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Cox Engine of The Month
November-2017
MauricioB's

"Cox Tee Dee .010"



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Crankcase shims

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Help! Crankcase shims

Post  Jerry M on Thu Nov 05, 2015 3:38 pm

I'm new here, so please treat me gently!

I am reliving my youth and rebuilding a .010 Tee Dee to go on a highly modified Hobbyking Ministick and see that the cylinder head has the not uncommon issue of misaligned bypass ports. The head tightens up about 45 degrees out in a clockwise direction from what I am sure is the optimum gas porting position of perpendicular to the fore and aft position. Shimming the cylinder head with say one or more 0.001" shims would seem to be the simple solution and I wonder whether anyone has any ideas where I might source these as I don't have the tooling to punch them out?

I measured up a .020 head shim, but the ID was just too small.


Last edited by Jerry M on Thu Nov 05, 2015 3:47 pm; edited 1 time in total (Reason for editing : typo)
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Help! Re: Crankcase shims

Post  TDbandit on Thu Nov 05, 2015 3:49 pm

Hi Jerry, welcome to the forum!! Glad to have ya here Smile
About the .010 if you are referring to the position of the exhaust ports there is no real need to worry about them being off center it will run just fine also be careful when you tighten the cylinder its so tiny that it's easy to distort it by twisting it just snug it down no need for shims unless the top of the piston is going past the seat for the glow head.
Hope this helps! Take care and again welcome to the forum! (Bandit)
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Help! Re: Crankcase shims

Post  RknRusty on Thu Nov 05, 2015 4:52 pm

Jerry, welcome to CEF. We've talked about both the exhaust ports and the internal bypass port position, including a couple of our members who are avid successful mouse racers. No definitive difference has ever been proven. However shimming the cylinder up will change the combustion timing as well as lower compression, the latter can be compensated, but we've always come full circle back to the original configuration as being best.

Once when I did align the ports on an .049, I removed the cylinder and used 800 grit sandpaper on a piece of glass to slightly deck the raised boss on the crankcase so the cylinder threads bottomed out a fraction of a turn farther around. As predicted, I detected no difference in performance, but it looked cooler to me.
Rusty

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Help! Bypass ports position and shims

Post  Jerry M on Fri Nov 06, 2015 7:03 am

Thank you all for the feedback. I have some experience in race engine flow porting and whilst Cox engines are new to me, it does strike me that the range of Tee Dee engines were designed to have the bypass ports lying directly above the open section of the crankcase, thus avoiding any shrouding from the crankshaft at the front and the fuel tank intrusion at the rear. A quick look down through the cylinder with the head and piston removed clearly shows up these potential shrouding areas. I have looked at a number of the sectional drawings on the original Cox instruction sheets and they all confirm this correct position of the cylinder. I do however see that the real world engines out there do have the cylinders at every conceivable position and I assume that this is due to the factory not considering this to be an important enough issue for them to tighten up on manufacturing tolerances.

However, I admit to being a nut when it come to maximising power output from my engines and hence my over exuberant interest in sorting this one out. Decking the raised boss as Rusty has done in the past would work if the cylinder needed to continue a slight rotation clockwise, but in my case it needs to go 45 degrees anti-clockwise. Hence my thoughts on shimming it back. Rusty is is spot on when he says that this will alter the bypass and exhaust port timing, but we are only talking about a 0.001" shim and the payoff of getting improved gas flow would I think more that compensate for the altered timing.

I also wonder whether designed in crankcase induction would be improved by getting the exhaust ports back to a position of directly facing the prop airflow?
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Help! Re: Crankcase shims

Post  RknRusty on Fri Nov 06, 2015 11:11 am

I've wondered that too, but I don't think I wondered it as hard as you are. My engine theory is all derived from tinkering with no formal training, so I'll wait and see what others say.

I do wonder something else... that is, how I'm replying to your second post, yet at the time I'm writing this, your post-count is still 1.
Rusty

P.S. I gave your top post a greenie for opening with a good question.

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Help! Re: Crankcase shims

Post  OVERLORD on Fri Nov 06, 2015 12:53 pm

Jerry M wrote: it does strike me that the range of Tee Dee engines were designed to have the bypass ports lying directly above the open section of the crankcase, thus avoiding any shrouding from the crankshaft at the front and the fuel tank intrusion at the rear.

Hi Jerry, Welcome to the forum. I've read your posts and the comments with great interest. Just can you explain what you wrote above -or make a drawing-  because I do not understand that very well. That is probably because of my limited understanding or (American) English. I think you mean that the bypass ports should be located on the sides of the crank case and not fore/ aft where the crank web rotates. Aft, there is only a plug.

If that's the case, I don't think it doesn't matter in a Cox engine as the mixture is pushed under pressure in the cylinder by the down stroke of the piston. Wherever the bypass ports are, the mixture will find its way up. This also goes for the exhaust ports. The exhaust gasses leave the cylinder once the piston opens the ports due to the reigning pressure in the combustion chambre. The position of the exhaust ports is of no importance. The piston goes further down a few mm and the bypass ports open. The pushed in fresh mixture scavenges the cylinder further and during the down stroke of the piston, the cylinder is filled until the bypass ports close during the up stroke. Hope this helps
Don’t rely too much on dimensional drawings to deduct any particular engine operation. Possibly, people who made those drawings have only a limited understanding of how engines work. Just as the people who assemble them from parts in the factory.

Lieven
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Help! Re: Crankcase shims

Post  Jerry M on Sat Nov 07, 2015 4:37 am

Lieven

'Wherever the bypass ports are, the mixture will find its way up'........... yes some will. But at 20,000rpm+ there are very rapid directional airflow changes and thus pressure differentials are created. The crankcase pressure will not be sufficient to get all the mixture up into the cylinder in the short time available and this is why engine race tuners spend so much time improving port flow dynamics.

I have (hopefully) attached an extract from the Cox 010 instructions and as you can see, the bypass ports are on the sides of the crankcase.

My research of 'old tricks' is not conclusive, but does seem to point to this being the optimum position and from a gas flow perspective I would certainly concur. Attaining this position by rotating (thus raising or lowering) the cylinder does however have the unwanted effect of altering the induction and exhaust timing and unlike compression height, this cannot be shimmed out of the equation. This is perhaps why in the past there has been no general consensus on the point, due to these two variables being inextricably attached and perhaps skewing any perceived improvements.

For the particular Tee Dee I am working on, a 0.004" crankcase shim would bring the bypass ports in alignment and also get the piston level with the top deck of the cylinder at TDC (which I am led to believe is the correct position for optimum induction/exhaust timing). So I am still looking for some shims at ID 0.325". Anyone have any sources? Crankcase shims are out there for the 049 but no luck so far for the 010.

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