FOr cb750s I usually take the entire fork apart and inspect for any bends or cracks and work the seals in by hand. However if you do not want to do this take the seal with you to home depot and match it up to some pvc pipe. There is a size that works I just forget what size it is. I like this better than metal exhaust adaptors because 1) you can get it in any length (including taller than your forks so you can get a hammer on the end) and 2) it won't mar the seal if the seal hangs up and you have to use a little extra force.