Back in the day, putting in 61mm CB750 pistons was an option for hot-rodding a CB500 or CB550. The crowns had to be machined slightly, but not a lot and it worked. This was a 600cc (actually 591.5) solution. Best of all, the factory rings ensured good sealing and long life. Beyond the expected improvement inherent in any new machining, boring out the 500/550 really does offer a welcome performance boost. The broadened torque spread not only adds to ridability, it also permits less throttle to be used, thus reducing the four-cylinder buzz.