The Evolution style breather valve replaces the standard rotary breather valve in Evolution Big Twin style engines Installation is fast and simple. No aligning timing marks or rotating parts Unlike the rotary breather valve, the S&S crankcase breather reed valve is not dependent on rotation of the crankshaft to open and close The reed opens when the pressure in the crankcase becomes greater than the pressure in the cam chest Installation of the breather reed has actually stopped high rpm oil carryover in test engines with marginal piston ring seal