These are original Big Block Chevy Aluminum Mark IV part # 3919838 casting 3919842 closed chamber Rectangular port heads as used on 67-69 L88 and ZL1 corvettes. They did not come on Chevelles or Camaros except for aftermarket special additions. 3904392 not 3919842 heads were used on L89 engines contrary to what you might read elsewhere. Chamber of 3919842 head is 107cc with corresponding high dome pistons had a compression ratio of 12.4:1. The rectangular port intake runner volume is 325cc valve sizes are 2.19" and 1.84". Exhaust port exits are round. DO NOT use these heads with stock production Generation V blocks. These are high RPM heads used on engines producing in excess of 500 HP and do not have much in the way of low end torque especially when used with a high rise open plenum intake manifold. Typically a stock L88 or Zl1 produced 550-600 hp though advertised as 425hp and in actuality were full race engines sold as production vehicles to qualify for some production race classes.