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[Norm]

Bryan, don't be hard on yourself early ones had a slot between the cases up high not a hole

[Beezabryan]

Thank you very much Norm. ... our 1992 Bullet came to us with a trashed engine, con rod through cases. ... it was replaced by a 1994 engine I got from the then importer who had spare 500 lumps sitting around. ... eventually got round to stripping the wreckage for any salvageable bits and that would be the one with the slots.

[Norm]

Marcel, as to your question why? they used the top section of the oil tank as an expansion chamber to even out the pulsing caused by the up and down strokes of the piston

[Marcel]

Thank you all! small hole long thread!
Maybe: the duckbill is creating aan avarage low pessure at the crankcase wich is also an advantage for the oil reservois against leaking?!
nice weekend!

[PeteF]

Marcel, that's exactly what the duckbill does. Some people put a better NRV in instead.