Messing about with the CB500 this afternoon, I fitted some aftermarket Velocity stacks to the bike, with screens, so that should help the bike breath better and I also managed to find a 15mm Die and tap the threads to a course range and the New petcocks I bought, screwed right on.
No idea if they will leak but should be fine and on the weekend I shall fit the gas tank, buy some 97 octane gas and see if I can ride the old girl. Still more to do, I need to buy some Throttle cables so I have a Push Pull set up but she should run ok for the time being.