Cox TD Run In / Break In Stick

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.

GaryB
'Yabby'

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...