S&S® Super Stock® 3-5/8" Bore O-Ring Style Single Plug Cylinder Head Kit For 1966-'78 HD® Big Twins - Natural Aluminum FinishS&S Super Stock o ring style cylinder heads for 1966 '78 HD big twins feature a stock like appearance, but flow much better than stock heads. Heads come assembled with premium valves and S&S valve springs that can handle cams with lifts up to . 590". These heads are machined for one spark plug per head and accept 3 5 8" bore cylinders, and are original equipment on S&S SH Series engines, and are a great upgrade for stock shovelheads. Rocker boxes are