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Old 11-27-2013, 06:59 PM   #1
R-dubb OP
Dubbious Adventurer
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Joined: Aug 2001
Location: San Francisco
Oddometer: 5,014
R90/6 top end overhaul

I'm restoring a bike I owned 35 years ago. I had the heads redone by SJBMW when the motor had about 40k on it. It now has 30k more and has been sitting some. Compression is at 115/120 cold without carbs. That's a little low. There is also some oil getting to the plugs, but no smoking except on hard acceleration. The bores are clean, but pistons seem slightly loose. Heads look just like they did 25 years ago but for some carbon.

I'd like to hone and fit new rings. Does anyone make oversize rings? Or, do I need to buy bigger pistons? Haven't taken measurements yet to see if the bores are straight. Pretty sure they are as nothing is visible that would indicate uneven wear.

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Old 11-27-2013, 09:38 PM   #2
Joined: Oct 2010
Location: Nor Cal, USA
Oddometer: 2,336
I went through this not long ago. First step...take the cylinders and pistons to a good machine shop and pay them to take good measurements. Max clearance is 0.120mm, which is basically huge--4.7 thousandths. If you're at 3 thou or less, flex hone them, buy next size up rings and trim ends to fit.

Oversize pistons are only for reboring to next size up.

Unless it's a race motor, button it up and run it.
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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