I agree with ukeeku. Talk to Bill. The size will depend on the thickness of the headstock and the size of the holes from the current tuners. Bill himself installed mine and I got to watch him. The Pegheads have a slight taper and are threaded. He used a reamer to create the tapered, threaded holes necessary before installation. They are also left and right threaded, so some screw in clockwise and the others counterclockwise. From my experience with bike pedals, I'm guessing that they screw into the headstock that way so that the string tension doesn't loosen them over time. I could be wrong about that. Hopefully, someone with more knowledge than myself can clarify it.