Have a pair of cylinders & heads on standard bore size. Will put them on TradeMe soon & put a link up here. Ok not vmx but still old Yamaha. These were before reed valve arrived on Yamahas. They have the rear transfers just as slots in the side of the liner but they still went well despite that. Great days those & remember assembling these models new & the heading of around town to give it a run [damn good thrashing lol], never went past 1/3 throttle Boss, as you instructed.
https://www.trademe.co.nz/Browse/Listing.aspx?id=1684776887&ed=true
