umm ok....basically you have your piston and piston sleeve....the sleevegradually gets smaller as it goes to the top...this is the"pinch" ...this gives your engine compression....as your engine heats up the sleeve expands at a much greater rate then the piston...this is why its important to get your engine to above operating temps asap (over 200 F.) this keeps your engine from wearing prematurly (yes running to rich will destroy an engine just as fast as running it to lean)....

what putting your engine at bottom dead center means is you put the piston at the bottom of its stroke (as far down the sleeve as it can go)..by doing this it helps preserve the "pinch" of your sleeve...if your engine starts cooling after its been running with the piston at the top of its stroke the sleeve dispinses heat faster then the piston which means it gets smaller faster,,,end result the piston that dosnt dispense heat as fast knocks the "pinch" out of the sleeve

a way to know your at bottom dead center is take out the glowplug, take a zip-tie and lightly stick it into the combustion chamber...rotate the engine using the flywheel until the piston goesall the way down...hold that point, flip the rc over and mark your flywheel with a permanant marker. (there is a hole in the chassis that allows you to see the flywheel)