Yes having spent my life working around motorcycle engines I was aware of that, the original post suggested the 500cc engines were "restricted to 250cc" by putting a sleeve in the inlet manifold, I was clearly stating this was not the case, it is a power restriction, not a cubic capacity restriction, it is much what the sports mopeds did to get around the law when a BHP and speed restriction was added to the law to prevent manufacturers making 70MPH "mopeds" by tacking pedals onto sports 50cc motorcycles , these were usually restricted by smaller inlets/carbs and restriction in the exhaust, easy to circumvent if you knew what you were doing !!!