Breather Filters

A breather filter caps the open end of a vent line from the crankcase, rocker cover or a catch tank. It lets gas and pressure escape while keeping dirt out. This category holds small breather and vent filters, with neck sizes such as 9, 12, 15, 18 and 25 mm.

Some combustion gas always slips past the piston rings into the crankcase. This blow-by carries oil mist and water vapour, and it must escape, or crankcase pressure pushes oil past seals and gaskets. A factory system feeds it back into the intake to be burnt; an open breather vents it to air instead.

Blow-by rises with load, revs and engine wear, and boost adds to it. A filter that is too small, or saturated with oil, restricts the vent and lets pressure build again. Many race rules ask for the vapour to pass through a catch tank, so the filter often sits on the tank’s vent rather than on the engine.

Choose by connection and capacity:

• Neck size: crankcase breathers with 9, 12, 15, 18 or 25 mm necks, and vent filters with 13, 16 or 19 mm necks. The neck clamps over a pipe or fitting of that outside diameter.
• Body size: the crankcase breathers measure 60 mm long and 51 mm across, and those vent filters are 51 mm across and 38 mm high. More media area copes with more blow-by before it restricts.
• Part-numbered filters: other vent filters are listed by part number, so compare the neck size and body dimensions in each listing.
• Finish: the crankcase breathers have a black finish.

Mount the filter above the oil level, with the hose rising from the engine so that condensed oil drains back. Keep it away from the exhaust and turbo, as oil mist on hot metal is a fire risk. On a road car, venting the crankcase to air usually breaches emissions rules, so keep the closed system and add a separator inside it if needed. Clean or replace a filter once it is oil-soaked.