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Seems to be some confusion about the difference :sneaky: but just so y'all spot the difference;

Your crankcase breathes to and from an area before the carburetor or Fi throttle body and a Positive Crankcase Ventilation (PCV) system if so equipped is fitted with a one way valve that draws gas vapour into the intake manifold somewhere after the carburetor of Fi throttle body.

Your engine might have a PCV system or maybe not, but if your engine is of 4-cycle design it will Always have a crankcase vent that needs to function. Materials spewed out of that crankcase vent include: Water (from condensation) Carbon, Engine Oil (a completely normal condition even in a brand new engine) and Fuel (that finds its way into the crankcase via piston blow-by or leakage past the valve stem. Air intake through the crankcase breather should be filtered and located where there in no possibility to intake water (like a well designed air box)

If your crankcase breather vent becomes restricted for any reason your engine will start to pump oil through places oil is not suppose to go, like out of the engine and all down the road.
 

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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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