So a good friend @Jflori217 went to do some dyno work on his never run before (0.0hrs), 23' 300sx after he got his ECU remapped and discovered that the rear skirt of the piston was hitting the crank journals...
I'm not sure if this is related to the 23' 250sx, so I contacted a good friend that saves his top ends and he has done (2) and the bottom of his pistons look ok and not like they have hit anything.
The video shows the rear of the piston skirt piston hitting the journals, so 2mm was taken off the base of the skirt to clearance it. The edges were chamfered and cleaned of sharp edges and all looks good.
I have a working theory about the carnage that has been posted about the 300sx engine failures and why if this is a problem, why isn't every one doing it.
Production tolerances, some made it through, some didn't, some riders are slower and don't rev it hard and some do, causing it to smack the crank halves.
https://youtube.com/shorts/TUIOwW2Q6A8?feature=share
(won't let me share the video so have to copy and paste in a browser, sorry)
pics of the piston skirt finished mod ontop, bottom is stock with the small shiney area on the right side is where it was hitting.
Pretty sure this is why the engine are having such carnage...skirt smacks crank, cracks skirt, pieces break off, wedged around the motor, motor locks up, want to keep running, cranks snap, etc..I did not discover this myself, Jeremy and John Ross had discussed some stuff and it was discovered...I hope this helps people, check it out, inspect it and take 2mm off the base of the skirt at the rear and clean up the edges and you should be good to go.
https://www.youtube.com/@themotocrossmodchannel/videos
Yikes!
Couple of questions Jbone. How did he notice that the piston skirt was hitting the crank webs? Did he hear a noise with the engine running? Did he detect it when turning the flywheel at BDC and he felt it hitting? Just curious how he thought to look for it. I know I’ve worked on more than a few two strokes that the intake skirts would interfere it you held the piston vertical (or rotated back slightly) and moved the piston center forward. If I recall, the last generation CR250’s had minimal clearance between the skirt to crank webs at BDC.
I’m not doubting there was an issue, just curious what led him to find it on a brand new bike unless he was specifically looking for it. Also, that bike survived a dyno run at the factory already as all new bikes are dyno’d prior to being crated up. I’ve also watched a few friends do topends and I’ve seen them smack the intake skirt on the webs while trying to put the cylinder on. I also noticed similar marks on the intake skirt after I looked at it.
I'd like to see a video taken in through the reed cage area with the cylinder on to see what kind of clearance there is.
I think that Ross called him and somehow it got bright up and they checked it..maybe @Jflori217 can comment.
I am not sure if every bike they sell is put on a dyno before it’s crated, but this one was not run had smack marks on it.
So however it was run, it hit..also, this may seem not possible but the gap can get closer at higher rpm’s than lower..total side convo but maybe at slower rpm’s when it was run it kissed it, but under a load and wide open it cracks it, etc.
as for the intake reed area, 2mm did the trick so I would say it’s like 1mm or and 2mm fixes the clearance safely.
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Had the exact same thing happen on mine. It let loose at 12 hrs. After speaking with John Ross about it as he is far more familiar with the Austrian bikes then I am called it as well. Said it has been happening for years. Under high load rpm situations.
I also spoke to ktm about it. The first person acted oblivious about it then talked with another tech and admitted they have had issues with this for awhile.
I trimmed 2mm off the intake side of the skirt and having been running it hard for a few hours now and no issues and the motor actually seems quieter.
Wiseco is about to release there piston and they have confirmed it is 2mm shorter.
There is a thread about this over on ktmtalk as well.
Thank you for posting this!!
Do you know how to get the 300 remapped?
is it just me but this OEM piston on that picture looks like a poor-cheap quality?
That pictures are from my TC300 it’s a Wössner Piston with almost 40 Hours on it
Is Woesner forged?
Yes the Wössner is forged run them since 20 years without any Problem on all my Smokers
The piston skirt on the oem piston looks like the casting is off. Look at the two sides. The left is different than the right.
KTM had a recall for this issue back in 2018.
I had talked to John Ross as I was putting the reflashed ecu in my ktm300 and told him I was going to Dyno it. He immediately called me and informed me of this issue and he mentioned it’s been an issue ever since ktm introduced the counterbalance. So we pulled it apart and that’s what we found. I believe it becomes much more of an issue if you are a very fast rider and ring the bike out constantly. Speaking with dealerships they said the ones that are coming in grenaded almost always have a broken skirt on the backside of the piston. I haven’t run it other than on the Dyno but I feel more comfortable knowing this could very well be the issue. Fingers crossed
Looks like total crap
Stock pistons are cast Vertex correct? Usually Vertex is a good product, maybe some quality control issues? Wonder if Vertex is aware of these issues?
They are vertex. Hoping that the aftermarket companies take this into consideration or if ktm might have a revised piston under a new part number
Does anyone know if anyone is making an updated piston to correct this issue yet?
From what I’m told vertex is. They look to be sold out but ordering directly from them you will get the 2mm shorter. If you order from ktm they still have old stock so they are longer
Pit Row
Wossner makes a forged, heard Wiseco but haven't seen the part yet.
THIS
Wossner USA confirmed to me by email yesterday (quick response too) that they indeed have the corrected piston (forged). The problem is not so much the intake skirt length, but the thickness of the casting, which can allow crank contact on the inner part of the skirt. The Wossner is machined in that area to allow clearance. Ordered one yesterday and due here tomorrow. Seem like good folks.
Any update or pics of the new piston?
Wossner (forged) on left vs Vertex. They look the same externally, but you can see the huge internal differences.
Can you measure and compare intake skirt length and thickness?
The forging is machined several mm to provide clearance for the crank. The difference is plainly visible. Skirt length is same.
Yes, but the bottom of the skirt is what gets the closest to the crank. Picture of mine at BDC.
There's several possible measurements there, depending on exactly where you look. Hard for me to make out exactly what is what in your pic. Wossner skirt bottom appears about half as thick overall and doesn't have the thick stress risers, with the forged construction and machining. They assured me they were aware of the problem and their piston was designed to correct it.
Just measured it. Wossner intake skirt is almost exactly 2mm thick from the bottom of the skirt to the top of the intake port, all along the intake side of the piston.
Dyno sheet?
Did you ever install the Wossner?
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