Primary Gear Noise and Solution

After four days of riding in Moab over thanksgiving, my bike has begun to make the "Primary Gear" noise. It sounds exactly like a loose cam chain coming from low on the right side of the motor. I'm assuming it's the same noise as the others were having. (2000 YZ426)

So, the questions are:

1.) How difficult was the fix? (Scale from 1-10, 10 being hardest)

2.) Did you have to remove the clutch basket?

3.) Did you have to remove the entire right side cover to inlcude water pump and filter or just the clutch cover?

4.) How long did it take?

5.) Should this be a warranty item?

6.) Do you have to replace any gaskets?

7.) Should you replace any parts while you are already in there?

You see where I'm going? I have limited time so if it's pretty easy and does not require the clutch basket to come off I'll take it on myself. If it's pretty much a bitch, I'll have my buddy at the local shop do it. In my opinion it should be a warranty item though so the shop should do it for free. Please be specific as possible in your response.


Roostn in Denver

Follow this link below to a picture of the parts in question. Please identify if the nut in question is item 16 or something else.

Also, I have heard of several keys needing replaced. Is the key in question item 9?

These are on two separate gears so which one is the culprit? I'm assuming you are talking about item 12 as the CB gear and item 8 being the Primary Gear.

What I am hearing is there is a bolt that holds on item 8 and it comes loose resulting in the rattling noise.

Several of you have had the nut (item 16) come loose which takes out its key and results in a similar noise.

Am I on the right track?

Roostn in Denver

Key is # 9, nut is not shown in this picture.

Nut and washer are # 2 & # 3, respectively, in this picture

Item 12 is the CB gear but it is held on w/ splines and doesn’t have the play problem, the gear that drives it does (the CB drive gear), it is # 8 in your picture (but don’t appear in the pic I linked above). So # 8 in your pic is the one you need to check for play, and the main gear retaining nut and washer are (#2 & 3 in above pic) what you need to check for tightness.

There are only two keys on the whole bike, both on the crank, one is woodruff on the flywheel and the other is the straight key that secures the CB drive gear.

Now you should be on the right track.

Hope this helps!

Does the clutch have to come off to get to the C.B. and primary gear?

[This message has been edited by ART (edited 11-28-2000).]

Originally posted by ART:

Does the clutch have to come off to get to the C.B. and primary gear?


You can check the retaining nut for tightness, visually inspect the lock washer and check the CB drive gear for play with the clutch on. But since the main drive gear drives the large gear behind the basket this needs to come off if either gear on the main (crank) shaft is to be removed.

By looking at the pictures if the problem is the Primary Drive Gear (Item 1) then it appears the entire clutch basket must come off to tighten it. These pictures are funny because you would think that Item 8, Drive Gear, and Item 1 on the other image, also Drive Gear, are the same. I don't think they are as they have different part numbers.

I guess I'll have to look at my manual tonight before I get any real data. I do know the oil, and water must be drained and the big cover must be removed in order to get in there and change anything.

Hopefully someone who has actually done this before can get on and add some clarity.

ROOSTN in Denver


Sorry to have only muddled the picture for you, but I have done this before (twice). I’ll try my best to make some sense of the mess I’ve made…

Item # 1 is the main gear which drives the clutch and tranny. Item # 8 is the CB drive gear, or the gear on the crankshaft that drives the counterbalancer. This gear rides on the crankshaft behind (or inside) the main gear. The main gear is held in place with splines. The CB drive gear is held in place with a straight key (but this is changed to splines also for ’01. Gee, I wonder why? :) ).

To remove right cover:

1) Drain water and oil

2) Remove brake pedal (don’t forget to remove retaining pin in the end of the bolt before unthreading) and spring.

3) Remove kickstarter

4) Remove water pump cover, secure it out of the way with a zip tie or safety wire.

5) Remove oil supply line, dowel and o-ring. This is at bottom of cover below filter housing.

6) Remove chrome oil delivery pipe at pressure end, this is a banjo bolt w/ two copper washers on top of case next to cylinder just above filter housing. You can pry the pipe up a bit to get the bottom washer out.

7) Remove oil filter cover and filter

8) Remove all case bolts

9) Remove ONLY two clutch cover bolts at forward, top side of cover (these also secure main cover).

10) Remove cover gently to save the gasket.

There are two, solid and very small dowels in the water pump cover. Don’t lose them. There are two, hollow, larger (~ 10 mm) dowels in case or cover, bottom right and top left. There is also a dowel and o-ring that go from the filter housing (in the cover) to the oil pump (in the case), if this dowel and o-ring came out with the cover remove and insert them into the pump so you don’t lose them (running the motor w/out this dowel & o-ring would be disastrous).

I just leave the bolts in their holes to ease reinstall, but the manual is pretty good at pointing out which length fastener goes into which holes if you want to do it that way.

To the right of the clutch you will see the crankshaft, mounted on it is a nut, lockwasher (w/ bent over tab), main gear, CB drive gear (in that order front to back) and straight key that holds it. Immediately to the right of this is the CB shaft which has one gear (with a bolted on counterweight) secured by splines and also held on with a nut and lockwasher.

What creates the racket is a rounded straight key under the CB drive gear which allows the counterbalancer to oscillate to and fro relative to the crankshaft (instead of staying “in phase.”). A rounded key is usually accompanied by a loose main gear retaining nut but the opposite may not always be true. If, once you have the cover off, you can rotate the CB drive gear, even slightly, w/out the crank moving then it is time to take off the clutch, remove the two gears and replace the key. If this happens make sure you inspect the CB drive gear, its keyway and the keyway in the shaft as well as replace the lock washer, it doesn’t clear the basket by a whole lot and you don’t want an overly bent washer making contact here.

When reinstalling the gears make sure the punch marks on the CB gear and CB drive gear align and make sure you bend the lock washer tab flush against the nut. When reinstalling the cover align the kickstarter hole and gently apply pressure while rotating the water pump impeller to align it with the notch in the counterbalancer. You shouldn’t feel much resistance.

Or you can get your dealer to do this for you and, if this is the case, be thankful you have a competent mechanic at your disposal whom you can trust because not everybody enjoys this convenience :D

I hope this helps (I’m tryin’ anyway…).

[This message has been edited by Hick (edited 11-28-2000).]

Dude, you are the man! I picked up a new gasket tonight on the way home. I have not decided if I'm going to tackle it myself or not yet. It doesn't seem to bad. I'm thinking about doing a Hinson Basket since I'll have to have everything off anyway. Also, if I do it, I'll document it with my digital camera from step 1 to the end and post it on my website.

Again, thanks for the great posting, that REALLY cleared things up!

ROOSTN in Denver

Great explanation Hick. I'll be sure to return to this post, if I hear any unusual noises.

I suspected lower end noise and did everthing as Hick described, however, I also use a medium lock tite on both gears, the straight key and both gear nuts on assembly of my WZ 420. However, in my case nothing was loose but now it will never be loose and I have some piece of mind.

When you reassemble everthing make sure you have the correct Counter balancer timing by aigning the puch marks on the gears, etc. and make sure you lock tite everthing and bend over the locking feature on each of the locking washers.


[This message has been edited by Clark Mason (edited 11-28-2000).]

after the last couple of rides, i have noticed a cam chain like noise comming from the right side case. if i put my hand on the clutch cover it lowers the noise considerably. it is quite loud. could this be the same condition as referenced in this post? or just an increase in noise from mileage? the bike is '99 yz400 with probably 500-700 miles.

So I've got the right side cover off my '99 WR - can't 'feel' any slop in the CB drive gear but the descriptions fit my bike to a T. So I'm going to pull the clutch assy and check all the keys, etc.

What did you use to hold the clutch basket while removing the screws/nuts?


[This message has been edited by Brian Meadows (edited 05-08-2001).]


You can either use an impact wrench to remove the clutch hub nut, or you can put a long screwdriver or something similar to put between the rear wheel lock and the swingarm.

It's easier to put a small screwdriver in between the teeth of the primary gear and the clutch gear.

I finally pulled the CB drive gear off the bike. I could not feel any play in the gear prior to removing the nut. W/O the nut there is just a tiny bit of play between the gear/square key/crank shaft - guess it is time for a new square key. I sure hope this solves the problem.

Clark - did you loctite the CB gear to the shaft?


Brian, if the key has little dents in the sides, it is the cause of the infamous CB gear noise. I suggest making your own key with slightly tighter tolerances.


I bought a new key ($1.20) and it looks about the same as the one I removed - corners are not sharp. I'll measure the keys to see if there is any difference. I'm not quite motivated enough to make my own key... I'll try the loctite route first.


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