am i doing the right thing?

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#1
2002 cr 250 melted piston

ok so here are some pictures of my cylinder and head and piston.





i gave it to a local shop to send off and re-nikkisil or what ever i had that option or add a sleeve. did i do the right thing? everything else looks fine like my crank and bottom end. what could cause this it looks like it was running lean i was in wet sand but only for 5 minutes? i have been running this way all summer so why now? also i have an fmf sst pipe stock silencer, v-force 3 reeds and stock jetting and im at around 4,800 feet and run premium pump gas at 40:1 with amsoil. i am planning on buying the pwk to re-jet my carb but even with the stock mikuni i should still be rich right? but this looks lean because it's melted and i deffinetly know that my pilot jet is rich.
 
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#2
I hope they sent it to us chrome! What are you doing to the head? Are you going to split the cases? There is a good chance that was caused by a bad right crank seal.
 
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#3
SST pipes need to be run with race gas. I don't know about 4800 feet, but at around 1000 feet 02 CR's running 91 octane with that pipe will detonate severely on the top end. Audible detonation is a bad thing.

How does your bottom end look? Locking up an engine can do damage to the crank bearings as well. Give the flywheel a shake, if it moves at all you need new crank bearings.

Getting the cylinder Nikasiled instead of sleeved is definately the better choice. NIkasil cylinders run cooler which keeps the piston crown cooler and prevents similar incidents in the future.
 
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#4
GM,where did you see signs of detonation? That looks like the worst bridge seizure I have seen! Detonation pits the piston and then starts melting the exhaust side of the piston off, or melts a hole in it. The sst,my favorite, does advance the timing on some models, not all. 4800 feet would need to be leaned out, from stock settings.
 
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#5
It definetly got hot at the exhaust bridge (of course it is about the hottest spot in the cylinder anyway) but what about the top edge of the piston being burned off or broken off at an angle? I don't think bridge clearence initiated this problem, but more over got hot and siezed due to what ever started happening above the ring grooves. IMHO

What does the plug look like?
 
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#6
The expanded bridge cracked the edge of the deck off and shoved it into the head. Worn ring, right seal, or bridge relief insufficient. Detonation does not crack parts off, it melts.
 

dirt bike dave

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#7
yamarider88 said:
i have been running this way all summer so why now?
Riding in wet sand is going to put a very heavy load on the engine. Sustained heavy load = lots more heat, so you would want more fuel and lubrication for this condition.

Also, the weather was probably cooler and dryer than in summer, and maybe you were having high barometric pressure. Cooler, more dense air = more oxygen, so you need more fuel and lubrication or you will be running lean.

Chances are your jetting was OK for summer, but you just did not have enough fuel/oil for the type of conditions you were riding, so you had a failure at a known hotspot.
 
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#8
alright sweet thanks guys for the answers i don't think it seized because it never completely stopped it quit idling right so i let it die and it kicked just fine just no compression. also there was a lot of oil build up around the cylinder to transmission area. the plug is a real crisp brown.