This Cylinder head fits all 1965-1996 Mark IV AND GEN V and VI 396 thu 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 315cc long intake runner and 300cc short runners enhance mid-range throttle response and torque. The exhaust runner volume is 110cc. The 118cc semi-open combustion chamber design will make a very streetable compression ratio. Spring DIA is 1.55", with 7/16" rocker stud holes. Thse accept all stock accessory brackets except GEN V where the bracket holes have been enlarged to 7/16". The rocker studs and exhaust bolt holes are heli-coiled for strength. Intake and exhaust ports are in stock locations. Technical Notes: These heads are made from 356-T6 aluminum alloy. The 9/16" deck surface is for maximum strength and great 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 1965-1990 with standard bore to 4.370" use P/N 12363414 and with 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 12511789.