Hey Dick, is it true we shouldn't overheat an engine while breaking it in? Over propping is the surest way to cause overheating. That's why I use a smaller prop and run very rich during break in.

Yup Cox had the tightest tolerances of any engine manufacturer.

When Clarence Lee makes one of his custom K&B61's part of the procedure is to measure the pistons and cylinders and get the biggest piston in the smallest cylinder with out over doing it. Ie, the piston still has to be a shade smaller than the cylinder. Ganging of tolerances is what makes a poorly fit engine. Cox had the best fitting piton/cylinder fit overall.

Some of the old lapped Enyas had very tight piston/cylinder fits but when properly broken it had unbelievable compression. They could sit a TDC for 5 minutes and still pop out when turned. Johnson's were like that too.

You are right. Some of the old Fox engines were horrible and the Chinese stuff was one step above junk.