CMS-Japanese Motorcycle Supply Parts

What is a crankcase breather and why do you need one?

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On some CX500s you will find crankcase breathers which look like this:


In any combustion engine there is an imperfect seal between the piston and the cylinder walls, and this gap - albeit very slight - allows a small amount of the combustion gases to blow by the piston rings and into the crankcase. This variation in pressure needs somewhere to "breathe" to - hence the crankcase breather. Another cause of pressure variation is the motion of the pistons, conrods and crankshaft creates a significant amount of turbulence - which is greater in some engines types than others. For emissions reasons crankcase breathers are usually vented back into the intake somewhere - on the stock CX500 they vent back into the airbox.

It's important that the crankcase has a path to breathe through, wherever it vents to - if you block these vents up the pressure will attempt to find an alternate path - be it via a gasket or other undesirable (and possibly expensive) path...

It's also important that there is some way of preventing debris and water from entering through the vent - having either of those things enter the engine case directly is potentially a very bad thing!

In some areas (like NSW) modifying the breather setup counts as altering the emissions system of the vehicle, which theoretically requires an engineering certificate for the vehicle to be registerable. You could still plumb the crankcase breather into pod filters if you removed the stock airbox, though it would probable be advisable to set up the system so that it attached to both pod filters to prevent the extra airflow and oil/blowby gases from unbalancing the carb performance.

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