hi,for some reason the bis exhaust likes to do that,they also shed internal baffles,the spot welds break from the casing.....aircooled ones, if properly installed seem more resiliant to the vibrations.......talking of engine mods,even the aircooled as standard is in a high state of tune.....23bhp,they started at 16bhp in the early 500........the cam in the polish 650 would hve been considered 'high lift fast road' back in the '60's,so the bis is right up there as well!!!....and tuned engines do break things :wink: