Interesting topic...New Yamaha oversize ring end gap. I had a well known machine shop do a Yamaha cylinder up 2 sizes and when just for the heck of it I measured the end gap when assembling and was at or near max tolerance! Although I never said anything to the shop I was kinda honked off. But, many miles later no issues as Mark T suggested. But I've never abused it either.

Put it together & she'll be fine. End gap isn't important within reason. It's how it seals at the ports where the power is lost. On my "86 KX125 i had from new, i soon knew when rings were needed as lost its sharp edge. Every time i could slide the old rings down the Nikasil bore & see past them in line with the ports. New rings bought it back to life big time. To add one more, on my RM125S 1975 the ring peg was the full size of the ring land & that's how it was manufactured with original single ring piston. The ring was gapped to the peg, so if you slid it in the bore it had ring peg thickness gap+. But was a perfect circle past the ports. Thought of one more that only shows me up--but-- on my yz465 motor in my IT250K or L it was getting puffed on fast tracks when hot & could feel it had gone soft on the kickstart. It was on first over but i only had a New set of Std rings. So in they went & never looked back--rocket ship. Sorry--gettin carried away there---or should be!!.

after discovering a too small main jet that likely contributed to the meltdown I cannot assume anything is "ok"
therefore to demonstrate just how little I know, what is the proper placement of the piston rings? e.g. where does the dykes ring goes, which way is up? which ring does the expander go under?

thank you

Keystone ring goes in top groove with lip facing up, very top of piston is smaller diameter to allow for the lip . Expander ring goes in bottom groove.

thanks Jack, getting close to final assembly and pressure testing hopefully this coming week.

got to get carb jetting set properly so I don't melt again.

big lesson learned here is to never assume that a very old bike hasn't been modified along the way, I really screwed up by not inspecting jet sizes against manual when I cleaned it up. On the hunt for my next 2 stroke project!

Pressure testing. You probably know this but just in case you don't, seals can be blown out very easily with too much pressure. If I hadn't read otherwise I'd have used my air compressor which would have blown out my seals in about .00001 second. I put together a homemade tester using a blood pressure bulb and hose and made an adapter out of an old sparkplug. Going to check mine at 7lbs. Still have to come up with plugs for exhaust and intake, but shouldn't be a problem.
Are you going to run a vacuum check on it as well? Mark and others here recommend it. I'm going to.

Yes - absolutely
I was just wondering how much pressure and vacuum to use to validate that crankcase is hermetically sealed without causing damage.

If you have these values I would be grateful for you sharing.

This time around I check every detail. I am a 55 yr old fat ass who just wants to go off-roading at a low pace so I don’t accelerate my demise!

Thanks again Jack and everyone, I love this website cause no snarky pricks criticizing low knowledge highly capable types such as me!

Lot's of opinions on how much pressure to use but from what I've read, most go with between 5 and 7 lbs on pressure test, I've read some go as high as 9 but no point in pushing your luck. I'm planning to go with 7lbs but then again, 5 would be safer so.. I may chicken out and only go with 5. I'm going with 5 inches on the vacuum.

I use 5 psi pressure and 10-15 in hg vacuum.

I always check pressure first. With soapy water it's usually not too difficult to achieve a virtual "zero" leakage. Maybe 0.5 psi loss over several hours. Make sure to check your test rig hoses and connections for leaks too!

The big bore decompression valves can be a pain but it's still possible to achieve a good seal. Leaks tend to get worse, not better over time and with heat vibration. So zero is good goal.

Once you have zero pressure leaks, any vacuum leak pretty much has to be a seal. Packing thick grease around the shaft/seal lip will tell you which crank seal is leaking and how much.

Maximum leakage you can allow is one psi/minute or two inches hg/min... those are still bad leaks but engine will likely run okay.

Thanks
Between my mitivac and German brake bleeder and a few odds and ends I can do this.

I got some 1/4” rubber left over from my ‘68 Honda ct 90 trail project that I bought to fabricate a $60 handlebar damper from that I can use to block intake and exhaust.