ORIGINAL: FatOrangeKat
I don't like the idea of using a torch/heat gun on an engine. Some hobby store proprietor just tried to sell me two engines that he said he had to do this to. They are built to be snug for a reason. If there is any premature movement of the material (expanding) not due to its own break-in process, you will start to lose compression and power.
While a blow torch or something which generates undue amounts of heat is a bad idea, using a hair dryer or medium heat gun is fine.
The heat temp barely gets over 160 this way, while normal operating temps are around 300, so you have an ample margin.
More important is to get the fuel flowing to lubricate the head, which this helps facilitate.
I do this merely to make the engine turn over so I can use an electric starter.
On some ABC/ABN engines such as the Tower .75, GMS .76 ( and up ), and larger SuperTigres, if I don't hit them with a heat gun first I'm apt to do damage to the engine.