There are bushing that will make a 5/8ths into a3/4ths--still you'd have to boar, hone, or turn the little bit extra to make 5/8ths to metric. There is only two things that would be a good reason to do such a thing-you have a 3/4's clutch, no 5/8ths and so you bush the 3/4ths-- or you want to put a torque-a-verter on a small motor--now there's a good reason.