I'm restoring a '75 DT250 with 6500 miles on it. Cylinder compression is about 90 psi. I pulled the head and cylinder and measured the cylinder bore at a very consistent 2.755 inches. Piston measures 2.751 inches. So I have about .004 clearance. Service manual recommends under .002 clearance. Bike is in really great condition and has been well cared for, motor runs good. I'm thinking I should cleanup the cylinder and piston, install new rings and call it good?? Any recommendations?

I'm going to apologize up front for the next couple of comments...

In my experience, stock bore is always right at or slightly larger than "spec bore".  Never smaller.  In this case stock bore was 70mm.  Your numbers (2.755" = 69.977mm) say your bore after 6000 miles is slightly under 70mm.   Makes me question the accuracy of the measurement. 

One-off compression readings are pretty useless...  many auto gauges read low on these bikes.  At a true 90 psi it would be a bitch to start at the very least and you say it runs good.  So I question that compression number as well.  (Say you got a fresh bore with new piston and rings done by a shop that knows what the hell they're doing like Bill Bune...  wouldn't surprise me if you still had about 90 psi with that gauge.)

That said, if it runs great and doesn't rattle...  and when you get to 0.004" to 0.005" clearance they tend to rattle pretty good...  that also is evidence the measurements are probably not accurate? 

Best advice i have Is send cylinder to Bill Bune for measurement and evaluation.  Then make a decision.   Or if truly "restoring" have them bore it to the next size and start fresh.

If it was mine and If it's reasonably quiet and runs good, I'd just put it back together and run it.  New Yamaha rings are hard chrome and don't tend to seat well in used bores in my experience.  Plus it's super easy to pull the top end again in the future if/when you decide it does need a bore. 

 

P.S. If you read the manual you should find maximum allowable clearance is 0.004"...  spec of about 0.0016" to 0.0018" is for a fresh bore and new piston.

Mark: thanks for the reply. FYI, it does have a bit of a rattle at idle. Who is Bill Bune and what's his contact info? Is he the dealer in Minnesota? I'd just as soon get a clean bore and a new piston...

www.billbune.com/