Gas cleans good, but it's extremely flammable, making it too dangerous to handle. The best (and safest) way to clean an engine block and its parts for rebuilding is to wash all parts with cleaning solvent (or mineral spirits, which is better known as paint thinner). Or, use a heavy duty oven spray cleaner (such as EASY•OFF Oven Cleaner) to clean the stubborn oil and dirt mixture that gets deposited onto an engine. Just spray the parts thoroughly, let set overnight, then blast the grime off with a high pressure (water) washer. If this doesn't work, take the parts to an automotive machine shop and have them "hot tanked."