My question is, what O-ring material stands up best for model diesel engine contra piston sealing?
Got both of my 1950 McCoy .049 diesels running last week. These are the original radial mount version, not the later beam mount type (though I have one of those as well). Both apparently had worn O-rings because the compression was leaking down. Max RPM with a Cox gray 6 X 3 prop was just under 12K on Red Max fuel of unknown type (that was recommended by Ed Carlson for the MK17 Russian engines). The MK is also plain bearing like the McCoys.
After running both of these engines up to full operating temperature, I was able to unscrew the cylinder on one of them, but could not get the head off the cylinder. The other engines head and cylinder are still frozen solidly in place. Pushed the contra piston out the bottom of the removed cylinder, and found an original white rubber O-ring worn completely flat on the outside.
Made a quick trip to ACE hardware, and found some Buna-N faucet O-rings that fit perfectly. I'm hearing that those are not the best material for contra piston sealing, and would like to know what works better? Viton?
Anyway, the engine now turns about 13.2K, and holds the compression setting nicely.
I was amazed at the sloppy fit of the contra piston on this engine. Don't remember seeing that on my original, but it could have been the same way.
Bill