S&S Reed Valve Crankcase Breather for Evolution Big Twin – Engine

Patent pending reed valve crankcase breather from S&S that really helps to resolve oil carry over problems in Evolution Big Twin engines. Unlike the rotary breather valve that has been used in Big Twin engines since the 1930's, the S&S crankcase breather reed valve is not dependent on the rotation of the crankshaft to make it open and close. Therefore the opening and closing times are not determined by crankshaft position. Instead, the reeds open when the pressure in the crankcase becomes greater than the pressure in the cam chest. The reeds then close when the pressure in the cam chest is equal to or greater than the pressure in the crankcase. The difference in the way these breather valves function is important because the ideal breather timing actually changes with rpm and the condition of the engine. By opening and closing "on demand" the reed valves automatically compensate for the lag time associated with accelerating the mass of air as it is forced out of the crankcase. This is most significant at high rpm when the time available to get the air out is the shortest. In addition, "on demand" operation allows reed valves to accommodate greater amounts of compression blow-by. Installation of the S&S crankcase breather reed valve has actually stopped high rpm oil carryover in test engines with marginal piston ring seal. The breather reed valve replaces the standard rotary breather valve in Evolution Big Twin style engines. Installation is fast and simple. Remove the original breather gear and insert the new S&S reed valve assembly into the breather valve cavity. No aligning timing marks and no rotating parts. The S&S crankcase breather reed valve can be used to replace the stock style rotary breather valve in engines with minor breather cavity damage, and there is a +.030" oversized reed valve for engines with heavily damaged breather bores. Fits all 1993-2000 Evolution Big Twin engines.

+.030" oversize

S&S BT93-99 breather reed valve +.030"

• Category: Oil Pump