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I put a 64mm Pro x piston in it. The piston measured 63.58 at the top and 63.92 at the bottom. Pro X says it needs .06-.07mm clearance. Is it the skirt diameter that needs to be within clearance or another number?
I guess I figured it was still pretty close to stock bore so the a new piston should fit. I guess next time I need to be more careful with measurements.

I agree, that is what it seems like.
I measured it prior to honing, and got pretty consistently 63.95-63.98. I remember measuring after honing it and getting similar results (I didn't write them down, just compared). I just went through and re measured the bore, and again consistently got 63.96-63.98 in both directions.
Maybe there is some error in my measuring? I don't have an actual dial bore gage, so I'm using a cheap set of telescoping gages, and an ok set of digital calipers.

I've been having some problems with compression, and wanted to check that everything else is ok before I get the cylinders bored.
I started off with a little rust in one of the cylinders (previous post). I ended up honing the cylinders and replacing the pistons with new Pro X pistons. The hone did seem to clean up the cylinder walls nicely, and there was no indication of rust or pitting. When I test it now, I get slightly higher compression at about 100 (up from about 95 with the stock pistons/rings), but still not great.
When I measured the ring gap on the new piston rings, they were a bit larger than spec at Top: 0.43mm and Bottom: 0.48mm. I believe it's supposed to be .2 to .4mm. Would these being a bit over spec cause the low compression? Could it be anything else I should look for first? The top copper gasket is new and in good condition.
If it is the ring gap causing the low compression, what are the options? Could I put the next size rings on it (64.25), and file the ends to fit, or is it strictly a rebore?

Ya. That's what I was thought.
I ended up hitting it with a file and a little sandpaper to clean it up. I guess I will see how it works when I put it back together.
Attached are some before and after pictures. The before picture shows the worse of the wear.

Thanks. I took a quick look at Chariot and was only able to find the outer basket, not the inner.
I do have some small files I can hit it with.
Out of curiosity, how much wear is too much wear?

I'm in the middle of tearing down my whole engine, and inspecting things. When I took the clutch apart, I noticed a small amount of wear on the inner clutch boss (where the fingers of the steel plates grip the inner clutch). It doesn't seem like a ton of wear, but I can see it, and feel some of it when I run my finger over it. I know this can cause the clutch plates to stick and not function correctly.
Do I need to replace the clutch boss, or would it be ok for me to smooth out the marks with some fine sandpaper? I don't remember where, but I thought I heard someone say they fixed a similar issue smoothing out the wear marks with a dremel tool.
I can try to add a picture later if need be when I get home.

This is for a 75 RD350.

Thank you both.
The engine is currently torn apart so when I put it back together I'll get some cheap rings to put in it and see how it does.
After I know what I'm doing with the cylinder I'd like to throw some new pistons in it. Possibly pro x, but we will see.

I have a project basket case 75 RD350 I've been working on for a little bit. I recently pulled the top end apart and noticed a little bit of rust in the right-hand cylinder. There appears to be a little bit of pitting (highlighted in red).
I was able to measure the bore, and it came back at 63.98mm. I measured a few different places within the bore (both vertically and front to back/side to side) and it was all in spec.
After cleaning the cylinders, pistons and unsticking the rings, I did a compression test and got 95psi in both cylinders. I know that's still pretty low, but for an engine that hasn't run since 1998, I figured it wasn't too bad. Both the clean cylinder and the cylinder with the spot of rust tested the same.
I'm trying to figure out if I need to bore the cylinder out one, or if I can just hone the cylinder and replace the piston/rings. I would rather not spend a few hundred dollars to bore out the cylinders if I don't need to.
I'm wondering if anyone could offer some insight or advice on to bore or hone the cylinders.

Has there been any updates for this?
Have an old bike that needs a title, and don't know where to go anymore.