Trottle body STPS motor

[Hooli]

I remember my late Dad hypothesising that the older British motorcycles were designed to leak oil due to crankcase pressure. Having only ridden some of them for short distances I can confirm I had oil on the shins of my boots from even that. Laverda was an exception, as they had a crankcase breather all those years ago. 

All engines have crankcase pressure, but two big pistons moving together does move quite a lot of air around the crankcase. They do have breathers, older ones were timed to let out the excess pressure & create a partial vacuum as the pistons went up as less air in there results in slightly more power output. Later ones just had conventional breathers.

It's mostly poor quality parts and bad assembly that causes it. They are more prone to leaks due to the design but can be made to keep the oil inside.