I agree I think letting an engine warm up naturally is the best way.i have rebuilt many hydraulic control valves and holding the spools in my hand caused them to swell and not fit into the bores, these pistons are tight enough as it is they have tight tolerances and heating them from the outside wont warm them evenly as if you ran them. also if you take a engine from inside a warm house to cold temp outside or vice versa let them set to adjust for the ambient temp they will be started in. most abc engines have a pinch fit at the top an electric starter is the best way to get them going until they get broke in.