Countershaft - sprocket failure

I searched the forum and did not see where anyone had this issue.  I was on the 2016 Loosenut ride this past weekend in KY and my countershaft/front sprocket failed.  I was able to flip the sprocket around and tighten the nut to be able to make it 60 miles back to camp.  Just curious if this is a weak link or just my luck.  I plan on tig welding a new sprocket to the shaft until I need to tear down the engine to replace the shaft...

I've seen this posted on this forum a couple of times in the past, but no, it's not the norm, and practically all motorcycles attach their front sprockets in a similar method.
The staking down of the nut to the shaft is where some have written about tendencies to take a short cut (like not doing it at all).

My only guess is:
Was the nut properly tightened?

If the sprocket is allowed to rock back and forth on the splines due to insufficient tension of the nut, that's the result.
Similar to if you have ever seen the splines on the end of a shift lever shaft worn out due to an always-too-loose shift lever securing bolt.

Block it by welding doesnt hold a day.
So if you go D.I.Y.
Fabricate a heat shield.
Carefully and slow TIG weld new splines on the shaft with thin 0.035" or 0.045" wire. Just take 100ft of available MIG wire.
Just 1 pass let it cool. So will take centuries. Buid one tooth in 5 or may be 10 layers.
Dont weld the bottom of the ditch!!!
Machine the splines with a Dremel. Will take time again. Hold the tool in radial position.
Use a new sprocket for calibration.

I.M.O.
when changing sprocket
better use new nut every time
use cheap impact air wrench securing its torcked
loctite eats the plastic in the nut
using technical rubber sealent does the same job not eating the plastic

Securing the nut by denting it as ment by O.E.M. is not idiot proof, as shown on your bike.
The the nut having a dentable edge driving it into the big radial groove is tricky.
Why this O.E.M. designed a 36mm nut?
A 36mm tool isnt in anybodys toolbox i think.

The easy way other O.E.M. just use 6mm bolts and a simple locking plate being idiot proof.
The simple locking plate is used enlarged on hard rock tunnel boring machines securing axles of 10" diameter.

Of course there is more on this bike.
You can swap the axle spacers on the rear wheel without noticing, its just a difference of 1/16" eating you rear caliper.

TioJose wrote:

loctite eats the plastic in the nut

Which plastic? Loctite is fine...

TioJose wrote:

Why this O.E.M. designed a 36mm nut?

The didnt, it uses a 27mm just like the rear axle.

If, its a self locking nut, it contains polymer plastic.
Loctite isnt compatible to plastics.

You can do the rear axle 27mm radial tooled,
need a quality 27mm axial tool for torque-ing the sprocket.
Its lots of tricks blocking the the primairy axle, but none of them is solid, so i prefer the impact wrench, leaving it in neutral just put the chain on and push the brake pedal wont harm your internal drive train .

Please tell me where the plastic is:



You realise that loctite is a brand right? The make 100's of different items, for different application.
I have, however, not seen one that isnt compatible with a self locking nut. (not saying that theyre not out there, just havent seen it)
Theres a big difference with "not suited for plastic parts", and not beeing compatible with a a self locking nut.
2 plasticparts with threadlocker will almost plasticweld itself together. Theres no such thing when used with a self locking nut.


Most threadlockers actually turn in to a kind of plastic, sealing the threads form corrosion.

I didnt say you should use the rear axlewrench, just that the socket for the nut is a size 27mm, like the rear axle. Its a pretty common size for dualsports and dirtbikes.
Impact gun is fine for removal, but when you install it you have no idea what torque you end up with.

Why no display of the top side of the nut? Able seeing the specific edge for locking? If locked by denting it cant fail.
No idea wether the average guy is owning a full set of torque wrenches or not. For sure my biking friends have not despite they are well done. In general i see them overtighten the 27mm rear triple times afraid loosing the whole rear at warp speed. Explaining my friends its a self locking nut doesnt withhold them overtighting. Equel having a variant locking element.

Nut was properly torqued (and nut peened) but I had not checked it for some time, I do believe it backed off a little, not positive.  Still going to weld on for now, looks like this is pretty rare which is good, agree on the issues with the nut and keeping it secure.  I will be checking this more in the future once I change everything back to normal.  Yamaha shop wants ~$600 for labor only and this is me bringing them the engine.  I plan on doing this myself later on.  Thanks for the feedback.

Understand your choice. Give it a try. Possibly your in favour of your luck and it will hold.
Be aware, carbon equivalent of both materials sets it out of welding scope, so taboo in engineering.

TioJose wrote:

Why no display of the top side of the nut?...

You have no access to google or your own bike to look at? Youre the one posting false information, you might wanna check stuff like that before you post.

Theres plenty of pics on this site aswell, but none with the dreadful plastic-part you speak of (cant actually come up with any brand which uses self locking nuts with plastic inserts. Not saying their not outthere, but its not common)
https://wr250rforum.forumotion.com/t5961-any-lock-for-the-front-sprocket-nut

Now... Back to the subject.

WRDave wrote:

I searched the forum and did not see where anyone had this issue.  I was on the 2016 Loosenut ride this past weekend in KY and my countershaft/front sprocket failed.  I was able to flip the sprocket around and tighten the nut to be able to make it 60 miles back to camp.  Just curious if this is a weak link or just my luck.  I plan on tig welding a new sprocket to the shaft until I need to tear down the engine to replace the shaft...

Was this a stock sprocket? Or aftermarket?

Can you post some pictures farther out from the countershaft? That doesn't even look like a wr to me.