one is the overall length of the bolt, but more important is that you always used bolts which have the correct length on the threaded end, from the tip to the point at which the ball-seat flare begins, just past where the threads end. If that is too long, the bolts will tear up your parking brake mechanism on the rear, and in extreme cases, may interfere with suspension bolts on the front; if it is too short, the wheels may come off the car or some klutz with an air wrench may strip out the threads even more easily than usual. It is this critical dimension which varies a lot from one type of stock wheel's special bolts to another's.

A year or so ago, the appropriate bolts were still available on the 'net from some aftermarket suppliers, but made in Asia. Those may also be off the market now, but I'm not sure.