ttoks

every single nitro engine has a tapered piston sleeve instead of piston rings, not just the O.S engines, and the brass and chrome in the piston sleeve expand more then the alloy piston, in an automotive engine (in other words a car engine) with an iron block an alloy piston expands more then the block and causes it to seize but brass expands more then aluminum when heat is applied.

in short, you are much better off running the engine excessively rich during break in then you are running it even slightly towards lean, you need you extra lubrication and cooling of the extra fuel to keep the engine from overheating and so there is as little friction as possible, the temps during break in should, even running very rich get to around 220-240 simply due to the extra friction from the tight fit of the piston and sleeve.

also can you please try to get your information strait, what you have said there go's against just about every moral of a nitro engine, they are not tapered because the top of the engines gets hotter and thus expands more then the bottom, they are a tapered sleeve because they do not have piston rings to keep compression, so they use the tapered sleeve to get enough compression to allow the engine run efficiently.

-edit- [link=http://www.rc-trucks.org/rc-nitro-engines.htm]link[/link] as a reference.