i was reading an engine break-in thingy, and it said this:
what does piston at BDC mean?
thanks.

Tommy.

bottom dead center

ooooh, so 'TDC' means top dead center?

but that still doesnt make sense, how can it be at the bottom and the center at the same time?

Tommy.

and how can you tell when it's at bottom dead center?

Tommy.

I'll rephrase. It is when the piston is all the way down. OR when the crankpin is all the way down. Stick a "thing" into the cylinder and rotate until piston is lowest, then you can mark the flywheel for future reference.

ohhh! very clever with fly wheel thing. awesome.

so why do you want to kill the motor and then put it all the way down during breaak-in?

Tommy.

The engine will cool down faster if it's at bdc then any where else. While the psiton is at bdc there is no hot trapped exhaust gases. If the piston is covering the exhaust ports then there will be some hot exhaust gases and the only way for it to get out of there is to do a blow by. Blow by is when the exhaust gases pass between the piston and sleeve. This is what happens when your engine has been worn out and no longer has much compression. Because the engine is experiencing blow by.

Leaving it down keeps it from sticking/touching the pinch area in the sleeve during cooldown.