Top of 250R piston came apart, why?

[chevman51]

I bought this 250R broken, guy said head gasket is bad. I pulled the head off and a piece of the piston is gone. Looks like the chunk that came off is smashed against the cylinder head. What would make the piston come a part like that? Too hot? I was going to rebuild the top end, I have a spare cylinder and head and going to buy a new piston but wanted to make sure there was not other problems causing this. Any info is greatly appreciated!

Thanks Tony

[WilliamJ]

The usual cause is detonation. This is a very destructive event where the fuel ignites spontaneously before the spark occurs. It can be caused by bad fuel, overheating, lean combustion condition and bad ignition timing which leads to high combustion chamber temperatures.

Of course, if someone hit a mildly seized piston with a trying to free it off before running it again, it would probably crack. Perhaps be thankful it hasn’t collapsed, which leads almost instantly to total engine destruction. You may want to inspect the head and cylinder very closely for cracks and the rod for bending. They can easily bend if the piston hits something solid.
Bill

[Kenny_smallz]

Had same problem but my piston wasn't busted but cracked my head was peppered and the piston too well I found out my lower piston rod bearings come apart and torn everything up be careful and check everything good is there gold flakes in your motor?

[WilliamJ]

Maybe someone ran it without oil in the fuel? That is another possibility.
When things get that bad you probably need to find another motor.
Bill

[Kenny_smallz]

Check lower rod bearings mine did same

[chevman51]

Thanks guys!! That is a lot of great information. It kicks over smoothly. The tranny sifts in and out of the gears fine. I will inspect it a little more closely tonight. I will pull the head off and inspect the crank. I will also drop the oil too and see if there is any thing funny in that too. The guy I bought it from was pretty inexperienced to put it nicely about this. I would not put anything by him. Thanks again, I will update this when I find out more!

[RIDE-RED 250r]

Dropping the transmission oil won't tell you anything about the condition of the crankshaft and topend. That oil is only for the transmission and separate from the crankcase. The crankshaft and crank bearings get lubed by the oil in the premix fuel.

[chevman51]

I pulled the head off yesterday. I attached a picture of the piston. There is lot shavings around head and down the crank I see pieces too. Is there any way to get the pieces out without pulling the motor and splitting the cases? In response to Ride red, I was under the impression that the tranny oil was also oiling the crank. Thanks for your info guys!

[RIDE-RED 250r]

Nope, tranny oil ONLY lubes the transmission on a 2-stroke ATV/dirtbike engine.

Having a little trouble telling in your piston pic... which side of the piston has the damage, intake or exhaust??

Either way, if that were my engine, complete dis-assembly for inspection and repair of any further damage would be in order. Especially check the crank/rod bearings and the crank and rod themselves. Would be advisable to replace both. You can get a re-man crank for around $200, new crank bearings for around $50 or a little less. Full engine gasket set runs about $70. If you can do the work yourself, that and the cost of the new piston and bore-job is roughly what you are looking at.

There is alot of debris in that bottom end and it's very possible it could have suffered some trauma. I would be leery of just slapping a new topend on and running it without at least a thorough inspection. All that debris can easily make it's way into the lower rod bearing and if it did, that lower rod bearing won't last long. A rod/crank failure can easily ruin an entire crankcase which are quite expensive to replace.

FYI, I recently scored a freshly rebuilt crank for my R for $115 shipped to my door. There are some deals out there if you look around.

[fordnut79@hotma]

Looks like the exhaust side; looks like an extreme lean condition at WOT.

[RoscoW]

^^^^What he said, looks like it was running very lean and actually was hot enough to melt the aluminum piston. When that occurs all the little droplets of liquid aluminum swirl all thru the top end. You will have little aluminum balls in the rod bearings,crank bearings, bottom end, reeds, pipe etc. The only sure way to get it all out is a full tear down and rebuild with new bearings and seals.

Ross..

[chevman51]

Yes it is exhaust side for the piston. Thanks, this is very useful information. I really appreciate it.

[RIDE-RED 250r]

In a lean condition, the exhaust side of the piston is the first to fail because it gets the hottest.

Good luck with your overhaul and keep us posted..

[WilliamJ]

Yes it is heat but it is slightly more complex than that.
It is the lean condition which provokes detonation, and it is the detonation that causes all the pitting and cracking. If you had a lean mixture and adjusted the timing far enough back, then no detonation would occur and it would just be down on power. Standard air/fuel ratio is about 14 to 1 by weight. Most 2 strokes run at 12 to 1. A lot of modern cars run 17 or even 20 to one at certain throttle openings and engine loads. You try running a 250r at 17 to one and it will detonate. Another factor on a 2 stroke is the oil. If you have too much oil in the mix it will have a similar effect to lowering the octane, and your looking at detonation again. That is a good reason for running as little oil as the engine needs for lubrication. If it is a hot day the same thing applies. If your cooling system is not 100% same thing applies. If you have a high compression ratio it has a major effect as well.

Do a full bottom end rebuild or at least get someone who really knows these things to inspect it after you have stripped it out and thoroughly washed it. It must be so clean before you put it back together that it won’t leave a mark on a white glove.
It may be the light but that rod looks blue!!
Bill