The thread is an M20x1.5. A standard M20 flange nut has a flange OD of around 43 mm. That's the same as the flange diameter of an M20 flange bolt so the base of #21 ought to be nearly the same. A standard M20 flat washer is 37 mm OD and I'd expect it to work fine here.

 

So the bolt is 20mm?
37mm seems huge!
* Edit- Oh, the OD, not ID. OK, thanks.

I want to remove the nut on top, drop the rear while supported from above, put about 5-10mm of washers as a spacer on top of the bolt, lower it down again and torque/tighten the nut on top.
Right?

 

Buy this: Ride Height Shim Kit-20mm - Traxxion Dynamics

 

Buy this: Ride Height Shim Kit-20mm - Traxxion Dynamics

That looks great Chuck, but I'm too cheap to even go that route.
I'll use some washers as shims- as many others have done- it's where I got the idea.

 

Are you going to have that much thread available? I would think you can only shim till the nut is flush with the top of the threaded stud. In other words, the length of thread protruding from the top of the nut in the stock set up is the maximum you can shim.

 

There's plenty of room there for at least 5mm easy.

 

There's plenty if it's anything like my 05 600.

Todd, the problem you'll have with washers is the OD being too big to fit in the recess in the frame. $20 in the grand scheme of things gives you multiple options that fit correctly. You'll spend more than that in gas and frustration trying to find what you're looking for. Ask me how I know.....

 

Yoshimura used to sell this. I'm not a fan of the slip-in type spacer but it gives you an idea of the OD.

 

If you've ever tried to change it, you' be pretty thrilled about slip-in style.