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The aluminum block requires 1/4-inch-longer threads on the head studs (3/4 vs. 1/2 inch) than the cast-iron block studs to maintain the required torque rating. This is because the aluminum threads don't have the same load capability as the iron block threads. Notice all the different lengths and thread depths on the head studs alone! ARP is great in these situations as they can tell you what to use based on their experience.