Piston size

[hondaREX]

I pulled my top end apart to check why i had very little compression. Well i found why. There was a hole in the top of the piston!!! and cracks going 3 ways from it. I could never see it through the plug hole so it went undiagnosed for awhile. My question is, what size is it? Or actually, just to verify my findings. And also where to go from here. Top of piston has number 200. The underside of the piston has HM25084 JAPAN and PROX. Looking on PRO-X website looks like its a 68MM 86' piston. Which if my memory is correct is what i remember ordering years back. It is a 87-89 long rod crank so an 86' piston makes sense as well. PRO-X number is 01.1309.200 for an 86' piston set. The cylinder looks great and has a KRANK IT racing port job. Calipers at 68.02 but thats just a caliper. And the hole in the piston looks like it was detonating for awhile and kinda wore or burned the hole through and in my opinion looks like no "shrapnel" came "out of" the hole. Head has some marks in it but they could have been there before this problem. Should i throw a 68MM kit at it and get it running? Maybe hone it to freshen it up? Should i be concerned with the crank bearings? No grinding or noise while spinning the crank by hand with the rod. Thats not to say there isnt stuff in them. Also, washed the crank out to remove what might be in there. And last how many bores are left on this cylinder and what does 68MM work out to in CC's? Any and all advice is appreciated

[C-LEIGH RACING]

Where is that hole located, on the edge next to the exhaust port or right in the middle.
By you saying you see .200 on the top, kinda leads to think it is on the edge.

If on the edge, several things can cause it to happen there, but for the most part it is high heat, hot enough to melt the piston.

Some things that can cause it,
Ignition timing set to high,

Octane of gas to low for the amount of compression the engine has,

Pipe not sealed around the exhaust flange & raw air being sucked back in along with the returning pulse wave,

Cylinder base gasket, crankcase center gasket or seals leaking & raw air being sucked in, but normally it will seize the piston if that should happen, could burn a hole in the piston from running lean though.

Cooling system not cooling the engine good enough & running the cylinder hot.
Neil

Which ever it was caused it to happen, if the hole is completely through the piston, that means that melted aluminum has been spit & splattered all down in & around the crankshaft, getting into the bearings & you would be taking a big chance just boring the cylinder & installing a new piston kit, because that aluminum from the piston will probably come out & chew up another piston.
Neil

[hondaREX]

The hole is located just off center, still in the middle of the piston tho. At i believe it is left off center side if that makes sense. Not sure about the . in .200 but def 200. And the hole is larger on the top with barely a hole on the underside. At least a third of the size. Tapered. I had an aftermarket ignition on it when it happened so im assuming that contributed to it. Any info on the size of the piston to verify the numbers?

[C-LEIGH RACING]

.200 on the top would pretty much say it is a 68.00mm & the fact it is only those three numbers, you could just about say it is a cast piston for sure, but, only way to tell the year model of the piston is to pull the cylinder.
If it was a Wiseco or Wossnor piston, it would have a long number right in the middle on top.
Neil

[hondaREX]

Cylinder is out. Underside of the piston says HM25084 JAPAN and PROX. Pro-x site says 86.

[C-LEIGH RACING]

There you go then, 85/86 TRX250R or CR250R piston kit.
If the bore dont look messed up, you might could just put the same size new piston back in, but get the bore measured real close first to see how much it is worn or out of round so it will be a good tight fit.

I know it will be tuff going through it, pulling that bottom end apart, but do you want to take a chance by just washing out the bottom end & thinking it is clean, only to find out in a short while of running, that melted aluminum that was hiding down in the case has ate up that new piston you just put in.
Seen it happen many times to guys & even to myself many years ago, but buying new pistons & bore jobs learned me what to do when I get a messed up piston.
Neil

[hondaREX]

Thanks Neil. Do you think there is more of a concern of the shrapnel ruining a piston or the crank and rod bearings or all of the above? Will the stroke just pull the stuff into the cylinder/piston area? I may just swap in another cylinder i have from my ATC and worry about this one later. Or should i be worried about this crank also and not take a chance on that as well? I have no problem with a lower end tear down but def dont want to if i can avoid it. If i tear it down, just clean everything better while its all out and easier to thoroughly clean?

[C-LEIGH RACING]

Well, I would like to think none of the melted piston went down into the case, just got blown out the exhaust, but chances are a good amount did & probably hiding behind those main bearings or in that rod bearing.

If any did go down & behind those bearings, then how did it get behind the bearings, how much went down through the oil hole or how much went through the side of the bearings themself & has already damaged the bearings.

I've seen lots of guys wash out the bottom end & keep on going, I guess its up to you how much you got to depend on the engine to live.

Myself, I would like to take the cheap route & just wash it out, but I know from my years it will bite me in the end if not taken care of.

If your just going to use another cylinder & run it as is for now, I would say wash it out good & bolt it on & ride.
Neil