ORIGINAL: twomanytoys

Here is an article that I pulled explaining what could happen when running a engine too cool.

What causes the connecting rod on my ABC engine to wear when it's run very rich for long periods? Most would think that extra oil in the fuel would help, not cause harm. The worst thing you can do to an ABC-type engine-other than running it lean-is to run it rich for long periods. When new, most ABC (aluminumalloy piston with chrome-plated brass cylinder) engines exhibit an interference fit (components that actually touch) as the piston crown passes top dead center (TDC) when turned over by hand. In many cases, you can actually hear the piston squeak; this is normal. Originally designed for fast, 2-cycle operation, the piston and cylinder expand similarly (slightly more for the cylinder), maintaining a good gas seal without enduring the mechanical drag of a piston ring. Unfortunately, when operated at a rich 4-cycle (engine firing once every other revolution), the piston and cylinder never reach design temperature and the piston rubs at TDC, causing great wearing loads (pounding) on the piston wristpin bosses and both connecting-rod holes. The piston fit also is worn away at its crown, further degrading the engine's mechanical condition. In the future, always try to run ABC-type engines (including the AAC and ABN types) in the 2-cycling mode, especially when they're new.