Cox Olympic Rear Drum Valve engine details. Now with Video!!!!

Thanks for the look inside. Funny that cox? Would polish up the transfer ports like that. Gotta atomize nitro as fine as possible to extract max power per lb.

Does the same crank pin drive both the piston and the valve rotor?

Yes -- the lower end of the rod also serves as a spacer to keep the rotor against the backplate and the thickness of the rotor disk serves to keep the rod in place. The pin is sufficiently long to index in the hole in the rotor disk.

I wonder why COX chose to go the route of a rotary valve rather than the more common slotted disk and backplate. I would think that the rotary valve would require more power than a simple slotted disk. Would it be because they already had the carb body/needle valve/venturis from the Tee Dee that would fit?

What's the purpose of three holes in the valve disk? I can understand having two to allow for CW and CCW rotation, but not three.

The Tee Dee series wasn't introduced until the end of 1960. The Olympic rear drum was prototyped in 1959 and some were given away at the 1960 NATs. The carb body is different as the pressure nipple is on the opposite side as a Tee Dee carb body.

The crank pin is stepped and a smaller diameter sticks out to drive the valve.

I think the 3 holes were there so the venturi could point to one side or the other. Maybe someone here truly knows.

Jason

Is the rotary valve body pressed into the backplate ring or is it free to be rotated? If it can be rotated, does it have any form of indexing to ensure that it goes in with the same orientation each time. The reason for asking is that if it may be freely rotated, then induction timing is fully adjustable -- at least to the point that the degrees at which it opens and closes may be changed. The duration is set by the size of the milled opening in the valve housing or in the valve itself.

The converse is if it is pressed in, then timing is determined by how far the backplate is screwed into the case -- and that could be hard to maintain in a production environment. From the pics, I couldn't tell if the valve body had a flat on it or not.

The valve body is free to rotate 360 degrees. There's no indexing and is locked in place by the threaded collar and the end cap. Timing is fully adjustable on this engine.

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Jason_WI wrote:................Timing is fully adjustable on this engine.

That's pretty slick.

Would you talk a little more about how your mold for the carb housing is made? It's quite impressive, especially being able to maintain concentricity of the part. Is the silicone material pretty thin when added -- I assume it must be to match the fine detail. Is your mold a one time use or are you able to get the part free without destroying it.

Being able to reproduce small intricate parts is quite a skill -- one I wished I had.

andrew

EDIT: mudhen posted while I was typing, so you have impressed two of us with your abilities.

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Im not sure how it will react with nitro, but I have used JB weld for the longest time as a moulding agent. specially when i needed something to be fuel proof. Tough as nails too

I never thought of using JB weld as a mold agent. I'll have to see if it can be thinned since it needs to be able to flow into all the fine details.

The silicone that is used to make the mold is thin and even picked up the cracks in the carb body and were reproduced in the first attempt. I ended up with a carb body with the same cracks as the original piece. On the second mold I used modeling clay to fill in the cracks in the original carb body and ended up with a mold that is nearly perfect.

As far as shape and concentricity the part fit on the the valve body with only minor scraping on the carb body with an xacto knife. The venturi threads in without having to run a tap into it.

The mold may last another attempt or 30 attempts. The carb body does come out of the mold hard where the threads are in the venturi. I suspect that this will eventually tear and the mold will be destroyed.


Mud,

I suspect yours is glued together with castor as mine was. Some heat with a monokote heat gun freed mine up. It took some persuasion with a wood dowel to push the carb body off of the valve.

Jason

Jason_WI wrote:I never thought of using JB weld as a mold agent. I'll have to see if it can be thinned since it needs to be able to flow into all the fine details.

I considered mentioning JB Weld, but it's pretty thick. I don't know what solvent could be used without affecting the properties of the mixture. I'm guessing that being the confined in a mold, the solvent would not be able to gas off and that either the surface of the pull would be pocked or that it would end up being very porous and filled with a lot of gas bubbles.

If it would work, when cured, according to company information, JB Weld is petroleum, chemical and acid resistant. I'm not sure what resistant actually means -- it's pretty open-ended.

Another characteristic that is advantageous is that it has 0.0% shrinkage.

That's probably the reason for the 3 holes on the valve disk, timing adjustment.

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Here is a exploded view picture of the drum valve. There is no indexing between the collar and the valve body.

Here is a picture of the first carb body I made with the first mold attempt. You can see that the silicone picks up the fine details of the crack from the original carb body.



Inside detail of the threads and crack.



Here is a picture of the engine with the black carb body installed.

The below link has videos on making molds from parts.

http://www.freemansupply.com/video/masscasting/complex.htm


This is the basic process that was used to make the 2 piece mold with a parting line. I did not machine a recess in the wood but used clay. Speeds up the process a bit.

http://www.freemansupply.com/video/masscasting/complexqt500.htm

It is a long detailed process but worth it if no parts are available.

Jason

Here it is in all its glory! Better than fireworks! Enjoy.

Beautiful! Congratulations on the great job! Worth every second of the wait to see it running.

Enjoy it,

Bob

Well done Jason.

That really is awesome! That is better than fireworks! I would guess that you are the first to start one of those Olympic rear drums in at least 30 if not 40 or more years! The molding job is great and it even worked great too!

I have an HD video taken with my iPad but it is having problems loading to YouTube.

What do the Olympics normally do with an 8x4 APC prop? So far the valve body I made held up. It did soften up a little at the venturi threads. I have 2 runs on it and that will probably be it. Don't want to break the crank pin off!

HD video loaded up. This is the second run on SIG champion 25% with an all purpose cutter (APC) prop. The engine kicked back and cut the foam tip of my chicken stick off!