Gary, If you don't have a 1/2" dia. bar, I'd consider getting the 5/8", since you can't bore a 3/4" hole with a 3/4" bar, but you can bore a 7/8" hole with a 5/8" dia. bar. I would also give serious consideration to forking over the extra money for a solid carbide boring bar. I've bought several on "that auction site". I have a preference for "Circle" brand solid carbide bars.