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Old 05-16-2012, 10:18 AM   #10
DiabloADV OP
Joined: Oct 2010
Location: Nor Cal, USA
Oddometer: 2,322
Originally Posted by sigpe57 View Post
Keep it original.
I hear you, but original is my most expensive option since I already own the 93.97 pistons. They are known good pistons and I'm reluctant to buy used (and more possible trouble).

I have it on good authority that what the bike came with (97mm-spigot R90's bored to 1000) is problematic on a number of levels. The spigot is too thin, mainly. Plus, the cylinders don't line up with the squish band in the R90 heads.

After a visit with the machinist (veteran airhead guy) yesterday, and after picking up two very nice 99mm-spigot R90/6 cylinders with new SS pushrod tubes, the plan is to...

Bore the case to 99mm. Easy and cheap to do.
Bore the new cylinders to match my nice 93.97 dome-top pistons. New rings and pins (which I needed anyway).
Machine the squish band ring on my nice R90 heads to match the new cylinder diameter. Doing a valve job anyway, so this is simple to do while it's apart.
Run it with the compression-reducing cylinder-case gasket.

This shop work will only run me about $300. Total cost of the repair, over what I had to spend on refurb anyway, is about $400. To change piston size would cost a lot more in parts and only save half the shop costs.

A bonus is that the cylinder and head work include having them fine-grade bead blasted, cleaned and returned to me looking like new.
1974 R90/6 motor w/1050 kit; twin plugged; installed in...1970 R60/5 frame; RM125 front end; KZ1000 Police Tank.
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