[Rat_Pack]

JC, just what Tom said. It does not have to do with the diameter of the wheel but the design of the spokes and center "spider" ( lug nut section of the wheel). When the GM disc brakes came out in 1967 on the A, B, and F body cars they used a similar design of the Corvette large four piston caliper. Aftermarket wheels of that time were not made to clear those calipers so companies had to make a new mold just for those applications. American made their Torque Thrust D and other companies did nothing or told you to use spacers. I will send you some pictures of the differences in the center sections of some American's. Also be sure to read the post on Yenko Chevelle wheels under this same section as it may answer some of your questions..........RatPack.................