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'01 CR250 Piston Exploded!

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...well, not exactly "exploded," but it definitely doesn't have the same mass as a new one! First off, my ’01 CR250 has been pampered, fine tuned and dialed in like a champ. 80 hours of operation (45 hours on this piston/rings) and never a fouled plug, only a slight bit of spooge on the silencer, and the thing chugs and rips like it should.

So, I had just started on a cold, semi-moist morning ride (~40 deg, ~4,500 feet elevation). I let the bike warm up for a few minutes and then started on the ride when about 2 minutes in, the bike shut off like a light switch. Luckily, I had friends and a rope with me, so a quick tow back to camp was in order. Once I got back, I took a look at the spark plug (figuring it was something electrical as the bike shut down like a light switch). Sure enough, the electrode gap was 0.0 mm! My first thought was that something broke inside the combustion chamber and smashed into the electrode, but as I peered into the cylinder through the spark plug hole, I didn't see anything unusual (like scoring/damage to the top of the piston). So, why the heck did the electrode get smashed? Hmmmm…. Well, I put a fresh spark plug back in, gas tank on, and with one swift kick, it fired right up! ***? Well, I let it idle for a bit, put my helmet on and went for a quick ride around the campground. It ran fine with the exception of a serious bog when cracking the throttle wide open. OK, so I knew something was wrong, so I parked the dirt scooter for the rest of the trip.

When I returned home, I tore the bike down and checked compression and was greeted with 150 psi. Nice! 45 hours on the top end and still have 150 psi… But ***? Something must have hit the spark plug. And the bike’s not running right.

So, off with the cylinder head and I noticed that the front edge of the piston was missing!

Piston2_2011.jpg

So, off with the cylinder and here is what I see:

Piston1_2011.jpg

Here are some details:

Wiseco piston/rings with 45 hours on them

Maxima Castor 927 @ 32:1 ratio

FMF Fatty pipe

V-Force Reeds

2000 Mikuni carb, jetted almost exactly to the FMF specs

The teardown showed no damage to the cylinder bore, cylinder head or any other internal parts. I haven’t found the “missing” piece of the piston yet. So what I surmised was that the edge of the piston broke of (for some reason) floated in the combustion chamber for a nanosecond, hit the spark plug and got spit out the exhaust port.

Question: Why the hell did the piston break like that? Has anyone else seen something like this before?

Edited by omega_jeff

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were you loosing coolant? there are 2 things that could cause that. in my opinion either

1) you had a head gasket leak develop on that side (not sure about that as youd think it would clean some of that carbon off, but then again, ive only seen that on cars. not sure if it reacts the same on a 2 stroke with all the oil in the fuel)

or

2) you had a very lean condition

im guessing you had a lean condition. you can see the failure happened on the exhaust side where the temperatures are of course going to be the highest. your piston wash doesnt look that bad though, maybe a bit lean.

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As far as I know, I didn't have a coolant leak. There was no white smoke coming out of the exhaust, and I hadn't lost any coolant as the level was 100% when I started the tear-down. Coolant = Water Wetter and Distilled Water at the Red Line prescribed ratio.

Exact Jetting Settings (from my log book):

Main: 178

Slow: 45

Needle: Stock

Needle Position: 1 position (clip closest to the top)

Air Screw: ~ 1.25 turns out

Edited by omega_jeff

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You said you had it pretty tuned in. What temp was it jetted for? If you jetted when it was say, 75 deg outside, then running at a 40 deg would definitely lean it out a bit. Especially if it was jetted on the lean side to begin with.

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Looks like the piston broke, not seeing any signs of seizing on the piston skirt. They do that some times.

The edge of the ring land causes a stress riser, and yes this is a common piston failure.

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ps. check the exhaust port is debured correctly. Could have caught the top of the piston. Check the top of the port carefully.

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You said you had it pretty tuned in. What temp was it jetted for? If you jetted when it was say, 75 deg outside, then running at a 40 deg would definitely lean it out a bit. Especially if it was jetted on the lean side to begin with.

True, though if your jetted for sea level, it would be fat at 4500 feet. Another potential factor, at 40 degrees is your dangerously close to where the 927 premix will stop being properly suspended in your fuel.

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Here's my question: I'm not overly familiar with the 92-01 cylinders--do they have an exhaust bridge? If so your piston has not been drilled for relief and this meltdown looks to be a result of that. Wiseco makes a good product however forged pistons require extra attention compared to cast when fitting them into a 2T cylinder.

If your cylinder doesn't have a bridge throw everything I've said out the window :busted:

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The ring land broke due to detonation. Use better fuel next time.

This is why I don't use pump gas in any of my 2-strokes.

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That's a pretty clean break for detonation and usually you would see more damage since the small pieces that break off normally get into the cylinder and take out the piston and cylinder. I would suspect something in your bottom end like a rod bearing etc. is gone which caused the piston to smack the plug and the piston hit the top end and broke at it's weakest point.

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Good ideas...keep 'em coming!!!

Here are answers to some of the other questions/comments:

* The bike was jetted when temps were between 60 to 70F at an elevation of roughly 0 - 500 feet.

* I inspected that area of the cylinder pretty closely for any damage, and I neither saw nor felt any burrs on the exhaust port. If there were burrs, I would assume the failure would have happened on my first piston, or at least a bit earlier on this (the second) piston.

No exhaust bridge on my cylinder, so no need to drill the piston.

Fuel = 87 octane + octane booster at the prescribe ratio to generate ~ 100 octane.

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Good ideas...keep 'em coming!!!

Here are answers to some of the other questions/comments:

* The bike was jetted when temps were between 60 to 70F at an elevation of roughly 0 - 500 feet.

* I inspected that area of the cylinder pretty closely for any damage, and I neither saw nor felt any burrs on the exhaust port. If there were burrs, I would assume the failure would have happened on my first piston, or at least a bit earlier on this (the second) piston.

No exhaust bridge on my cylinder, so no need to drill the piston.

Fuel = 87 octane + octane booster at the prescribe ratio to generate ~ 100 octane.

I think that's a really bad plan. If you require better fuel than pump premium, your better off using avgas/race gas alone or mixed with pump gas.

I have a hard time beleiving that snake oil (probably toluene) raises the octane 13 points, and since your starting with a base fuel thats completely insufficient...

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Same exact thing happened to my RM250, not quite to the same extent as yours, but close. I was too lean. Also, it happened on about my 4th trip to the dunes, I'm not sure why nothing happened on the first three trips.

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Fuel = 87 octane + octane booster at the prescribe ratio to generate ~ 100 octane.

That's the cause of your piston failure. Detonation from insufficient fuel octane.

Octane booster points are actually measured in 0.1 points, so if you added enough booster to raise your 87 octane fuel 13 points, you ended up with 88.3 octane gasoline. If you are using pump gas, always use the highest grade gasoline at the pump.

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That's the cause of your piston failure. Detonation from insufficient fuel octane.

Octane booster points are actually measured in 0.1 points, so if you added enough booster to raise your 87 octane fuel 13 points, you ended up with 88.3 octane gasoline. If you are using pump gas, always use the highest grade gasoline at the pump.

So, when I was running straight pump gas, I heard knocking/pinging as clear as a bell. When I added the octane boost, the knocking/pinging clearly when away. But you still think it was detonation? I'm not so sure I buy it, but I think I'll step up my game anyways and run a higher octane fuel w/o the snake oil octane boost! :busted:

Does this change anyone's opinion of my problem? This is (obviously) the underside of the piston with the exhaust-side of the piston at the bottom of the picture. What I believe this is telling me is that the exhaust gas temp was much higher than the intake side causing oil burn on the under-side of the piston. So, that side of the piston was "really hot." My thought is this: Because of an extra lean condition, could the piston of heated up much quicker than the cylinder (especially on the exhaust port side) causing the piston to expand at a much faster rate than the cylinder (because the outside temp was 32 degF, thereby causing the coolant to be 32 degF to start with and keeping the jug too cool, too long)? So, because the piston expanded faster than the cylinder, it snagged on the exhaust port and broke? Now, I didn't see any damage on the exhaust port of the cylinder, so I kind of debunked this theory, but it's just an idea...what do you think?

Piston3_2011.jpg

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im looking at the back of a 927 container as i type this, be careful running that stuff at 32*f. the bottle says it can separate at temperatures <35*f.

that piston got hot man.

lame: sorry had to edit my post. had a nice reply coming and then remembered you were at 4500ft. at that altitude and temp you should be closer to a 172 main with the clip on 2. i dont know how hard you ride your bike, but if you run it WOT a lot, then an overly rich main can compensate for a lean needle as at WOT youre not really on the needle anymore.

i think you may have had a lean condition caused by an air leak. i mean a 178 should be pretty rich, and that piston shows HOT

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Those octane boosters are deceptive in that what they claim is decimal points, not whole points. It's cheaper to buy Av gas or race gas and use that to spike your pump gas.

Regardless, the cheapest octane rating per dollar is premium pump gas as a base. Using 93 octane instead of 87 is a 6 point bump for only another $0.30 per gallon.

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Noticed your running castor based oil and the piston has a fair amount of carbon build up on the crown and on the side near the ring lands. When you run a combustion chamber hard with the build up....the properties of the metel break down which probably cause your piston metel become brittle.

I run castor based oil in my vintage Bultaco 125 and is the best for not seizing up but must tear down after every run or the piston would weaken and break.

Your silencer screen is probably covered in hard carbon from the oil......try putting a propane torch to burn it off and you will notice the carbon start glowing and spark.

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So, when I was running straight pump gas, I heard knocking/pinging as clear as a bell. When I added the octane boost, the knocking/pinging clearly when away. But you still think it was detonation? I'm not so sure I buy it, but I think I'll step up my game anyways and run a higher octane fuel w/o the snake oil octane boost! :busted:

Yes, you detonated the piston due to insufficient octane. Detonation is the most common cause of broken ring lands.

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I run castor based oil in my vintage Bultaco 125 and is the best for not seizing up but must tear down after every run or the piston would weaken and break.

nonsense. why would tear down a bike after everyrun? its not even a rocket engine.

anyways,

did the OP wipe clean the piston on these pictures?

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