Broke piston skirt - 2017 KTM 300 XC-W

11/17/2019 9:57am Edited Date/Time 11/19/2019 5:36pm
My buddies bike just died when we were riding. Got it started but it had no power. Get it back home and look in the exhaust port. Here’s what we saw. What are the chances that the prices are NOT in the bottom of the engine? No clanking or grinding at all when the engine is running. Any chance the cylinder is ok? Haven’t had a chance to pull the head off yet


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11/17/2019 4:00pm
SLOWTIME wrote:
Rebuild.
Whole thing top to bottom? I pulled the head off and the cylinder looks perfect. Still has hatch marks
kb228
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11/17/2019 4:10pm
Yes rebuild top to bottom. You have aluminum pieces everywhere in there. Its going to act as sand paper until it wears out every bearing or worse causes even more catastrophic damage.
1
garagedog
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Fantasy
802nd
11/17/2019 5:17pm
You probably have some large pieces laying in the bottom of the crankcase and as mentioned by kb228, you have alum flakes all in your bearings too. That 17' is a nice bike so you'll want to rebuild it right.

Mic the cylinder to check to see if its in specs. If its not, send it off the USchrome or Millennial for a new coating.

I would replace the crank bearings and seals and check the lower con-rod bearing real close. A good chance you'll need to replace that if out of spec or now is not buttery smooth because of damage caused by the long lost skirt. A new KTM crank is pricey so just replacing the rod is a less expensive option with an aftermarket kit like ProX.

The Shop

YZed250
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11/18/2019 1:05pm
How many hours on that piston? I don't hear of too many stock pistons breaking the skirt. I'm thinking it's not an OEM piston?

And there might be more to this story than 'just wore out and broke'.

I know that there was some issue with Vertex KTM 300 pistons and their part numbers between the old and new gen KTMs … maybe the piston was the wrong one? There are a number of stories of the skirt hitting the crank webs when the wrong piston was used. That might have caused some cracking to the skirt?
11/18/2019 1:13pm
YZed250 wrote:
How many hours on that piston? I don't hear of too many stock pistons breaking the skirt. I'm thinking it's not an OEM piston? And there...
How many hours on that piston? I don't hear of too many stock pistons breaking the skirt. I'm thinking it's not an OEM piston?

And there might be more to this story than 'just wore out and broke'.

I know that there was some issue with Vertex KTM 300 pistons and their part numbers between the old and new gen KTMs … maybe the piston was the wrong one? There are a number of stories of the skirt hitting the crank webs when the wrong piston was used. That might have caused some cracking to the skirt?
The previous owner said it was replaced by Millenium at 55 hours. Has about 130 now.
SLOWTIME
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11/18/2019 2:46pm
There’s your answer.
4
YZed250
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11/18/2019 4:33pm
As others have said, split the cases, replace any worn parts, press a new rod onto the stock crank, new piston (vertex/OEM/Wiseco (my opinion)), replate cylinder (if its not thru the plating, it will be close to needing it), set x and z dimension when putting it back together and reface the cylinder head if there is any pitting.

If it were me, I'd also cryo the trans, vapor blast the cases, replace any shafts or bearings that were worn in the bottom end while I had it apart. Also service the starter motor, repack the silencer, replace the worn parts of the clutch.
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but that's just me wanting to do things the right way and not spend 25 minutes putting a Namura piston in it and crossing your fingers that it will go another 300 hours.

Expect a week of evenings in the garage and spend the money on the proper tools, please thank you, good luck!
Bearuno
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11/19/2019 8:54am Edited Date/Time 11/19/2019 8:57am
I'm not recomending not doing a full rebuild, But, sometimes you can be lucky:

A fair few years ago, at the A4DE, a mate of mine was on his TM300, when the fresh piston he'd put in, carked it on the second day. Luckily, they'd introduced a rule where you could keep riding, as a non finisher, if you could repair your bike.

Well, the barrel and head were trashed, and the crank and rod was locking, with alloy and barrel coating particles, but, as he really wanted to keep going, I said, " let's have a go at flushing the cases ". We didn't have the time ( or weren't allowed to? - I can't recall) to pull the engine, so, there we were, two blokes filling the bottom end with petrol, picking the bike up, jiggling it around, then tipping it upside down, rotating the crank and swinging the conrod, over catch tubs on 'enviro mats', with officials making sure there were no spillages. How many times we did that - 20/30? - until the crank spun freely and cleanly, and, the same with the big end. Got a 250 top end from the team, got it going, he did the next 2 days, and, got second in the "non finishers" final moto, which had a Lot of riders in it.

We checked the bottom end after the event, and everything was working perfectly. We ran the 250 top end until a 300 top end was available - we just hit a time when there was no stock in OZ - the OZ distributors generally had great parts availability, better than the big distributors most of the time. The bike ran perfectly for the rest of the year after that, with a few ring and piston changes. He's an arsey bastard Woohoo - even in his bad luck, he has better luck than most people.

Once again, I'm not recomending this at all, just showing how flukey things can be.

kb228
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11/19/2019 9:34am
YZed250 wrote:
How many hours on that piston? I don't hear of too many stock pistons breaking the skirt. I'm thinking it's not an OEM piston? And there...
How many hours on that piston? I don't hear of too many stock pistons breaking the skirt. I'm thinking it's not an OEM piston?

And there might be more to this story than 'just wore out and broke'.

I know that there was some issue with Vertex KTM 300 pistons and their part numbers between the old and new gen KTMs … maybe the piston was the wrong one? There are a number of stories of the skirt hitting the crank webs when the wrong piston was used. That might have caused some cracking to the skirt?
The previous owner said it was replaced by Millenium at 55 hours. Has about 130 now.
40-50 hours max on a 250+ cc 2 stroke piston man.

Thats why it broke.
1
ctb108
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Austin, TX US
11/19/2019 12:15pm
I know several people that push these 300s well over the hourly range you are in. People that think you need to change pistons every 40 hours on a modern woods bike have never owned and worked on one themselves. That being said you would need to ensure it's properly installed oem quality parts. Stick with vertex piston (oem) and full synthetic pre-mix.

Anytime an engine fails, I want to know why. To me it looks like there could be a lean detonation issue going on. This year range ktm is notorious for wet side crank seal failure. Take your time and do things right.

More photos needed to property diagnose.
1
kb228
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11/19/2019 5:36pm
ctb108 wrote:
I know several people that push these 300s well over the hourly range you are in. People that think you need to change pistons every 40...
I know several people that push these 300s well over the hourly range you are in. People that think you need to change pistons every 40 hours on a modern woods bike have never owned and worked on one themselves. That being said you would need to ensure it's properly installed oem quality parts. Stick with vertex piston (oem) and full synthetic pre-mix.

Anytime an engine fails, I want to know why. To me it looks like there could be a lean detonation issue going on. This year range ktm is notorious for wet side crank seal failure. Take your time and do things right.

More photos needed to property diagnose.
Theres tons of people who get more life out of their engines yes. That doesnt mean its the rule for everyone. After 40-50 hours your luck goes way down and thats a fact. Esp with cheap cast pistons.
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