Determine engine wear?

Mudhen's diagnosis is reliable,and particularly valid for older TeeDee049 and 51 engines with tapered cylinder bore. Later engines did not have tapered  bore and the piston of even a new cylinder/piston set may be easy to push over the TDC.

My diagnosis is very simple and the same as written by others above. Check compression by looking at/listening to the hissing blow-by at the exhaust ports when the piston wet in fuel moves upwards.

I have a COX TD 051 engine with near 300 hrs runtime on it that I retired because of its impaired compression. The engine starts with electric starter but drops speed under heavier loads e.g. when the plane climbs steeply.

BTW welcome to CEF.

I've had three engines that deserve comment on their wear.

1. OK Cub 0.049A. Worn out in a single afternoon of flying. Castor oil fuel, snow on the ground so no overheating or dust. Just a soft cylinder or piston.

2. Pee Wee 0.020. Had poor compression and would only run with added oil in the fuel. This engine had a silver tank and I suspect was pulled from a Cox rtf plane.

3. Fox Rocket 0.099. Just gradually lost compression after many hours of running under a combination of summer and winter conditions.

When there is no compression......it's worn out!

Ok guys. Thanks for the nice welcome and the info. I've got her an .049 profile trainer for she and I to throw together, and I've got the engines soaking in glow fuel. Yesterday I bought some 25% 1/2a glow fuel. Man, this is bringing back some fond old memories!

Thanks again,
AV8TOR

I'd just like to add that engines which use separate tanks are a lot more cooperative. The ones with built-on tanks have so many more opportunities to leak air and drive you mildly mad while your young protege stands by getting fidgety.
Good luck, thanks for joining CEF.
Rusty

I would also like to add if there's no compression, it's not always a worn out piston cylinder. That being said, a loose head, a damaged head gasket, a head not properly flattened which is leaking, a loose backplate, all will reveal signs of low compression which cause hard starting and problematic engine runs. A ball socket not properly set can punch a hole in the center of the piston as well. It doesn't mean the fits are worn out, just that a hole in the piston doesn't hold compression.  Bottom line is evaluate the simple things first before heaving the piston cylinder. some of my best running engines were the engines with the lightest compression.