ORIGINAL: 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.
This information is inaccurate. I was on the design team for Kangke’s SK line of engines, as well as several other well known brands.
The reason for cylinder taper IS to compensate for temperature differences from the top to the bottom during normal operating conditions, and almost all air cooled engines do this. The Lycoming IO-360 in my real plane uses taper.
Interference fit engines use more taper than ringed engine to keep piston to wall clearance tighter at the top to assist in sealing near TDC.
The reason an engine that runs too lean will over heat and die is because the piston does expand more than the cylinder, and it’s easy to tell. If the engine dies from lack of lubrication there will be score lines on the thrust surfaces on the piston, if it’s from an over heat, the score lines will be on the wrist pin boss sides.
Cylinder head temps are normally between 215 – 250 degrees but it’s not uncommon for the head of the piston to reach 325 – 350 during normal operation.
The single most common cause of rod or crank pin failure during break-in is 4-stroking, after that it’s a too lean over-heat seize. Many engines use what is know as “piston porting “to control the charge entering the crankcase intake port, or to the cylinder ports to keep a constant exchange of vapor under the piston head, this promotes cooling. Without this porting “pocketing occurs under the piston head resulting in very poor cooling. There are a growing number of engines on the market that do not recommend 4-stroking during break-in. We at SK recommend that you richen the mixture until it just starts to 4-stroke then lean it out until it just cleans up to a 2-stroke mode and start there. But follow the instructions that come with your engine, the factory usually knows what works best.