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09 300xc piston crack

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So I was out for a ride after messing with the jetting to no end since I bought the bike. It was running good but smoking terribly. Did a big climb and the bike didn't want to idle and the motor would roll over just sitting on a small incline. Said F it and cruised it back to the truck. Pulled the jug/piston and the cyl walls are not scored in the least but there is zero crosshatch left. I'm going to do a replate. However the piston had a small crack down on the skirt. Wondering why it would crack there? The rod has a lot of side to side movement but I can't really feel any up/down play. Am I doing a top AND bottom end?IMG_0221.JPG

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Its not a manufacturing defect on a eight year old bike. Its normal wear and tear.  As the piston wears, it gets smaller and can rock in the cylinder

Check the clearances. Replace the piston, rings, circlips, and top end bearing. If you don't know how many hours are on the engine, replace the bottom end too.

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GENERALLY the damage is caused by too small a piston in a worn bore

Edited by 7Tktm
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No idea what the hrs are on the bike. Easily 250hrs. It's not the original piston. The guy I bought it from said he put rings in it 20hrs before I bought it. I'm not buying that. Was just wondering what would cause the skirt to crack but rocking in the bore will do that I suppose? It's going to be $700 for a replate and new piston/rings/Conrod bearing up here in Canada. Wish I had done it when I bought it since the weather is just getting good for riding all winter. Only maybe got 6hrs on it since I bought it in October. Oh well lesson learned.

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If you want to avoid such difficulties and have maximal life in the future, I would suggest this:

a. Run a forged piston, as forging is stronger than cast.  It is common for cast pistons to crack at high hours.

b. Buy the piston first and send it with your cylinder.  A reputable re-plater will match the cylinder to the piston you send.  Make sure you set the ring gap per the PISTON MANUFACTURER'S instructions. 

c. run a reasonable oil ratio (KTM guys seem adverse to oil, but 40:1 should be fine)

d. when you get the cylinder back, make sure you completely clean all parts (full wash down in sinK)

Edited by rpt50
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Hmmm, If you don't want to lose 3 weeks of riding a new piston and gasket kit is only $150 and you are running. If the compression is still low, you can plan a replate over winter.

Forged are reputedly stronger, but also more prone to cold seize and scuff. Standard KTM pistons work great for enduro riding. Consider forged only if you are MX racing and even then I'd look at Wossner. I have run both over the years. 

 

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