**Always follow the instructions in your repair manual when doing repair or maintenance work on Outdoor Power Equipment. Manuals can be found at the manufacturers website.**
A small engine breather valve is a one way check valve. It works by allowing high pressure to escape the engine crankcase, but close when crankcase pressure is lower than the outside ambient air pressure. This has the overall effect of a crankcase pressure that is lower than the outside air pressure. The volume in the engine crankcase is not constant because the bottom of the piston is part of that area and the piston is moving up and down as the engine is running. When the piston moves from bottom dead center to top dead center the crankcase volume increases and the pressure decreases. When the piston moves from top dead center to bottom dead center the crankcase volume decreases and the presure increases, and during this stroke the crankcase breather opens up and allows the high pressure to escape.
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