KDXRIDER.NET

My buddy got a YZ125 the other day and after a little work we were ready for a quick drag race.
We take off, I smoke him no contest, but then as I'm pulling through 5th full throttle I lose all power and the bike sputters back fires a few times and makes a loud knock.
Seeing as I only have a few hours on the rebuild I was very upset but I still had compression and there were no nasty noises with light kicks.

Anyways my first guess was that I sheared the woodruff key and I was right.
I pulled the stator cover and the flywheel was literally just free-spinning on the crank.

This happened once before I rebuilt the engine but I thought it was from me messing something up.
This time though I made triple sure everything was lined up and it was torqued to spec with a new nylock nut.

Is there any reason this would have happened? I assume my nut somehow came loose and allowed the flywheel to slip.
I am thinking of installing the nut with loctite this time around. Any other suggestions?

you should have a big thick washer and a flange head nut i would check and make sure you have the correct parts also check the stator plate marks are lined up correctly the timming being out could cause it to shear.

Lap the rotor on. I have done this on other engines and eliminated the key altogether.

suzuki 166 wrote:you should have a big thick washer and a flange head nut i would check and make sure you have the correct parts also check the stator plate marks are lined up correctly the timming being out could cause it to shear.

It didn't come with the stock parts. The only thing on there was the nylock nut against the flywheel. Maybe not having the washer put too much pressure on the nut and made it loosen up.

Lap the rotor on. I have done this on other engines and eliminated the key altogether.

I was reading about this. I get the concept of lapping them together so they make 100% contact but people were saying to just put the compound on the shaft and spin the flywheel on it until the high spots went away.
This seems like it wouldn't really do anything since when you torque everything down the flywheel will actually get pushed up on the shaft where you couldn't get to with the lapping.

You have to be sure you have good metal-to-metal contact along the tapered surfaces. This can be checked with some chalk marks or a light smear of prussian blue paste on the shaft. Put the rotor on the shaft, rotate it a little bit, then see if it rubs off along the entire tapered surface. If not, the tapers are messed up.

When assembling, be sure the tapered surfaces are perfectly clean.

Once the flywheel is properly lapped and torqued down the woodruff key is almost not even needed to keep it from spinning. Just get some lapping compound and do it, you will feel it get harder and harder to turn as you go.

When you put a new key on it did you use a factory piece or one from a hardware store? Reason I ask is because the cheap hardware store keys have been known to shear rather easily. Only use the factory Kawasaki key.

SS109 wrote:When you put a new key on it did you use a factory piece or one from a hardware store? Reason I ask is because the cheap hardware store keys have been known to shear rather easily. Only use the factory Kawasaki key.

I used a hardware store key. It is SAE so it has to be filed down a little bit too.
I mean if everything is working like it should the key should have absolutely zero force on it anyways.
I was reading about people even cutting up an old washer to use temporarily so they could keep riding for the day.

I am going to clean up and lap the taper and flywheel together and then install a lock washer with the nut and put a little loctite on the nut as well.
If that doesn't work then idk what will.

What I'm pretty sure happened was not that the flywheel just spun on the taper, but that the nut came loose and allowed the flywheel to dislodge.
That is why I could spin it by hand and why the nut was not tight on the flywheel.

I would advise you to not use a lock washer, just a regular flat one. It will mess up your torque reading and grip on the taper. Nylock nuts are only good until they are removed once. I would get new OEM parts, check the taper, and torque to specs. The parts should be fairly cheap. No loctite should be needed or used.

If it the nut is torqued properly, the flywheel should never move until you pull it off. The taper will hold it even if the nut comes off. The only time I have ever had trouble is with used bikes that had the flywheels removed and the PO did not apply the correct torque to the nut.

Thrahl wrote:

SS109 wrote:When you put a new key on it did you use a factory piece or one from a hardware store? Reason I ask is because the cheap hardware store keys have been known to shear rather easily. Only use the factory Kawasaki key.

I used a hardware store key. It is SAE so it has to be filed down a little bit too.
I mean if everything is working like it should the key should have absolutely zero force on it anyways.
I was reading about people even cutting up an old washer to use temporarily so they could keep riding for the day.

I am going to clean up and lap the taper and flywheel together and then install a lock washer with the nut and put a little loctite on the nut as well.
If that doesn't work then idk what will.

What I'm pretty sure happened was not that the flywheel just spun on the taper, but that the nut came loose and allowed the flywheel to dislodge.
That is why I could spin it by hand and why the nut was not tight on the flywheel.

Well, I have heard of this happening before on KDX/KX bikes, people trying the cheap hardware store keys, and they kept shearing them until they finally bought a genuine Kawasaki key. I'm just sharing the knowledge but it is up to you what you do with it. Besides, how expensive is a genuine Kawasaki key, and maybe a new proper flanged flywheel nut, to actually fix the problem over all the other BS? You can get both parts together through Service Honda for less than $6.

Ok so an update. I went ahead and bought the stock key rather than use the hardware store one. It just fits a whole lot better and I didn't have to screw around filing it down so it was worth it.
They didn't have the oem nut in stock so I just bought a new nylock from the hardware store.
I lapped the flywheel to the shaft and it definitely had some high spots and now is perfectly smooth.

I cleaned the shaft really good and installed the flywheel with a regular washer behind the new nylock nut and torqued to 47 ft-lbs. I also threw a little blue loctite on for good measure.
Went for a ride last weekend and it rode great, no issues at all. When I got back home I checked the nut again and it was still just as tight so hopefully it is good to go.

Thanks for your help guys.