Piston, rings and bore question

A buddy of mine gave me a "79" KZ650SR D. He couldn't get it to run. It ran poorly than just wouldn't start one day. I originally thought it was the carbs and fuel. So I was on that forum for a bit. But it turns out to be the rings. I did a whole bunch of testing of stuff to find this out. Here's where my brain starts to hurt. The piston rings are worn. Okay I got that. The gap is around .027in. My problem is that the cylinder bore is smaller than what any manual says. So are the pistons. It's the original engin with 40k on it. I can see the pistons being smaller because of wear, but there isn't any sign of it. They are perfectly round and measure at 2.418in. The bore is also perfectly round and measures 2.438in. It was never worked on or taken apart. Oh yah, the ring grooves are .050in thick/wide if you need that info.

Okay, my questions are: How do I know what size rings to get? Why don't the measurements match any manual? Basically, whiskey tango hotel, over.

Jeff
I'll find a way, or I'll make one.
Halifax

Whiskey Tango Foxtrot?

You measured 61.9252mm bore.

Slick spec is 62mm

What does FSM say. 61.5? I don't know, I do not have one.

Did you check your measuring instrument against a gage block?

Ring end gap sounds like it's near/past limit.

I'm betting you've erred in measuring the bore.

Are you using a 'T' gage and measuring NS and EW at three depths or did you measure from the top with a slide caliper? If the latter, you likely measured the deposit ridge at teh tp of the bore where the rings never go.

-Duck

Hmm, 2.418 = 61.4172mm
2.438 = 61.925mm

by what you are saying you have .020 clearance between the piston and the cyl, if this is the case the pistons should be so loose that it would have made a hell of a noise in the top end.

as Duck said what did you use to check the bores and pistons with, and has it been checked to make sure its accurate?
snap gauges are not accurate at all, and you need to check the bores at several spots up and down in the cyl

I'm a machinist and take my home gauges into the shop on a regular basis and get the inspection guys check them. So I know they're all good.
I took the pistons and cylinders to work and used the inspection guys tools. I know they're accurate.
I checked the pistons with two different sets of micrometers and a set of calipers. They were all the same. (I wanted to make sure.)
I checked the bore with one of works inside micrometers and an inside dial gauge. I know they're right. I also used my calipers which have a longer than normal set of blades for measuring inside diameters. They were the same as well. There doesn't seem to be any ridge. Atleast none that I can see, feel or measure.
I realize there's a .020 difference between the bore and pistons, but I don't remember and my buddy doesn't remember hearing any clattering or strange noises when it ran.
This bike is total mystery. Nothing is the way it's supposed to be, measurement wise. Again, no work has ever been done to the engine except for basic maintenance.

We're in the twilight zone, I'm getting scared! I also have nothing useful to say.

wish you would have said you were a machinist, I would not have questiond the accuracy of your tools.

when you slide the piston, without the rings, do they rock back and forth in the bores?

I am kind of wondering if the motor had been over heated
and colapsed the pistons, since the bore is real close, that would also explain the ring gap also

I just checked. They ride up and down the bore fine. The wrist pin isn't binding and nothings stiff or floppy. Just the rings were really worn. A .027 gap on the top and second ring. Forget about measuring the oil rings. My feeler gauges for my bikes only go to .030. I had to use the ones for my truck to measure them. .038 and man were they thin.

as a standard rule of thumb, .007 per inch is what you are looking for on a stock application, I am puzzled on the piston to wall clearance.
try one thing, with the rings off, slide the piston in the bore with a feeler gauge, what is the largest one you get in it, just snug on tight