Countershaft splines sheared

I pulled my countershaft sprocket to replace it with a new one, and this is what I saw:



About half the splines sheared, and the same is true of the other half splines of the sprocket:



I can't imagine how the bike was working without any problems all this time, and I have no idea how this damage could have come about.

I searched the site and I could not find anyone else reporting such an incident, but it's hard to believe that I'm the first one to experience this.

Not sure how to proceed from here.

I believe the way this happens is when the nut is not tight and the sprocket rocks back and forth for many miles, eventually wearing down the splines.

Proper repair is to replace the shaft.

I have seen that issue on a couple other WRR's.

Normally the C/S nut is not peened or worn to the point peening will not secure it in place. After a bit the nut and sprocket are loose and things begin to wear.

I use to reuse the C/S nut on my bike when I would replace the sprocket or chain but during one of those times I noticed the nut and sprocket were loose. Replaced the nut and used some loctite as well. Now I pretty much order a new nut with each front sprocket - I don't believe they were terribly expensive.

In you case I think the shaft may have to be replaced.

YZEtc wrote:

I believe the way this happens is when the nut is not tight and the sprocket rocks back and forth for many miles, eventually wearing down the splines.

rsteiger wrote:

Normally the C/S nut is not peened or worn to the point peening will not secure it in place.  After a bit the nut and sprocket are loose and things begin to wear.

The nut was not peened; it had been kind of mashed onto the threads. The only other indication that something was wrong, was that it did not take a lot of effort to unthread it: it was definitely not torqued to 69 ft-lbs per specs.

So I can only imagine that the previous owner did not correctly install the nut, it got loose and the sprocket started eating the splines, at some point the damage was noticed, but he simply torqued the nut back on.

The torque of 69 ft-lbs Yamaha specifies is way higher than anything else I have ever encountered on other bikes, so I presume the sprocket is primarily held on in its position not by the splines, but by the nut.

I know that the only real solution is to replace the countershaft, but that means that I need to open the cases. For the time being I will install a new sprocket with a new nut (Loctited and peened) and see how long it lasts. Will re-evaluate come winter time; maybe I will take the opportunity to install a Hot Rods crank and a Thumper Racing overbore kit.

johnkol wrote:

YZEtc wrote:

I believe the way this happens is when the nut is not tight and the sprocket rocks back and forth for many miles, eventually wearing down the splines.

rsteiger wrote:

Normally the C/S nut is not peened or worn to the point peening will not secure it in place.  After a bit the nut and sprocket are loose and things begin to wear.

The nut was not peened; it had been kind of mashed onto the threads. The only other indication that something was wrong, was that it did not take a lot of effort to unthread it: it was definitely not torqued to 69 ft-lbs per specs.

So I can only imagine that the previous owner did not correctly install the nut, it got loose and the sprocket started eating the splines, at some point the damage was noticed, but he simply torqued the nut back on.

The torque of 69 ft-lbs Yamaha specifies is way higher than anything else I have ever encountered on other bikes, so I presume the sprocket is primarily held on in its position not by the splines, but by the nut.

I know that the only real solution is to replace the countershaft, but that means that I need to open the cases. For the time being I will install a new sprocket with a new nut (Loctited and peened) and see how long it lasts. Will re-evaluate come winter time; maybe I will take the opportunity to install a Hot Rods crank and a Thumper Racing overbore kit.

Yeah I would see how many more miles I could squeeze out of that before I start tearing apart the motor. You may be just fine with a new sprocket, torque wrench, loctite, and a punch.