These oval port cylinder head will fit all 1965 - 1996 Mark IV and Gen V or VI 396 thorough 502 cubic inch big block Chevrolet engines. They are fully machined with semi-finished Bronze Alloy guides and semi-finished valve seats that use 2.19/1.88" valves. These high velocity "290cc intake runners enhance mid range throttle response and torque. The exhaust runner volume is 110cc., the 110cc semi open combustion chamber design will make a very streetable compression ratio. Spring DIA is 1.55", with 7/16" rocker stud holes. These accept all stock accessory brackets except Gen V where the bracket holes have been enlared to 7/16". The roller studs and exhaust bolt holes are heli-coiled for strength. Technical Notes: These heads are made from 356-T6 aluminum alloy. The 9/16" deck surface is for maximum strength and greater head gasket sealing. Designed for 3/8" push rod diameter. These heads require cylinder head bolt kit P/N 12367779. Use the following performance head gasket 1965-90 with standard bore to 4.370" use P/N 12363412, with bores of 4.47" to 4.54" use gasket P/N 12363411. The semi-finished bronze guide can be finished either 11/32" or 3/8". The interlocking hardened valve seat will accept leaded or unleaded gas. Use intake gasket P/N 12366985.