I'm doing a conversion to the 4-piston discs and had to order new rotors (ouch!). My problem is that while the holes are on a 4" bolt pattern they are larger than my original studs. Do I have to find new studs and have the hubs drilled out? I've checked two different sets of rotors, different brand but still made in China, and these were exactly the same. Anyone with any experience here?

Do you mean the studs ar 1/2" instead of 7/16"? If so, drill the rims to 1/2".

On the early Kelsey-Hays discs the front hubs have to to set-in to the rotors by a machinist. In my case, the new rotors have a larger hole than the original ones. The 7/16th studs in the hubs therefore do not fit. Perhaps these will accept 1/2" studs, but I didn't want to chance redrilling my hubs (and possibly ruining them!) before I asked if anyone had run into this problem. Different wheels I got.

What brand/part# rotors did you get, and what car are they going on?

I put bendix brand KH rotors onto my 71 duster (I think the car the brake setup came out of was a 72) and had no troubles with the hubs/studs/wheels all matching up.

Assuming you have the right size parts, mating the rotors and hubs together doesn't require any special tools to do. If you have an air gun and/or hammers it just takes some time.