The CB500 that I'm working on has a big bore kit fitted. Have just put the gasket in place, set on the barrels and then put on the cylinder head. Horror! the gasket between the crank case and barrels had split and the ends were sticking out. My guess is that the gasket was a CB500 (I bought the gasket based on the bottom end) and the barrels are of a 550 because you can't otherwise bore them out for the big bore kit.
Could there be any other reasons for this to happen?