ORIGINAL: zeddfirstwizard
but if you don't have A back plate on there then that's another thing,, It keeps the internals in place in the motor itself.
But some engines are different as well and the back plate does nothing but provide A mount and place for the crank to stabilize. As for the leaking so what? The pressure is above that in your head anyway so it does not affect performance,
The big thing you're forgetting is that the crankcase is also where the air fuel mixture is introduced to the engine and is pressurized on the downward stroke and then goes into the combustion chamber when the intake port is exposed.
Personally I can't see that small leakage making much of a difference in your performance compared to the volume of air being sucked in and pumped through the engine.
That said, I did just buy rear non-pull start backs for my Force engines on my Ofna buggy - mostly because I hated the pull start, it always melted on the exhaust manifold, and the buggy looks cleaner without it.
Could have just run without the pull start mechanism on it (but retain the back) but the new backs were cheap.
Dave