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I agree with all that Tony wrote. One factor he did not mention, if I understand it correctly, is that pistons and cylinders are of dissimlar metals that expand and contract at different rates. At normal operating temperatures, operating clearances are correct, but if the engine is (too) cold, the cylinder is actually too small (having contracted more than the piston), creating increased friction and wear, as well as more difficult starting the colder the engine gets.
Oh, and I am told another risk from hot air preheat done inexpertly is melted electrical wiring.
In case I ever have to have this procedure done on my airplane, can anyone tell me, on an average winter day with ambient temperature of say 15 F, how long should you let them (make them) preheat your engine with a propane heater (sitting out on the ramp). How many minutes?
Kevin
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