Originally Posted by lrutt
Oil and oil pressure
I donít have evidence but Iím thinking if you overfill the oil the pressure can be great enough to blow seals and foul the plug. Maybe even the air expands when it's hot and causes to much pressure.
If you overfill it too much, you can get seal problems, but not from expansion (crankcase is vented).
What happens is there isn't enough air volume in the crankcase for when that big whoppin' piston comes down, and the crankcase pressure spikes, and out pops a seal.
Since the scavenger pump in the sump pumps oil up to the resevoir, and maintains a constant oil level in the sump, its not really overfilled unless the top oil-tank gets pretty much filled up while the engine is running.
Since the oil tank comes to a peak right at the top, there's not a lot of 'spare' volume up there.