The NSR crank has a labyrinth seal between the two sides, not a rubber seal. The cuts down on friction, whilst still performing admirably as a block to air. This explains why the oil will leak accross fom one side to the other. There is a slight air leak, but since the pressure down there is so low (about 5psi above ambient) it's not significant.
The O ring (as supplied by Andy) fits around the centre bearing, but the seal through the bearing itself is the labyrinth.
This came from Elliots, the grey bike importers, so I'm pretty sure it's true.
Anyone dispute this before I rebuild my engine? Matt?
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