Blew up my 300, need some advice.

[Corban_White]

So on Friday I did this:









I think the cylinder looks good but I don't have much experience with Nikasil stuff.







It looks like all the broken pieces went through the bottom end and up through the transfer ports. I have cleaned them out and the crank feels smooth (I think?). So the question is, can I replace the piston and run it? Should I send the cylinder off to get recoated? Do I need to split the cases? Obviously I would prefer to throw a piston at it and go but I don't like to walk. I know there are those who would say full rebuild and start fresh but I don't want to do that just because. If it's needed then sure but I don't want to spend money needlessly and I would love to get back on the snow ASAP.

Secondly, what caused this? Was it simply hours on the piston? It is a vertex piston which I don't believe was stock. I haven't replaced it in the 2 years I've owned it and I don't put a ton of hours on-probably 25-30 per year with ~70% of those on snow. The top of the piston looks good and I feel that the jetting is pretty good.



What are the thoughts?

 

[Cascadia]

I don't know how to help but my last 300 had 180 hours without a rebuild and I rode it hard. So I can't imagine it's hours unless that piston already had a lot of hours on it when you got it.

 

[Rot Box]

Ouch!!!


To be honest I would split the cases. Even the smallest piece of aluminum can wipe out a crank or rod bearing in no time. At very least I would pop out the crank seals and flush the crank case housing thoroughly. If you remove the crank (I would) it would not be a bad idea to have the crank balanced. Crank/rod bearings and seals should be OEM quality or better. Lots of inferior parts out there.. :-\

As for the nicasil a lot of the time those scuff marks can be cleaned up BUT make sure you can't catch a fingernail anywhere in the bore. Take a cotton swab or cotton ball around the bore/ports to check for nicks or burrs. If there's discoloration anywhere it could be a sign the coating is wearing thin. Millennium offers an exchange program (send them your worn jug and they send you a fresh re-plated one) provided there is no porting which really speeds up the process. Fwiw I can't say enough good things about millennium they do outstanding machine work and likely the best coating on planet earth.

Hard to say why it blew. (thinking out loud) I wonder if the timber sled puts more strain on the engine. Detonation and/or excessive heat will quickly destroy cast pistons. If my memory serves, didn't some years of KTM 300's have detonation issues? Might be worth having the head reshaped if this is applicable to your year.

 

[Corban_White]

The marks in the bore are only visible, nothing can be felt. I will try the cotton swab method as well. I was planning on doing a flush of the case. The kit adds some load but it's mostly the chest deep snow that adds load. Imagine climbing sand mountain. All the time. That's what it's like riding off trail in the snow. As you can probably see, the broken part is exactly lined up with the exhaust port. Not sure that that matters, just more info.

 

[anderson750]

First things first. From what I can see, the root of your problem lies in detonation. All that pitting in the top of the piston is caused by detonation. The detonation will cause the combustion temps to go up which further causes a lean condition. I would venture to guess that when you let off the gas you were getting the good old "ring a ding ding" that is seen in hot, lean conditions. If you are not already, I would recommend running at least 50% mixture of VP110 and pump gas during the winter. I think the stress of the timbersled on the 2 stroke motor just showed itself.

If it was me, I would split the cases and replace the crank and main bearings. It may feel smooth right now, but it will not take long for small amounts of metal shavings to take their toll on them. As far as the cylinder goes, if there are not big gouges in it, you would probably be fine, but if it was me and I was this far into it, I would go ahead and make it completely fresh and have Millenium replate it.

As a side note, I saw this exact thing happen to 2 bikes (250sx, 300xc) 2 years ago at the Mesquite desert race where we were in bottomless sand and the bikes were worked hard. When you lose a piston skirt, it is almost always on the exhaust side due to higher temps since the intake side is cooled by incoming fuel and air.

 

[rholbrook]

When your piston comes apart, you really need to do the bottom end or you are just asking for another failure. Do it right the first time so you aren't doing it again. Have Lindsy Machine do the bottom end for you so its done quickly and then you can put your piston back in it.

 

[Rot Box]

Imagine climbing sand mountain. All the time. That's what it's like riding off trail in the snow.

As you can probably see, the broken part is exactly lined up with the exhaust port. Not sure that that matters, just more info.

It does, the exhaust side will always be hotter. In this case it may be worth using a Forged piston like Wiseco or Wossner (extra care is needed when sizing/fitting forged pistons fyi) and an aftermarket head/radiators for increased cooling. I think an inline thermostat is something to consider as well due to the extreme temperature fluctuations a snow bike will see. My LC4 has one I can get you the KTM pn if you'd like.

Definitely get a second opinion though I'm not super familiar with the 300's or snow bikes.

Fwiw my CR had major meltdown/broken piston issues before I had started using forged pistons and had head work done to help with detonation. It was/is a wot all the time sand bike.. with air cooling which only makes it worse.

 

[Corban_White]

First things first. From what I can see, the root of your problem lies in detonation. All that pitting in the top of the piston is caused by detonation. The detonation will cause the combustion temps to go up which further causes a lean condition. I would venture to guess that when you let off the gas you were getting the good old "ring a ding ding" that is seen in hot, lean conditions. If you are not already, I would recommend running at least 50% mixture of VP110 and pump gas during the winter. I think the stress of the timbersled on the 2 stroke motor just showed itself.

If it was me, I would split the cases and replace the crank and main bearings. It may feel smooth right now, but it will not take long for small amounts of metal shavings to take their toll on them. As far as the cylinder goes, if there are not big gouges in it, you would probably be fine, but if it was me and I was this far into it, I would go ahead and make it completely fresh and have Millenium replate it.

As a side note, I saw this exact thing happen to 2 bikes (250sx, 300xc) 2 years ago at the Mesquite desert race where we were in bottomless sand and the bikes were worked hard. When you lose a piston skirt, it is almost always on the exhaust side due to higher temps since the intake side is cooled by incoming fuel and air.

Thanks for chiming in. I appreciate your knowledge and experience with dirt bikes. So the pitting was not caused by the skirt pieces bouncing around on top of the piston? I've been a little concerned about detonation but I've never had the sound pointed out to me by someone who knows so I'm not sure what to listen for. I figured I would be all right with pump gas since I am at 8000-10000 feet all winter?

When it let go I had just started after having the bike parked for a few minutes. I started the engine and let it run for 20 seconds or so while reving it up and down slightly and then took off (this is what I normally do after stopping for a few min in the snow to help get the piston warm again before loading it). I only made it about 50 yards before it went.

 

[anderson750]

So the pitting was not caused by the skirt pieces bouncing around on top of the piston? I've been a little concerned about detonation but I've never had the sound pointed out to me by someone who knows so I'm not sure what to listen for. I figured I would be all right with pump gas since I am at 8000-10000 feet all winter?

I am sure the pieces did some damage but if you closely examine the outer edges of the piston, that is where you will see detonation showing up the most. It will also pit the cylinder wall at the top of the stroke if examined closely. The higher elevation makes it even more sensitive to detonation. There is a reason that aviation fuel is high octane. A quick google search came up with this on detonation. At 8-10K feet you are dealing with much less oxygen in your combustion.

 

[ricsrx]

Thanks to all of you, this great information, have i ever said how much i love RME4x4, no wonder i am addicted!

 

[rholbrook]

Corban, how hard was it to get off the trail once it went on you? I couldn't imagine it was very fun

 

[Corban_White]

Once we got it to a flat spot and could get it moving it wasn't too bad. The tow vehicle wouldn't get it started with a rider on it until we got it to a flat solid spot. We had to push it while towing until then. Once going it was pretty easy to tow. We tied from rear bumper to the frame of my bike just above the pipe, running the ropes through the forks.