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Just picked up a clean one with 10 hours on the clock. It has a KTM cylinder and not the Kymco cylinder. Hard to decipher through posts here if the ktm piston was the issue or the cylinder was. Should i just play it safe and put a Wossner piston in?
I put 100hrs on mine before I serviced the top end…..when my bike was new and I used it on the aggressive map, I felt some detonation in deep sand… I started running 25%C12 with pump gas ….probably not needed for woods riding …. See thread “ 2023 ktm 300sx potential issue” ….
As I recall it was the OE piston’s skirt being a fuzz too long, and hitting the crank under high rpm’s and/or load. I believe the aftermarket piston mfrs shortened the skirt slightly. If it were mine I’d throw one in just to be safe.
The issue is said to with riders over-reving.
The piston hits the crank but it’s the rider. Got it.
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I have never believed the piston hitting the crank is the issue. I don't see how it could run 10 minutes that way let alone 10 hours before grenading. Does the piston break and then hit the crank? Yeah probably like every other bike does if the piston comes apart. I have seen a couple pictures of pistons that are not broken with signs of pretty severe detonation. Run one like that too long, and I bet the piston will not do well. It seems like some combination of fuel pump/voltage regulator issues will cause them to run too rich or too lean. I wonder if that is the real issue.
Well about all I can say is you can't over rev a 2 stroke and piston to crank contact cannot be a good.thing.
Hey, we're talking KTM here. They ignore these issues, so if there wasn't much of an issue before the SX(track bike), Therefore it's MX bros pinning it to win it. KTM logic, get on board. \s
Here is something I found interesting. The TBI 250s (sx/xc) use the same crank as the older XC/XCW. Pistons are the same too from what I have read. The 2022 SX has a different crank than the 2022 XC/XCW. We should have seen 250/300 XCs blowing up for years now with crank to piston contact if the crank or piston was the issue. It would not surprise me if a forged piston fixes the issue because they are stronger than cast.
Any piston to crank contact will be loud and quickly catastrophic. To say a bike ran fine for 10-20 hours and then the piston hit the crank with no warning? I think there is a different cause of failure. Could a bearing or rod go bad and then things hit? Yeah. Could there be a fuel delivery problem that causes detonation and/or excess heat that weakens the piston? Yeah. This is my theory. We know there have been bad voltage regulators that cause fueling problems. A weak fuel pump might also cause a bike to run lean.
Piston to crank contact is not something new or unique to this bike. You would be surprised to find sometimes they never fail.
I typed this out before in another thread.... there is a difference between static clearance and dynamic clearance that you are ignoring. Just because the piston clears when all parts are brand new and cold does not mean it will maintain that same clearance at 9,000rpm and 450 degrees when it is 45 hrs worn.
There are some other factors involved here. The TBI bikes have a rev limiter. The previous carb bikes did not. What harmonics or vibration are induced when the rev limiter kicks in? Another factor.. the OEM pistons have more piston to cylinder clearance than the aftermarket pistons do. This will allow more piston rock in the cylinder, taking up more of the crank clearance in extreme situations. Maybe putting in an aftermarket piston has as much or more to do with cylinder clearance than it being forged.
Some good info but either way got it. I am putting a new Wossner piston in it.
Can you post up some measurements of the Wossner compared to stock?
Just trimming the stocker is all that is needed.
I think Pinnit Racing shows them having the same measurement from wrist pin to piston bottom. Maybe the tighter fit prevents the rocking at BDC.
When it gets hot the metal stretches
Was a issue with the prior motor and has been documented with pics on numerous forums, same piston as the tbi. It’s just more prevalent now that more are being rode mx with the 300sx.
BTW, heat issues close to the PV is resolved by using a 1/4" drill bit on the left coolant passage.
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