Sounds like an expander...  yes stock.  But shouldn't be flat. 

Ok why is mine flat? It is so flat that It is almost invisible. Would be curious to see what it should look like.

Wonder if a ring set or second ring from wiseco or someone has been used & the ring is deeper in the groove, or equal to the groove depth & flattened the expander which measures next to nothing when flattened. Someone must've had fun getting the cylinder over the ring if that's the case.

Finally got the damn ring off without too much violence. Amazingly the rings "looks" straight and the ends still line up. Was some corrosion in there which mostly wiped off but was holding the 2nd half of the ring on there tightly. Used more heat, Blaster soak and a zip tie jammed underneath the ring to help it work out of the groove. Rubber hammer swatted the piston and rings and done.

Question now is if ring is reusable? Has an 8mm gap at rest. Also see numbers 75 and NR on the ring. What do those mean?

Thanks for all the replies.

Hate to burst your bubble… was wondering when you said it had a 1975 date on piston… 

What you have is a 3rd oversize piston and rings. I don’t know if it’s worth trying to reuse old parts or not.  

Ok that is good to know. Thought '8 5 75' was a manufacturer date. So does this look like a Yamaha piston based on numbers? Maybe will just get new rings because piston looks low mileage.

So what do you think was sticking it so tight. Me being me i'd probably slide it down the cylinder a bit in the position in line with where the ring peg was then hold it up to the light & see if you can see past it at any point. If so then replace it. I know ya don't want to spend money if you can get away with it. Looks like a Yam piston. I didn't realise 400's ran the L shape top ring. All good to know.

Yes, looks like Yamaha piston.

Ring grooves are critical so I'd inspect carefully.  Check with Enduronut on parts usually at best prices.  Not sure what new piston/rings would cost vs using old stuff. 

The ring seemed to be a really close fit in the groove and looks like what little space there was had just enough corrosion or carbon to glue it in place. Was just trying to remove without damaging anything. Once loosened and cleaned a little it would slide in and out alot easier.

Was hoping to clean the cylinder, piston and rings completely then reassemble to do a compression test. Then decide on a course of action from there. Piston and cylinder looked doable but rings were tight in piston. Compression seemed good as is but need to get a new gauge to test it. Seemed like a waste to replace everything if there is so much cross hatching still in the cylinder.

I'd toss that ring expander too as not any use really. Give it a run for a bit before comp test to let the ring get used to being back in there.