9 We wanted the 604 crate engine as a foundation for this project because of the forged steel crankshaft (the crank in the 602 engine is cast), but the aluminum cylinder heads aren’t legal in any Street Stock class. The original plan was to hunt up a pair of double-hump heads and machine them up. But those heads are as rare as the proverbial hen’s teeth and cost a fortune, so we went the more logical route and sourced a pair of stock replacement iron heads. The RHS iron Pro Torker heads are a great option. They are very cost effective, can be run right out of the box and have options available that put them close to a stock alignment. They arrived bare, and we paired them with a set of Manley Race Flo steel valves.