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Is this amount of movement normal? Unfortunately you have to click on the link I’m sorry.

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No sir. The output shaft likely has notches worn into it.

How did this happen?

I can't see the video, but how tight is your chain??

Can't tell from the video, but it could be play in the sprocket itself, or notches in the output shaft. Either way, those parts are a toothed mesh, and under some pretty serious strain, so eventually something is going to wear. Not sure what year your bike is or how many miles it's been. If you pull the sprocket off you'll be able to tell where the play is.

IMO, this wear is due to a loose fit in the splines of the output shaft and sprocket.

The constant change in forces between drive and braking or deceleration causes the sprocket to rock back and forth on the output shaft. Eventually this rocking causes the worn splines you now have.

IMO, this is a poor design. In addition to the splines, I would prefer to see a bolt and washer, or nut and lock washer arrangement to hold the sprocket in place. Many other bikes use the bolt or nut to hold the sprocket in place, but I'm afraid we're stuck with what we have now.

Only thing I can think of to help stop this rocking action is to use blue Loctite # 242 or equivalent on the splines when assembling the sprocket on the shaft. To clean and prime the surfaces for the Loctite, I would give the parts a thorough cleaning in Acetone first.

So should I replace the output shaft as soon as possible?

I'm not able to see the video, so can't comment further.

New shaft, sprocket, and snap ring are best repair, although there was a recent thread about welding sprocket to shaft as a temporary measure until sprocket teeth wore out. You should be able to find that thread using the search function.

It would be nice to see a pic of the shaft splines with the sprocket removed before making any solid recommendations.

Could it possibly be the sprocket worn?

KTM fuel injected 2 stroke might be my next bike. I’m disappointed in this type of repair.

Have you taken the circlip off to further inspcect it?

I turned on flash to watch the video. I am not sure on the condition, from just that video.

Do you have another front sprocket?

Are you the original owner? Do you have any pictures of the chain tension?

If you can afford a $9,699 bike, then why not drop it off at a dealership and have them replace the parts if they need to be replaced, or just sell it and get what you can to put towards the new bike. Post pics when you bring home the new bike, please. Right now, you have posted one video and no pictures. Without me posting a Jerry Maguire "help me, help you" meme, I don't know what else anyone else can do for you since you are at the "I'm going to go buy a new KTM stage".

Alright here is some pictures of the shaft and sprocket as well as a new video. Also I took the stator cover off and noticed this.. how could this happen??

When I see this happen, I wonder about the chain tension. I have seen several dirt bikes with chain tension like a BMX bike. Never good, especially over time.

I have just changed the output shaft due to worn splines and previous to that I welded on the sprocket which works but needs to be done very carefully. I can tell you the amount of wear you have on those splines is not that much,mine only had half of the splines left. I wouldn't stress too much, I have a fix for you which is guaranteed to work. You need to order a kx500 front sprocket which is 2.7mm thicker than standard then you machine 2.7mm off of your spacer so you pick up the none worn splines(2.7mm is just from memory it might not be that exact number) the amount you machine off is very important as you need to create enough pressure when the snap ring goes on that the o rings seal otherwise you will have a serious leak

There's only one picture of the shaft and I couldn't play the video. Its looks like the groove for the circlip might not be exactly square on the edges but that picture doesn't look bad to me. The part of the shaft that the sprocket rides on looks just like the end of the shaft....that the sprocket doesn't ride on.

A used sprocket rocks back and forth or a brand new sprocket? Please tell me we're not talking about a used sprocket from who knows where, who knows when.

Those splines don't look bad at all. I had a xr600r with 2/3 of the splines gone which took 25,000 miles. Just put a new sprocket on and Don't run the chain too tight, consensus is too tight promotes spline wear. Also use a light silicone spray (CRC won't attract dirt) on the chain and splines instead of regular chain lube. 2c

Okay here is the rest of the pictures and video that I said I was going to upload. Also how could this of happened to the stator cover? There was plastic in the stator. Watch the video at the end just click on view my video it’s a link.

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Thanks for the pics and effort to take a video and post pics.

Do you have a new front sprocket?

No that’s the stock sprocket.

I’m also concerned why that happened to the stator cover.