There are many ways to hold the jaws while you bore them, and much of it is pertainant to whether or not it is a through bore. I have numerous spiders that i made for just that. I have bored jaws for chucks up to 48" using spiders. if you must bore through then a good way is to put pins in the bolt holes and they must fit pretty good, then put a ring into these pins and close chuck. using differant size pins, rings, and shimms you can fit your jaws with good presision. it is imperitive to hold your jaws solid and bore them right to size, else your work could fly out of chuck with an improper fit. Good luck and have fun. oh and use a good quality chuck grease, and not too often, but when it is needed.