Postby LaceyDucati » Sun Jan 05, 2025 2:08 pm
George
In theory if it is 74 it should be a 30mm crank, but then again in 74 it should be Italian manufactured according to folklore! Maybe late 74 engines were also built in spain, when this started is maybe not January the 1st 1975.
I have seen Spanish built "Italian 1975" engines with both Italian 32mm cranks (denoted by an R on the rod) and 30mm cranks (denoted by LAF on the rod). I think the 30mm spanish cranks were the very last of the 1975 bikes. Mototrans models did not use the italian 32mm kit using the 35/43/22 bearing. Mototrans continued with 30mm cranks until much later (80's I think), then did their own thing using the bevel twin bearing 38/44/22. The size of your rod kit can easily be determined once the cylinder is lifted, by the width of the rod assembly including the shims (width between the crank thrust faces). 27mm and 30mm assemblies are 20mm wide including shims and the 32mm kit is 24mm and very visibly larger....no need to strip the cases to identify.
Your issue with vibration may not be the bigend, it could be a worn crank journal/bearing on the drive side. or even a loose main bearing. Either way probably time to investigate, pull the clutch cover off and get hold of the crank and try and pull it up and down. If you do strip the engine, at that stage even if the bigend "looks" okay I'd still split it to inspect. The "B" rod 32mm kits used a double row of rollers which have a tendency to tip and tear the pin in the middle, ultimately leading to pin surface failure and eventual bearing carnage. If you catch the failure before it damages the rod eye using a Yamaha bearing with single rollers can prolong the life.
Hope this helps
Best wishes Nigel
) vibration at high rpm so something is getting a bit worn in there somewhere. Changed oil recently and as always came out as clean as it went in... no odd noises and no smoke. (All engine bolts are tight).