Hello advrider members, I'm a new poster here... proir to now I've just read stuff. Hoping I can find some help however. I'm fixing up a 1980 Yz250, ( frame and motor prefix 3R4), which required major motor repairs. I found and bought a complete bottom-end from a 1981 yz250, ( a prefix 4V4 ). Opening up both motors I find the 1980 is a 6 speed.... the 1981 a 5 speed and parts lists support that. First question... can anyone verify that? Now what I'm really stuck on is the two cranks have different connecting rods in that the size of the wrist pin bearing. The 1980 con-rod, ( marked 5x5... a yamaha part #), measures 23mm ID. The motor I'm told was a 1981 yz250 has a con-rod measures 22mm ID ! ! Short story is : I can't fit my 1980 Weisco piston onto the 1981 motors crank... the wristpin bearing is too big to fit into the con-rod. A concern is... the 1981 con-rod has YAMAHA cast onto it's side where-as the 1980 cod has, like I said, 5X5... a std. yam part #. Maybe the rod is aftermarket ? ? Weisco doesn't list one. Or maybe the crank,and /or whole motor isn't a 1981 yz250 at all. Anyone have some insight into this? Thanks, dave Is there a better place to source this info?