCU

would'nt you need higher compression for a larger prop???i would think it takes more power to swing a big prop...not to mention the fuel consumption...

anyway why would want to lower the compression???are you wanting to swing a big prop at slower RPMs???like a WWI class airplane...

need more details...
john

ajcoholic

My Feedback: (10)

John,
When you want to swing a bigger prop you lower the compression - since the rpm will be less you need to adjust the timing (ignition point) or the engine will not run well (rough and overheat).

Andrew

jboy381-RCU

thanks AJ,
i had it backwards...so, lower the compression for bigger props...

john

DICKEYBIRD

Yup, think of it this way. Smooth burning in the combustion chamber is just as important on these tiny little engines as it is in a car engine. If we're turning a small prop at a very high rpm, we have to start the ignition process earlier so that the point of maximum cylinder pressure occurs at just the right point in the piston's movement for max. power. The ignition of the fuel/air mixture is controlled by (among other factors) the compression pressure so more compression = earlier ignition timing.

Conversely, big lumber needs the ignition point to occur much later or the point of max. cyl pressure occurs too early and is wasted trying to push the piston back down before it gets to TDC. That usually causes detonation and broken parts as well.

Me? I have much better luck under-compressing my Cox engines and giving up a little top end power. They smile at me when I run them.

Japanman

A bit of a spanner in the works here..
Er.. I got a decent rpm increase with my pee wee 020 swinging a cox grey 6by3 by making a high compression head!
mind you that was with 10% nitro fuel, but that was how it seemed to like to run...
I thought this was because in this case, the engine was running quite a bit slower that the cylinder was designed for. hence the charge leaping out of the transfers was having too much time to escape out of the exhaust port- hence, a lower actual (dynamic) compression ratio.
J.M

ajcoholic

My Feedback: (10)

Hey anythings possible. Perhaps due to the low nitro (only 10%) the ignition point was allready retarded enough. Too far (the ignition point) is no good either, you loose power.

In a glow engine, the relationship between the prop, the engine itself (timing #'s and porting, etc), the heat range of the plug/head, the nitro content of the fuel and the compression ratio all come into play. Its the last three that determine the timing and depending upin what prop you want to turn, you have to play with these to get max. rpm without predetonation.

I know you know all this JM, but probably a lot of newer modellers or those never dealing much with engines do not.

AJC

PS I sent the head shims today.

bsindel

AJ,

Your comment that you had limited success in punching out these little metal head gaskets got me wondering how in the heck Cox was able to turn these things out by the thousands. Do you have any idea of what kind of tooling they used?

Bill Sindel

DICKEYBIRD

Hundreds of tiny little elves running hundreds of tiny little lathes?

Seriously, when you think about the gazillions of tiny little super accurate parts that Leroy & co. turned out way back in the fifties, WITHOUT CNC, it's mind boggling! I've run across some posts here & there from ex-Cox employees that described Cox's automated screw machines, custom made equipment and "Tem-Trol" process but never in enough detail for this layman to get a grip on it. When you think of the low prices, it's even more amazing. I keep hoping that someone with first-hand knowledge will document all of the processes before they're all dead & gone![]

The closest I've seen is Dan Sitter's series of articles in Model Engine Collector's Journal but it's more about the finished product, when they were marketed and rare one-offs, etc. Not very much about the nitty-gritty details of production.[]

rcuser004

My Feedback: (287)

The shims are made with a series of punches from sheet or strip stock, similar to a cookie cutter. I originally asked if anyone has successfully done this to see if any simple methods were devised.

The way a manufacturer would stamp these is to have a male punch tool with a female die. Think of a thin sheet or strip of material passing over closely placed female dies. The male punches, also closely stacked, cycle up and down rapidly punching out these little "donuts".
The purpose of the female die is to support the material from the backside as the punch pops through so the material is cut cleanly without deforming.

To do this yourself in your work area, you would need to make a punch out of steel rod with a groove cut into the end if it. The groove width is equal to the width of the shim. The groove depth must be greater than the thickness of the shim material. The edged on each side of the groove must be a sharp, burr free right angle. The female die will be a mirror image of the punch with a slightly oversized groove so the punch could protrude into the die to enable a clean cut of material without deforming the material.

I am hopeful this makes sense to all of you.

Regards,

rcuser004

My Feedback: (287)

Andrew,

Thanks for the .020 shims.

Mike

ajcoholic

My Feedback: (10)

I have made my own punches, and punch out head gaskets and stuff on a block of hardwood. 75% turn out. Not bad, but its finicky.

AJC

DICKEYBIRD

I used to have a short blurb from an old time machinist on making thin shims but I can't find it right now. If memory serves, the I.D. was punched and a bunch of oversize blanks were stacked and clamped in a mandrel before turning the O.D. down to finished size.

bsindel

Red Baron Mike,

I can see how using a die punch set would let them cut out a clean shim, but how in the dickens do you suppose they could then fish the gasket out of the die in a mass production environment? Then think of doing this with head gaskets for the .010 Tee Dee! I wonder if Cox ever made a documentary about their engine production technique's? Is there anything left of the LM Cox company that might still be contacted?

Bill Sindel

No Dickeybird, I don't think they had thousands of little elves running around picking out the gaskets.....

rcuser004

My Feedback: (287)

To: bsindel

Good job for picking up a major issue. I was going to bring this up last night when I wrote the post but it was getting late and I was already concerned about confusing everyone. In a manufacturing situation, there would be an automatic operation whereby a plunger would protrude out through the punch to expell any shims stuck in the groove. For the home workshop, you could slot the sides of the punch so you could stick a small probe on each side of the punch to push out all stuck shims. Making this tooling is a lot of work but we do this in the
manufacturing world also.

If any of you make a punching device, please let me know how it works for you.

Regards,

ajcoholic

My Feedback: (10)

I have a new idea for a punch. Will get to it in a bit and post the results...

AJC

rcuser004

My Feedback: (287)

AJ,

Make a die set along the line of the Cox piston/rod reset tool.