I guess it varies from country to country what you can/cant get away with.......Its all cheating to a point......latest Terry Prince head, but very heavily re worked........welded up the valve guide bores and started again........The valves, well we thought we were the first to do what we've done, but turns out Steve Hamel beat us to it........Simply another round collet groove machined half way up the stem, then a pair of modified standard collets with the outside machined parallel with a small hat section left at the base for the stock collar to press down on to.......Had some ney sayers about how it will work, seems to be ok using 140 Lb seat pressure springs and 600 lift at the valves.......taken it to 7000 no problem.......Just kept pulling like a train.......We aren't doing anything special that hasn't been done before........just not sure its been done with an original type Vincent engine.....Maybe maybe not.......It's our own personal challenge.......My single is the guinea pig to my mates twin......If mine is anything to go by, his twin will be a monster......Main trouble we've had is modifying Titanium valves.......once the special coating is gone, they wear badly........on the stem, it chews the stem and guides out rather quickly, and on the valve face it starts to abrade the face quickly causing the tappet to close up........not a big deal, just needed to get the valves re coated by a tool company that supply machine cutting and lathe tools.......Vic should know what i;m talking about......Once these items are sorted, it should be all guns blazing........Cheers......G.