Sounds like you're having fun, or fun you could do without.
Not sure if i'm reading it right but do you 'not' have a piston but you've bought new rings?. Cylinder should be either 66mm std & up to 67 in 0.25 steps. Better get a accurate machine shop measurement & see if it needs reboring & order a piston kit for that. Or am i misunderstanding--i'm good at that when i'm tired.

I got it as just a barrel.
I ordered a standard piston kit as a starting point because it was sold as "used old stock" but looked practically unused, if a little rusty.

The rings from said piston kit are too small. Which means it cant be standard. (Should be 0.3mm is 1.0mm)

The bore was measured at 6.4mm with a bore gauge.
I don't know if its 66.25 plus 0.2 ish for rings. Or 66.5 rounded up from 66.4.
A machine shop is the ideal solution if I can find one.
0.5 over is cheap. (£35)
0.25 over is not. (£70)
Reboring to 0.5 is most definitely not cheap. (£120 and probably 6 weeks)
Getting 0.5 stuck in 0.25 is also not cheap. (see rebore cost)

I figure it might be worth trying 0.25 and 0.5 rings at £12 a set to see if either end gap matches the spec.

RT325 is correct....  the inner hub may be too short!   Good catch!

1978 and 1979 125 had shorter inner and outer hubs than 175, 80 and 81 only used the shorter inner.

Pressure plate was different too... something needs to compensate so pushrod works and I think Yamaha did that with the pressure plate. 

I've run a 125 with the 175 top end and didn't have a clutch issue, only did the clutch swap once on a 78 and simply used a complete 175 clutch.  (I'm pretty sure that the YZ125 used the 5 plate design until at least 1976 so it holds up fine with more power)

On the pistons/rings...  stop with the ring gap.  What is the piston to bore clearance at the skirt?  You can ballpark with feeler gauge. 

And stay away from the cheap Chinese piston kits...  might sound like a bargain but skimping may completely ruin the engine.  Here's a pic from Enduronut of a nearly new "bargain" piston...  both rings and piston were substandard and failed quickly doing lots of damage.





 

Does your cylinder have a wear lip at the top that can be felt?

Also, probably of more importance than outright wear figures is bore ovality. You can have a perfectly serviceable bore diameter, but if ovality is out of spec, you will get blowby. Lots of it because new rings will not be able to conform to the walls.

Max out of round spec for 175 is 0.01mm.
For comparison, the average diameter of human hair is considered to be 0.06mm.

Not to be in any way pouring doubt on your abilities, if you do not possess the equipment and experience required, you are best to take it to a MOTORCYCLE rebore specialist for a second opinion.

I took mine to a trusted local car and light commercial engineering company for a rebore and they completely trashed it.

I know that it is probably inevitable that it ends up at PJME.

The bore at present is straight with remnants of surface rust, that can't be felt with a fingernail.
Probing with a 60-70mm telescopic bore gauge it reads 66.4mm, at all angles and depths that I have tried so far.


I still need to sit down and clean it properly. It has been stored somewhere quite filthy.

I am exploring if I can get something that fits for much less than the price of a rebore.
It seems good enough that it might just work if I can figure out what size it is.

Its in better shape than my 125 bore... which has less rust stains but far more scoring from the last rebore that was DIY by the seller...

The pistons in question are Yambits supplied Kiwami standard and/or +0.5.
And a NOS 2A7 +0.25 piston on eBay.

Looking at that bore, it looks like it has been bored, but not yet honed to size.  That would explain the discrepency.

Getting a piston.  Of course the piston is required for honing to size.  + 0.5.  

You do not mention having a cylinder head.  Be aware that the pistons, rings and heads are different on the 80s 175s.
Presumably they made revisions to the squish area.  You cannot mix and match.  If you go for a 2A7 or 2N5 piston, it is my belief that you are stuck with a earlier head and visa versa.

A 3J1 piston must have a 3J1 head.  A 2N5 piston....

That might explain it.
As to why it was half done and shelved to rust for a bit...

I do have a 2A7 cylinder head. I got one of those 6 months ago. The barrel was much harder to find.

I appreciate all of the help so far.

Dug through the clutch box and sure enough the 2A6 hub is 5mm shorter 
photos.app.goo.gl/pg8a9KgPAf8cqT3S8

Short update.
+0.5 rings measure exactly 0.3mm end gap. Textbook spec I believe.
So I am thinking of putting in +0.5 piston and measuring again. Then if still good, going with it.
(On a side note, is Kiwami decent enough? Never heard of them, but they claim to be Japanese.)

The clutch seems to be tall enough to accommodate 6 plates. Both inner and outer components. The spring studs are flush, not protruding like the shorter one.
The 2A6 plate even fits on. But it lacks room on the push rod adjuster as expected. It will adjust to spec, but has a turn of thread hanging off which is no go.
A 1W1 plate has been acquired, however. It seems to be the same thickness but with a deeper recess in the middle. About to find out if it fits on the rest of the clutch.

Do not fit a piston in that bore. It is not a finished surface.