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most everything i ride leaks oil
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all my old british stuff runs open breathers, no filters, just a tube sufficiently long to exceed the tidal volume below the pistons. most breathe through a tube from the primary chaincase, which receives pulses through an open drive side main bearing.

on my race motors i drill the crankcase and rocker boxes for breather lines, and run them to a catch bottle behind the motor somewhere.

some people put a one-way valve in the breather hose, but it isnt there to ensure a one-way circulation, because there is no unvalved entrance to the crankcase. rather the idea is to pull a partial vacuum in the crankcase to reduce oil leakage.
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