On the parts list for a 1968 YG5T piston and rings there are 2 listed, Keystone and I guess non Keystone. What are the differences and can you mix parts?

For sure keystone rings won't fit a non keystone piston. Might only be the top ring thats tapered but not sure where i'm heading but thinking if you had a non key piston maybe two sets of key ring might work if you used the second ring from both. Don't read too much into that statement. Someone will sort it or sort 'me' lol.--if, in fact that is what the difference in part numbers means? Anyone?.

If your current piston has a K stamped on the crown , you need Keystone rings .
That's assuming it's a Yamaha piston .
Mixing between the two types won't end well . Don't ask how I know .

Someplace I remember seeing a list of rings showing the two types and part numbers. I will see if I can find it.

Unsure of the part number for the standard rings for a keystone piston. Piston has a K stamped in it. Babbits online shows the part numbers but so many variants on different sites.

YG5T came with keystone rings standard according to a Yamaha doc I found.

Prefix = 180

Body = 11601 = both chrome plated

Body = 11610 = top ring chrome, second ring cast iron

Suffix = 00 = standard size
Suffix = 10 = first over
and so on... to 40

As far as installation, Keystone rings are stamped with letters/numbers near the end gap. Numbers go up. The ring with a "1" in the number (1N or 1T) is the top ring. The ring with the "2" in the number (2N or 2T) is second ring. Oversize, if applicable, is supposed to be stamped on the other side of the gap.

This came from a 1969 document so it's possible things changed since then. If this doesn't make sense, post up the numbers and maybe someone like Enduronut can help.

I ordered 180-11601-01 hoping they are Keystone.

Noticed the 80 cylinders from mid 60's have different part numbers first three digits 168 and 180, even pistons have the different numbers also. What do you suppose the difference is? Are they interchangeable?

My guess is yes? But I don't know for sure. Later models have different porting and something like pipe flange could be different.

Pistons might only be ring difference and/or possibly ring locating pin difference. I know that many, many years ago when I needed a piston for my 1964 MG1T as a kid, the dealer didn't have the exact part so they looked through the parts bins and found a piston kit with a different number from a later model that worked fine. Might have been a YG5 or GS6 piston that we put in a YG1 cylinder? Too long ago, just my guess. If I remember right, one of the parts guys said something about the newer model piston working in the older engine but not the other way around... which sounds like ring gap pin location to me.