If you are making adapters, then I assume you are putting studs in those holes. If so, you need to drill the hole to the stud you will be using, which, is your choice. GM used several types and sizes of knurls on their studs. You just have to go pick out a size and style you like. You also need to look in the "buyers guide" the manufacturer has on the studs. You should be able to find the interference fit needed for the particular stud you use. Some buyer's guides are in the back of the catalog, others are seperate books. Dormann would probably be your best bet, Motormite probably won't be much help. I assume you will be using aluminum so make sure you find the interference fit BEFORE you mill the holes. I've seen guys put small tack welds on the back of steel adapters to keep studs from turning if they went a few .000 large on fit. I don't recommend this. Another possiblity is drilling and tapping your hubs to 5x 4.75 and using VW lug studs, providing you have enough material to thread into. This will eliminate proper interference fit worries. Either way, be careful, your life and maybe mine could depend upon it. If you don't have proper machining experiance or training, get help. I want to say either 7/16 or 1/2 but I can't remeber which.