Lovely engines. In actual fact it is a BSA A65 lump with an overhead cam. The guy that designed it worked at Ulster University, took the design to BSA where they promptly sucked on their pipes, sighed deeply and said "The British motorcyclists would never go for such a thing", so of to yamaha he went who nearly bit his hands off to get the design,(he later designed the RD200 two stroke for them)and with the cunningness of a jap that knows he`s onto a good thing Mr Yamaha decided to restrict the making of parts for the XS650 so there would be no pattern parts available, hence a pair of pistons are nigh on £200.
Just thought you might like to know this.
