ORIGINAL: Pat Roy
Seating the ring IS NOT the most important part of a break in, it's truly only a very small part. Keeping the engine cold by using a rich mix actually hinders the break in process by preventing the proper thermal cycling of a cast engine case and cylinder head. THAT'S the most important part of a break in, and the part that requires the most time.
This is exactly correct, and here is why: the cast iron ring in its initial state is much softer than the cylinder, dimensionally unstable, and somewhat rough around the machined edges. When heat is applied to the cast iron it expands slightly and wears to a smooth surface while it is still soft. When allowed to cool it hardens somewhat. Repeating the cycle sequentially (and completely!) will yield a ring with a glassy surface which seals extremely well with the cylinder, and which becomes somewhat harder and is very stable dimensionally with the application of heat.
The first few runs are the most important. The ring MUST be brought up to operating temperature, allowed to expand and wear off the rough surfaces, and finally MUST be allowed to cool completely thus stabilizing the ring dimensionally.
Heat to expand (run at regular operating temp), run under moderate pressure (slightly smaller prop in moderate-to-high speed flight) to smooth the ring, cool to harden and stabilize (allow to cool COMPLETELY to ambient). Repeat. Repeat again.
Most of the effects are seen during the first several heat cycles. As Pat said, continue this for 3 hours of flying time.
Great stuff in this thread! Thanks everybody.
Brett