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Cox TD Run In / Break In Stick

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Happy Re: Cox TD Run In / Break In Stick

Post  balogh Fri Feb 10, 2023 7:32 am

Yabby wrote:.....Andras  @balogh often says he believes the machining tolerances on the older TDs are tighter. Mind you I must say none of this has caused a power/performance problem for me with the TDs I own which are mostly made/assembled by myself from parts. They all run really hard and serve me very well in my modelling and CL flying! I am really hopeful Bernie is able to get new ones being produced. I own plenty, but some of the parts would be very helpful to me and I would like to see the TD continue as a truly available engine. They are unique! And Half A is the thing I really enjoy above all other CL sizes.

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Yabby, yes, and I am not the only one to state that tighter tolerances are better. As long as 2 TD engines have a good to medium compression, and their other components are also functional, there really is no reason for the 2  to deliver different performance..the blow-by from the cylinder is limited above a certain pressure ratio inside/outside the engine (a Laval nozzle is required to increase the blow-by if the ratio is above 1/0.56 or similar, and the contour of  parallel gap between the piston and cylinder is by no means a Laval nozzle..so for a given engine whether the combustion space pressure is 3 bars or 30 bars will result in the same blow-by loss..if the gap between cylinder and piston further grows as the engine wears, however, the blow-by will also increase, but still remain independent from the pressure ratio).

What I believe the tighter tolerance of old stock COX engines is good for, is their longevity. A tightly fitted old stock piston and cylinder combo will break in to result in a still tight fit with still good compression maintained by the well mating surfaces, that will continue their wear only at a lower rate once run-in, so the point of time when they loose compression will come much later......a loose fit  new engine will have even sloppier c/p fit after break-in, and will reach the end of its useful life due to excessive blow-by in fewer hours than a well fit engine...

Unfortunately this is what I see proven in practice..my story about my old stock 051 with 300 hours on its clock is boring (even to me, too), and I had late production Estes COX engines that had to retire after only a few hours of run...

But my approach to engine parameters might be different than that of others, in my evaluation system not only the high performance, but dependable, long life is equally important, because my access to these engines in Central Europe is more limited than those of e.g. our US friends, who still have good scores of nice old-stock engines  on e.g. swap meets...


Last edited by balogh on Fri Feb 10, 2023 9:11 am; edited 1 time in total
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Happy Re: Cox TD Run In / Break In Stick

Post  Yabby Fri Feb 10, 2023 8:49 am

Nicely explained Andras! It explains your concern as to tolerances nicely. I interpret what you are saying as given 2 engines, one tight, one not as tight, once broken in, both run and perform comparably, but the difference is how long they each hold their performance. Start off loose, only going to get looser and wear out sooner.

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Happy just a thought..

Post  roddie Fri Feb 10, 2023 12:08 pm

Just a thought.. I haven't tried this, but it would be non-destructive to the inner-face of the backplate. "What if" you were to use thread-locker (the type that permits removal using hand-tools) on the backplate threads.. and only partially screwing it into the engine's case? You could set the desired-gap between the crank-pin using your feeler-guages.
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Happy Re: Cox TD Run In / Break In Stick

Post  balogh Fri Feb 10, 2023 12:30 pm

How about cutting a shim washer from a beercan? Pressing the TD crankcase against the soft beercan material will leave the mark that you can cut with a fine manicure scissors?

Or, winding a few layers of plumbers' teflon ribbon around the backplate neck that will increase the clearance between the crankpin and the inner face of the backpkate?
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Happy Re: Cox TD Run In / Break In Stick

Post  Yabby Fri Feb 10, 2023 2:39 pm

Good Idea Andras Smile I willl try the plumbers taoe as I feel that is more likely to seal properly. mindy you cutting the shim from a beercan and using plumbers tape may work together. Certainly an easier way to get a bit more gap without removing metal. I will give it a try Thumbs Up

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