orings can be had at Mcmaster Carr as mentioned above if you are making your own, they are typically glued together with cyanoacrylate glue AKA, super glue, crazy glue ect. and as mentioned cut with razor blade,
looking at the plug it looks like aluminum stuck to it from burning a piston, hard to say without being there. good luck on the rebuild, seems like a straight forward fix and you'll be running in no time

Everybody better start talking about that piston and plug -- RIGHT NOW !!
I don't hear one peep about it here. Why is that ??
It's clearly telling a story -- at least to me it is.

Lets start at the beginning and Redskinman comments:
1) Pilot ran great for two days with no problems.
2) On my last ride of the day Friday I could tell it did not have the top end and the power that it previously had
3) Before I started tearing it apart I did a radiator pressure test to make sure the head gasket did not blow and it passed with flying colors.
4) For those of you that don’t know I am running a 400 cylinder with a sleeve from LA sleeve bored out to take CR 500 piston,currently running 89.50 mm

Here is how I see it:
1) Machine ran great for two days so there is no fn way there was anything wrong with that Engine before he left the house.
2/3) On the last day of riding he noticed a power drop and did a rad pressure test -- it passed he says.
He also did a compression test and of coarse it failed.
4) He is running a CR500 pilot -- that means this is a kick azz Engine and putting out some power.

Now lets look at that piston and sparkplug because there is a lot of info there.
There is only 5 things that would cause a piston and plug to look like that in my opinion.
1) timing
2) fuel additives
3) leaking coolant water into combustion chamber/ over heating
4) jetting and lean out
5) detonation

We can discount #1 because our machines are keyed and there is no adjustment on timing.
It's possible that #2 is relevant because we have no idea what kind of fuel he ran.
We can discount #3 because he said he did a pressure check and it passed. If he was going up a dune and not gaining but working the Engine hard I would say it over heated because no air flow through rad but that is not the case here as he said he was chasing others at full throttle.
It's possible that #4 is a culprit but it ran good for two days so I will take a pass on this one. Did you look at the carb and verify no jet came loose or fell out ??
It's a possibility that #5 could have happened because we don't know what kind of fuel he was running and secondly he was out there for three days which means he fueled up several times and may have put some shyte fuel in by accident.

It you look at that piston I see detonation signs. That piston got real hot and the aluminum started to melt and flake off and it deposited on the sparkplug. If he would have run it any longer it would have holed the piston. Detonation will also bust a ring, especially on a kick azz Engine.
Just my opinions CO

I was running av fuel that I bought before I left my house.No I have not examined carb

Then the way I see it right now is:
1) Jetting was off -- I doubt it and highly unlikely because it ran for two days, but possible
2) You developed an air leak -- check your seals and intake

It could have been detonation I guess, but it sure looks more like the top ring snagged the exhaust port as mentioned.

What was your squish band clearance?

How long has this Engine been running, years? Hours? It might have just been time for a new topend

Its too late now, but whenever I think I'm having running or fuel issues I always stop and run a flow test. All you need is a water bottle really and a new zip-tie for the fuel line at the carb.

Been running great since 2015,look below at the other related topics and you’ll see when hoser rebuilt it.

He did mention he changed pipe with out rejet. whether he put in new plug for trip or not no idea. At dunes there is a lot of WOT (Wide Open Throttle) ridding going on. No matter what it snaged a ring what caused it is the issue. What went first?

There is damage to the cylinder at the port and above the port. The cylinder should be able to be honed out. The port damge is what will depend if it need bored to clean up and re chamfer.

The lower exhaust port damage can be cleaned up pretty easy as long as it does allow for short cycle. (Exposed exhaust port at TDC (Top Dead Center))
The upper exhaust port lip will require a little more work to clean and radius out with chamfer.

Check your rings for a sharp edge in place of a rounded edge. That will tell you if ring twist was taking place.
since its a big bore maybe send to CW racing for proper fix

forgot, nice to see ya posting CO

Heck as much carbon build up as there was it may have locked the ring and snag. piece of carbon getting into ring area
pic of exhaust manafold please

Plus excess carbon can get hot and detonate the fuel charge.

I am still around and will comment once in awhile.
My money right now is on a blown crank seal or fuel pump not putting out enough fuel and it leaned out.
The piston got real hot and started to melt. It deposited this liquid aluminum on the sparkplug. See pics.
This deformed the ring land and stuck the ring. It then snagged the port and busted. I really don't think that ring broke first.
CO

some pics
plug heat range?
how long plug was run?

If that is the jetting it requires to run good then keep taps on carbon and keep it cleaned up. I have that type of set up before. The difference may be I run a hotter plug. Im not saying do it just mentioning it.

couple of pic from one of mine "rich"set up. took apart to clean up and check.

What you take from this the difference of the way the two look. yours and mine. Both rich or rich at one time. why do they have such a different colors to them. One running hot and one not? One went lean and the carbon was present prior.

read up on carbon build up and coolant leaks.

here is an excert from a web page about coolant leak effect on piston

The ring that broke in half is bent and a little twisted,the ring left in piston has sharp edges.
All the ports edges feel smooth except the bottom side of exhaust port.
It has a small sharp lip ,with the lip headed towards the exhaust exit.
You can probably see it in the pic where it’s shiny.
I don’t know how old the plug is, I can’t remember when the last time was I changed it,Its a br9es. But the pic you’re seeing of the plug is how it looks after I took it out and wiped it off a little when I did compression check,it looked worse when I pulled it out the first time

One more pic

-------------

Coolant leak was my first thought...

I had this same setup on my BAD500 - remember George is NOT running a CR500 cylinder.
It is a punched out FL400 cylinder with a CR500 sleeve, when you do this the cylinder wall is VERY thin...
Thus a very small area to keep head gasket or rubber rings in place.

I'm curious George was the coolant still topped off in the radiator when you did you pressure test ?

One other thing to note the two days prior it was rainy and much cooler than the last day he rode.
I could tell that my machines ran harder with the cooler air.

Also I have seen piston top coating kinda flake off some - From top piston picts I could not tell if it was excess carbon buildup or just the piston flaking...

When I rebuilt mine I had BORE TECH carbon bore process the cylinder and had the piston crown, head and exhaust port ceramic coated, piston to cylinder clearance was .0045, I also had the cool head cut to 60cc (I think 50 or 55cc was stock)

I ran it that way for 5 years before selling it...

Just my .02
Dennis

Everyone take a look at that plug pic on page 1 and that piston pic on page 1.
To me if an Engine was running rich then wouldn't the plug and piston be black ??
The plug and piston are ash white. That screams lean out to me.
Redskinman said it passed the rad pressure test "with flying colors" so we have to take his word on it.
What FL670R commented -- was coolant still topped off before test ??
Can we get a pic of your exhaust pipe inlet and outlet ??
I stand by my previous statement above -- lean out.

This could be another horror novel.
Think I will follow this one.
CO

Yes coolant was still topped off

Here's a pic of pipe,can you tell the lame man what this will help determine?
Ignore the wet looking spot on inlet of pipe,that was a bubble of radiator fluid I just wiped off,which one hr radiator fluid got on it because I just had pipe laying below cylinder,and when I pulled cylinder off it still had fluid left in it which spilled onto pipe.

Been thinking about it, I don’t know if this will muddle the equation or help solve the mystery. Since this buggy has aftermarket fuel tank that takes away the stock airbox I have made one out of a Tupperware basket, that also has a pre-filter on top of it .
First run of the day my airbox had the lid on it and ran great. The second time we went out, which was probably four hours later, I can’t remember, it ran like crap, took the lid off the airbox and it came to life and ran great again. The next three or four times out it slowly started running worse and worse till the last run,but it was ever so slightly.I would leave before the others because I wanted more time to get out there and let it warm up, it would not come up to speed for at least five minutes of me running it on the dunes,and even though it still seemed like it had the power and I was keeping up with everyone,at WOT (Wide Open Throttle) there was some slight sputtering that wasn't there before.
After the run that I finally noticed a big difference I came back to camp and removed the prefilter completely to see if that would help(prefilter was mounted directly on top of airbox,so now theres just a hole in airbox where prefilter was) so I head back to dunes like that,I was only out there for a couple minutes because On my way out the last two members of the party were coming in,and it did run better with prefilter off,but of course not as good as it had previously,It was after this run that I came in and check the compression

Also this pilot with exact same setup and jetting EXCEPT for the pipe survived 3 says of Lttle Sahara 3 years ago.

I don't remember if this was first or 2nd day but I know it wasn't the first run of the day because I had not hooked up gopro yet
I don't hear any tell tell signs but maybe someone else does

Ohhhhhhhh !!!
That answers it for me.
ANYTIME you get a backfire or sputter on a two stroke, get out of the fn throttle NOW !
I bet that fuel pump is not putting out, or you lost a seal.



If you look at the outlet pic of that pipe you can see that it is blackish.
That means it was running rich or good for most of its life.
Now look at the inlet pic of that pipe.
You can see that it is whitish.
That means it was running real lean in the last few minutes at the end of its life.


I may be off on my theories according to you guys but I know what I know because I have blown up many engines in my oddy life. I know what I saw.
Please feel free to shoot at me but then you must also explain your theories.
CO

Edit: your video don't work

Ok thanks for reply
The video is on YouTube,it works for me on my phone and on computer so I would say the problem is on your end,try watching on YouTube
https://www.youtube.com/watch?v=OAme_vSNrpc

That vid still didn't work for me so I went to your youboob site to view it.
I did here something at the 2:46 mark. But I don't really know what it means right now. It seemed to run fine after that though. Engine sounded good to me.
What an awesome place to ride. It looks dangerous to me. You could crest a hill at full speed and then there will be a 100' drop off that you can't see until it is to late .
I can only dream of holding full throttle that long where I ride. You would be real lucky to hold for 10 sec around here before you fly off a cliff or hit some trees.
CO

Video worked for me

I know one thing,I am Going to see if I can get the old pipe welded back up and stick it back on.
Pipe on the the bottom is the old one,The one on the top is the one I just took off.
CO I did Get off the gas when I heard the sputtering, but then I would get right back on it! Lol
It would not shock me but I would be a little surprised if they still went out since hoser just completely rebuilt this bottom end 4 years ago, I would say more likely the jetting was off caused by putting on the different pipe(If indeed this is what caused the mishap)