Blow-By

Blow-by is combustion gas and unburnt mixture that leak past the piston rings into the crankcase. A small amount is normal in every engine. A crankcase breather system routes these gases back into the airbox to be burned, rather than releasing them into the air.

Excessive blow-by means the rings are worn, stuck or broken, or the cylinder is scored or worn. Symptoms include oil mist or vapour coming from the breather, an oily airbox, higher oil consumption, blue smoke, loss of compression and power, and harder starting.

A compression test or a leak-down test shows how well the rings and valves seal. On off-road four-strokes and two-strokes, a worn piston and rings are a normal service part, replaced at intervals given in the manual.