It's what you get when a bike builder builds your crank, he can't understand why we need shims, he says bikes don't need them so why do Lambrettas? I couldn't answer his question. He's probably the biggest UK supplier of cranks and pistons for 2 stroke bikes.
I've changed the crank now, I'd tell you what the new one is but too scared of all the "they're s**t they all break" replies. I know it's going to break ..... I break everything given time.
Chris Sadd built one for me last year using MecEur webs and a Yam 110 rod...no shims and its absolutely fine
So something in the rod, piston, or bore isn't sitting central? If you've got enough parts to swap back and forth you may be able to find the bad part. My guess is either the bore or the crank.
When I measured the cylinder stud spacings on a milling machine, I found that the bottom 2 cylinder studs (exhaust side) were the same distance down from the center of the bore, and the same distance either side of the bore. May not be fully accurate, but you could check the rod central-ness by installing the crank with no piston or barrel, lay the rod down on the exhaust side and measure the distance from rod to stud. That would tell you if the rod sits central to the studs/casing. Slide the barrel down with no piston on the rod and see how the rod sits in relation to the bore itself. If both of those are dead on, then check gudeon pin support thickness' on either side of the piston. I would think one of those 3 measurements would have to be off in some way.
I'd certainly trust a bike builder to put a crank together for me.
RE "they all break" I'm still usimg the 60mm crank I bought from you. It's one that they have started saying breaks, but it's wotking fine in my engine. I've had 3 "supercranks go bad on me (one twisting and 2 losing the weights carving the rods)
Wonder if there's an offset on the big end eye of the rod you had fitted.
I know it's going to break ..... I break everything given time.
