The reason I ask is that my bike seems to pressurise the crankcase. On the last run it blew quite a bit of oil vapour out through a small gap in the rocker inspection cover gasket. It's not a stuck ring or anything like that: there's plenty of compression and it still "chuffs" out of the rocker cover when kicked over with the plug out.
It's not entirely clear to me from the exploded diagram exactly how the breather works (and there seem to be at least three different variants). Mine is the later type, which I don't think can come loose (unlike the earlier type).
I don't really fancy stripping the engine for what seems like a relatively minor issue. Everything else seems fine, so I might just make up a Webco-style breather onto the inspection cover...