Fitments
L77-99 Big Twin (Excl. TC)
Part Description
S&S, steel breather valve. STD
High quality steel breather valve for use as stock or high performance replacement. Note: S&S breather valves come with elongated breather holes to increase the duration of the vacuum, allowing more oil to be drawn from the cylinder heads. This has no disadvantage in any model year, either stock or Big Inch motor. Note: that steel breathers require steel breather valve spacers. Plastic breathers require plastic spacers. Do not intermix steel and plastic parts. Note: A .030" oversize breather is available to save worn or damaged crankcases as 518662. A JIMS breather hole reamer (MCS 978343) will ream the damaged hole to +.030" so a +.030" breather can be used. Tech tip: The timing for S&S breather valves, only for late 77-84 Shovel and 84-99 Evo Big Twin models, has been changed by S&S in the year 2000. This is done to improve oil scavenging and reduce oil carry-over in high performance / large displacement engines. Old opening was at TDC and closing at 65° after BDC. New opening is 17° after TDC and closing at 82° after BDC. The old timing specification can be obtained by installing the timing mark on the breather gear one tooth clockwise from the timing mark on the cam gear. Also the breather's cavity oil evacuation hole (opposite of the drive gear), which used to be a round hole, has been elongated (made longer) to increase the duration of suction to the crankcase breather 'settling' cavity. The new specifications are not useful in most stock engines, but harmless. Breather windows in S&S cases will typically open at 7° after TDC and close 92° after BDC. Builders using other crankcases may modify crankcase breather opening to obtain these specifications for maximum breather efficiency and performance.
