The Evolution style S&S breather reed valve replaces the standard rotary breather valve in 1993→ Evolution Big Twin style engines. Installation is fast and simple. No aligning timing marks, and no rotating parts. There is a +.030“ oversized reed valve for engines with heavily damaged breather bores.
How does it work?Unlike the rotary breather valve that has been used in Harley-Davidson big twin engines since the 1920‘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. The functional part of the reed valve consists of three steel reeds that are normally in the closed position. When the pistons travel downward in the cylinders, the air in the crankcase is compressed and pressure in the crankcase below the pistons increases. This pressure causes the reeds to open, allowing the air, oil and oil mist in the crankcase to escape into the cam chest. When the pistons change direction and begin traveling back upward in the cylinders, a vacuum is created in the crankcase. The vacuum causes the reeds to close, preventing air from entering the crankcase. The effect is that a slight vacuum is maintained in the crankcase and oil scavenging is greatly improved. This is most significant at high rpm when the time available to get the air out is the shortest.
- fits Evo Big Twin 1993→
- Crank case
- Over-/Undersize: +0.000 ”
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