Kawasaki 650 Crankshaft Question

Hello, new to the forum here. I've been a member of the forum for a llittle while now but always seemed to have my questions answered in previous threads. I have now come across a question that doesn't seem to have any information about it on this forum.

My question is about Kawasaki's crankshafts, I'm in need of a new one for 91 Kawasaki X2. I've been looking around and found that SBT has 2 options for 650 crankshafts. They sell a 18mm and 20mm, the dimension comes from the outer diameter of the treads on the back of the motor. This means that in order to get one you would need to remove the engine from the ski and take off the driveshaft coupler (which can be a really bad job from what I've seen...). I would rather not tear my brand new motor rebuild apart mid-season for this project.

I'm wondering if there is any other way to tell without removing the motor?
-I called SBT and the sales guy said that measuring the diameter was the only way to tell. But maybe some of you guys stumbled across another way to find out.
I'm also curious of why the two different crankshafts?
-I didn't think Kawasaki ever changed the crankshaft design on their 650 motors. I also asked the sales guy from SBT, he only said the two are not interchangeable. It doesn't make sense to me why Kawasaki would do this. Other part dealerships only have 1 crankshaft. And from what I've seen they only sell 1 size end seals.

Any info on this would be great!
 
I think they changed it so that they could just use the 750 crank (20mm) in the 650 (18mm). They then don't have to make two different cranks.

You just have to measure it. You just need to pull the engine far enough forward to be able to spin the coupler a few threads out.

Also, why are you replacing a crank in a brand new rebuild? This part doesn't make sense.
 
That would make sense, similar parts keeps building costs low. The motor was new to me, it was my negligence not to split the cases when I went through the motor.
 
Random thought - It's very possible the PTO portion of the crank was changed / standardized over the years, but FYI to the other Noobs besides myself - 750 cranks have a 74 mm stroke & 650 cranks have 70 mm stroke. They are NOT interchangeable as an assy.
 
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