I just got done restoring my dt360, when I got it there was no speedometer so mileage is unknown, but it was standard bore, i threw a NOS piston at it with a light hone and it sounds pretty loud after the first ride.
Heres a little video of the sound, anyone have any thoughts on how loud a big bore 2 stroke top end is supposed to get before you should rebore? I shattered a piston once in my yz250 many years ago and it sure was a pain in the butt.

Mmmmmmphhh....hate to say it, but that's a loose one. At idle it sounded OK but revved and on decel not so much. I tend to push rattly pistons, as you do have a bit of room when you run the bike hard...and I've had a couple of pistons let go on me as well, it's interesting at 50 MPH let me tell you. IMHO, if you're going to ride it a fair amount, I would go oversize with a fresh bore.

I have same bike
2800 miles and a tight original bore and is sounds like this. And when I am hauling ass it sounds like a detonation popcorn maker. And timing and mixture no air leaks. So I would make sure oil pump is properly functioning and adjusted and ride your ass off!

And of course just my opinion!

Get a few compression readings hot and cold. At first I thought it was a rattle box. But as somebody else mentioned when the bike was revved up it seemed more solid? It's hard to tell from a video. The piston on that bike is extremely costly but you should pay close attention to it.

I take it you measured the clearances when you had the barrel off ?
Rather than using it just now,it's worth the effort of pulling the barrel and checking it out.
The time you have obviously spent sorting it ,it would be a shame to wreck all that hard work.
All you would really lose at this stage is some time and a few bucks for gaskets. If it shatters a piston you will boot yourself in the balls and say " i wish id done it".
Incidentally,that's what i would do as it would bug the living day lights out of me if i chanced it.
Regards Ted

Can I use this RT1 piston and drill two holes in the side of it?

You need to have a machine shop check it out before you buy more parts . My RT1 project looked good to me on standard bore but cylinder was getting ovalled. Took 2nd oversize to clean it up. I wouldn’t guess and re-do it a third time!

Took the jug to machinist today he said .25 might work but .5 would be best.
2nd oversize RT1 pistons are on ebay for 20 bucks, anyone know if this will work if I make a window in the side?

Bad idea. Very likely to develop cracks if you bore holes in skirt of cast piston.

Contact Enduronut for the correct piston/rings at a decent price.

Or if you want to go cheap, use RT1 piston with no holes (as long as it's not cheap aftermarket copy or questionable origin/quality). Reed valve allow for holes in skirt but not required... and not all reed valve bikes had the holes.

P.S. Piston might be $20... what about rings? Cheap piston without correct rings available at good price is like finding Part A Epoxy on sale for 10 cents... with Part B hard to find and at a price of $199.95... Part A is worthless without Part B just like a cheap piston is no bargain without correct rings available at a good price.

Thanks for the tips everyone! I contacted enduronut. I can get 1st or 2nd oversize rings for RT1 for around 8-10 dollars shipped.
So im looking at 35 dollars shipped for a 1st or 2nd oversize RT1 piston and ringset, Can anyone comment on the performance difference there would be between the two?
I am using a 32mm carb, stock ports, sleeper pipe.
Thanks!!