How do I determine if I need oversize rings/pistons?

[Guest]

A different way of looking at it would be: How do I determine if I should just get a different set of cylinders, since they're easier to come by and less expensive?

Here's the situation: I'm currently piecing together the necessary parts to convert my 750 to an 850. I picked up a set of jugs and pistons for next to nothing on eBay. One of the cylinder walls had a scratch that wasn't deep enough to catch my fingernail, but still a bit worrisome. I honed most of the scratch out with a ball hone, but left a hint of it as I didn't want to go too oversized.

Looking at the 850 service manual, I see that the standard cylinder bore is 69.000mm-69.015mm and the limit is 69.080mm. So where do the oversizes fit within that range? Is 69.000mm - 69.015 mm the range in which I can still use standard piston rings and then anything above that, up to 69.080mm, would need oversize rings?

The fiche on Parts Outlaw lists oversizes at 0.5 and 1.0, but they only have pistons in OS 0.5 and rings in OS 1.0. I'm assuming oversize rings need to be matched with pistons of the same oversize and that you can't put oversized rings on standard pistons or any other such combination, correct?

Also, is the OS 0.5 and OS 1.0 nomenclature an actual measurement in millimeters, or is it more akin to size measurements like small, medium and large? If it's the latter, what are the actual bore size measurements to determine if one would need OS 0.5 vs OS 1.0?

I haven't had a chance to measure what the diameter of the scratched bore is now, but I will be borrowing a bore gauge soon.

I know this is a bunch of questions about something that is sort of a moot point in this particular case, but I would still like to acquire the knowledge of how all this works.

Thanks in advance for any input.

[Nessism]

You don't use oversize rings in a standard bore, regardless of wear.

If the piston/cylinder specs fall outside the service limit then you need to bore to the next size. OS pistons and rings are hard to come by for some bikes though.

[fastbysuzuki]

To do this correctly you need to have the bores measured to see how they compare to spec before making any decisions.
as said in another post you do not fit oversize rings in a standard bore.

[Grimly]

The standard bore is 69.00mm, with tolerance for manufacturing and wear in service tacked on to that.
The oversizes are 69.5mm and 70mm and you need the specific rings for those sizes (which will also have a tolerance quoted for those sizes in the factory manual).

[tkent02]

The 1mm oversize rings for the 850 are the standard rings for the GS1000. Same part number.
But you still need oversized pistons.

[80GS850GBob]

Once you hit 69.080 they suggest you go either a half or hole MM oversize...for you this will be most likely determined by which one you can actually find the combo in - matched .05mm or 1mm OS rings and pistons, but not a combination of one of each{that won't work}.
Folks with the 850s are sort of left hanging for viable overbore choices now a days...sucks, but it is what it is.

When you check bore taper, check top, mid travel and bottom....biggest spec wins for wear spec. I've seen some rather affordable locking ball gauges on ebay...use a mic to measure the width....

[80GS850GBob]

As I recall, folks with 550s can go to 650s, 650s can go to 750s, 750s can go to 850s....850s, well, no such luck to jump to 950 or 1000. {that I know or heard of}