These precision made steel breather valves replace the stock steel ones in 1936-early 1947, late 1947-1976 or 1977-1989 Big Twin, as well as the plastic ones in 1990-1999 Big Twin. The version for 1936-early 1947 is a high-evacuation version of the stock breather valve. All these S&S breather valves are also available in a .030" oversize. These can be used when the breather hole is worn and reamed with ZPN 756304 Jims reamer tool. We also supply steel precision ground breather valve gear shims to set correct end play in 1977-1999 models.