These S&S manifolds for 1955 thru 1965 Panhead engines with "o-ring" style cylinder heads are 1 7/8" bore. Knuckleheads and Panheads with 1936 thru 1954 cylinder heads that have "plumber" style intake mounting nipples must be converted to accept a 1955 to 1965 o-ring manifold. Manifold can be used with S&S ES, LS, A, Super B, Super E, H-D Tillotson, Bendix, and Keihin carburetors.