A crankcase breather filter is used to prevent dirt from entering the engine through the crankcase breather. On standard motorcycles, the crankcase breather sits in the air filter box, and a separate crankcase breather filter would be a superfluous extra. However, if the original air filter and air filter housing are replaced with a K&N universal air filter, the installation of a crankcase breather filter could become necessary.
The size of the crankcase breather that fits your motorcycle is determined by the inside diameter of the filter flange, marked by "F" in the drawing below. This dimensions must correspond to the outside diameter of the crankcase ventilation. With this value you can set the filter on the left of this page, in order to find the correct crankcase breather filter. The other dimensions can be found on the product page of the individual filter under the tab 'technical specifications'.