Some years ago we had a club member demonstrate how he torques down the head bolts on a Fox 35 CL stunt engine. Phillips-type bolts replaced with allen-head bolts. He removes the glow plug and mounts the engine in a test stand with a prop attached. Then takes a propane torch, a can of WD40 and sprays the internal parts of the engine liberally with the WD40 then heats the upper cylinder evenly with the torch and spins the prop by hand. With the cylinder heated to produce nearly the clearance of an engine at running temperature he tightens/slackens all the allen bolts to where the engine spins over freely. This is the only way he can guarantee there is no cylinder distortion introduces and therefore cause a bind in the piston/sleeve fit which must be ground away by hours of break-in running.

It seems to work quite well. Of course, the Fox is old sintered iron piston and steel sleeve (quite thin) which must go through many heat cycles before the fit is stabilized and can be called properly broken in.

Still, nothing quite like the nostalgic song of a Fox stunt breaking perfectly between a 2 cycle and 4 cycle during different points of each maneuver.