Designed for non-emissions 302, 327, 350 and 400 c.i.d. engines up to 1986, these heads made 420 horsepower with the Performer RPM Power Package for small-block Chevys. Performer RPM heads with 64cc combustion chambers improve performance and retain stock compression ratio on 1970 and earlier small-blocks. Hardened pushrods are required. Installation Notes: When using stock rockers on S/B Chevy Performer & Performer RPM heads, .100'' longer than stock pushrods are required. Due to valve-to-cylinder bore interference, SB-Chevy heads may not be used on engines with less than 4'' bore (262, 265, 283, 305, 307 c.i.d.) except with camshafts having less than .450'' valve lift. Tech Tip: To gain more spark plug clearance, Edelbrock recommends using Champion RC Series plugs such as the RC12YC (P/N 192-49776). They are 0.230'' shorter than a conventional spark plug and have a 5/8'' hex for easier access.