Any guesses what broke my piston?

[junior636]

Heres a pic of my piston, I haven't pulled the cylinder yet because I haven't ordered a piston.

http://i120.photobucket.com/albums/o176/junior636_bucket/IMAG0222.jpg


You can see up in the top left at the valve relief that there is a small crack formed to the edge, then bottom left a small section missing at the valve relief, the finally at the bottom right a large chunk missing at the valve relief. My guess would be the valves hitting but my timing was right on? I don't see any marks where it looks like the valves were making contact...?

[junior636]

By the way it ran fine last time I rode it, I only tore it apart to fix the oil leak from the valve cover. I don't want to slap it back together and have the same problem but everything looks ok other than the piston, nothing wrong with the head or valves or anything.

[CJM]

Possible bad valves, but I dont see any damage there either. Valves and head look good?

Could be due to lack of oil, but that would cause catastrophic failure more likely.

Could be that the bore was worn out and the piston was rattling around in there

[junior636]

Yeah, I have thought all along those lines too but this has been one of the more quiet 400's I have heard, not much if any rattle. I guess I will just inspect everything very carefully when its going back together.

[xcracer416]

from the look of the picture i see vertical lines on the cylinder wall. how did it run when this happened?

my guess would be heat. the piston was actually coming apart when this happened from the looks of it. how many hrs are on the piston?

i would definatly check the cylinder for taper and out of round.

but then again if it was mine i would do a valve job and bore it so then i know the cylinder is squared away. it doesnt look like hd headstuds either. i would do that while i was in it b4 it was even thought about putting back together.

[xtreme762]

Detonation can cause this as well. Detonation can do much worse and much stranger things to pistons and the head if it gets REAL bad.

Just a guess though.

+1 to xcracer416's post above.

[dxcody]

Originally posted by xtreme762
Detonation can cause this as well. Detonation can do much worse and much stranger things to pistons and the head if it gets REAL bad.


I think your on the right track, since its been pretty hot outside, and that looks like a high compression, like 12:1?

Im going with pre det.

[junior636]

Maybe I had some bad fuel, I believe this is an 11:1 piston, the guy before me said he ran 93.

[cheater13]

I have a 12.5:1 piston and i run 93 which i been told i shouldn't because i should be running atleast 100 octane. I been running it for a year now with no problems but i'm hoping nothing does happen. I should of put a 11:1 piston in mine...

[dxcody]

Originally posted by cheater13
I have a 12.5:1 piston and i run 93 which i been told i shouldn't because i should be running atleast 100 octane. I been running it for a year now with no problems but i'm hoping nothing does happen. I should of put a 11:1 piston in mine...

I think your fine as long as your running a stage 2 cam.

I was gonna do a 12:1 + stage 2 cam, just to see if it would work... but then i got a good deal on a complete stock setup so i am gonna run that.. :grr:

I dont want a stocker motor...

[b0rn c0nfused]

Originally posted by dxcody

I dont want a stocker motor...
You need a stock motor..You blow all your built motors up :D

[dxcody]

Thats how us goons roll
:macho

[CJM]

Reason you can get away with pump gas on a higher comp motor is due to the actual compression your making./ Although the piston is advertised as 12.5:1 or whatever, your making less than that due to gasket thickness and cam duration.

See Here for More Info (http://www.exriders.com/vbb/showthread.php?s=&threadid=487458&highlight=high+compression)

Even so still not a good idea to run pump gas.

[Pittsburgh400ex]

i'd say 100% detonation. Thats looks almost identical to my old JE 12.5:1 kit i had from sparks. It was cracked on 3 places and the one side of the pistion wore against my cylinder, looked like it started melting. I ran pump in it, i knew i shoulda have been mixing in some 110 but i didn't, lasted me a week tops. Did you hold it wide open near or on the redline? I tink that's what finally cooked mine. It almost immediately started pouring blue smoke. I could hear what sounded like someone tapping my header, let off the gas and looked behind me and i bet i could see maybe 20-30 feet back, too much smoke.

[Pittsburgh400ex]

12.5:1 JE

[Pittsburgh400ex]

sorry they are just thumbs.

[xtreme762]

So, do ya think an 11:1 piston would be OK on high octane (93 or better) pump? Those pistons aren't cheap to say the least. The good ones aren't anyway.

[CJM]

I and another buddy of mine run 11:1 on 93 with no issues. Been thru 2 bores and only used 11:1 with no issues.

Proper jetting is key!

[Pittsburgh400ex]

Should be fine 11:1 I mix 50/50 pump and 110 but i know it's overkill. That JE piston scared me so i run better fuel. I have run out though and had to run 93, no knock or pimg to it that i noticed. I think compression ends up around 10.8:1 with the gasket on there. My 450 has 13:1 with 2 gaskets to run pump.

[xcracer416]

what bore are you going to have to go up to? was that the intake side of the piston with all the vertical lines? if so take a look at your intake boot to see if it is separating at the head. ive had two of mine do that and cause it to run lean and overheat.

it was crazy b/c a plug chop would reveal it to be spot on, but it was lean ennough to create a problem like you have there.

[junior636]

Well, I took the cylinder off tonight and took the piston off, it looks identical to the pics above. I can't upload pics right now but after taking the piston out I'm a little confused. The guy told me this motor was a 440 but the piston is an 88mm and the cylinder sleeve looks pretty thin. What do you think I have really? The piston was a Ross, is there any way to identify what ratio piston it was?

[junior636]

Got some pics uploaded, also forgot to mention that the cylinder measures 88.5mm. I havent even seen an 88mm piston available when looking.

http://i120.photobucket.com/albums/o176/junior636_bucket/IMAG0229.jpg

http://i120.photobucket.com/albums/o176/junior636_bucket/IMAG0230.jpg

http://i120.photobucket.com/albums/o176/junior636_bucket/IMAG0231.jpg

[junior636]

Well I guess after looking on ebay at pistons this is a 426 unless I have a stroker rod. Is .50mm too much clearance between the piston and cylinder wall?

[Pittsburgh400ex]

Wow, almost uncanny, that piston looks almsot the same as mine. 87 is 416 so 88 should be 426 yea. Never heard of Ross before. I'd take a guess and say that one you have is over 12:1. Blew apart like mine did. I run 11:1 426 now, no issues. If your sleeve isn't screwed up just polish it and slap a wiseco in there. What did your wristpin look like? Mine was all discolored black from heat.

[junior636]

Im pretty sure it got too hot, I went trail riding with my brother in law and wanted to be sure he couldnt keep up on his Ktm 450, I won but lost in the end since i killed my quad lol. When I was taking it apart you could smell the burnt oil....cylinder doesnt look bad at least.

[Pittsburgh400ex]

i could smell oil for a day or so but it wasn't smoking, didn't think much of it at the time. Looks like your rings held up better than mine, top of mine was all black What's a sleeve run? I dont think they are too bad, could press a new one in and just put a stock bore, high comp in it. When this one of mine goes that's what i'm doing, dont want a 440.

[xcracer416]

from the looks of that piston i would highly recommend a sleeve or cylinder. 426 is about the biggest bore on the stock sleeve. where did the spare chunks of piston end up?

[CJM]

Originally posted by xcracer416
from the looks of that piston i would highly recommend a sleeve or cylinder. 426 is about the biggest bore on the stock sleeve. where did the spare chunks of piston end up?

Agreed.

Id do a stock bore sleeve and hi comp 11:1 piston.