Loud knocking sound in YZ426F

This motor WAS sounding perfectly quiet and normal, until I gave it an oil change. When I was done, I fired it up, and it made a loud knocking sound that sounds very serious, like a bad rod or something. I can't tell where it is coming from, and it is so scary, I hate to run the motor for fear of making it worse. It is a constant bang, bang, bang, and I have not revved it beyond idle. It starts easily with one kick, as usual, and it turns over smoothly with the kickstarter.

Before I start tearing the motor apart, I sure could use some advice from others with similar experiences. My mind is burned up with thoughts of a major catastrophe. Help!

After searching Ttalk, I started it again, and the knocking sound seems to be coming from the clutch/balancer area. I decided to drain the oil, and I got about 1600cc. It seems I have no choice but to tear down the clutch side and see what I can find. Any suggestions?

Hi Scott

Sound like the CB Gear thing again.Stange just after a oil change. Did you kick it a few times and check that there is oil going to the cams. They say you can start it and then check if the oil presure is ok but I prefure the other way.

Chech out the mail from Hick. He always gives excellent advice.Oh yes check the CBgear on the cranck. I did the Lock tite thing to the CB gear to hte right of the crank and have to srip every thing again.

Hope you this helps

Hang on the cable

426 Monster

A friend of mine had a similar problem and it was because he cracked the rist pin.

When you say “bang, bang, bang” I picture an unoiled rod bearing. Maybe something happened when you changed the oil that is now restricting or blocking oil delivery to the bottom end.

The rod and crank are oiled via a passage in the right case. Oil goes from the tank, to the pump, through the filter, through the filter cover to this passage which houses a seal which fits over the end of the crank (which is all in the house that Jack built).

If you’re gonna yank this cover anyway to check out that side I would clear out this passage and the accompanying passage in the crank with some compressed air. You should be able to blow air through here with little resistance.

You could also check for oil pressure with the bike running but from your post (“bang, bang, bang”) I’m guessing you may balk at that. To do this just crack the banjo bolt on the chrome oil delivery pipe (oil should squirt out). But this won’t tell you whether or not the crank is getting oil (just the tranny and valvetrain) but will show that oil is getting to, and past, the first pump.

Good luck and I hope this helps!

Thanks Hick, I think I will try the oil pressure check, but I'm very afraid I will blow or seize this motor. I would hate to go from a "minor" repair to a $1300 repair.

I'm wondering if the oil pump is cavitating. Should it be self priming? The oil pressure test should tell me if it's pumping. If it is, then I have a mechanical problem somewhere.

When I did the oil change, I screwed up and only put about 1200cc of oil in it. Could this cause immediate damage? I then added 500cc but no improvement.

I just started it again to check the oil pressure. It leaked quite a bit of oil at the banjo fitting. I had a rag stuffed in there, but it still made a mess on the motor. It sounded almost normal at first, but when I turned off the choke, it idled down and sounded real bad again. I revved it slightly, and the noise seemed to quiet down a bit. Maybe it is just the CB gear loose on the crank, and when it revs a bit it smooths out.

It seems my next move is to pull the clutch cover and see what might be loose.

I just can't believe that this motor ran perfectly before the oil change, and now it has a problem?!? Talk about getting a lump of coal for Christmas. I just totally detailed out this bike, getting it ready to sell. It was perfect.

I guess the ol' 426 doesn't want you to get rid of it - you must have been feeding it well.

If it's a broken clutch basket, you should be able to get out of it for about 200$ and a long evenings work.

I hope I found the cause. As some suggested, and searching the archive indicated, I tore it down to the crank to inspect the CB gear.

The clutch ring gear had a bit of play, but that may be normal. Otherwise, the clutch was in excellent condition after a year of use, since I am very easy on it. The fingers were not grooved at all, and were barely marked by the plates.

The CB gear on the crank had a lot of rotational play, about 1/8". As others have found, the key was mashed and must be replaced. Now it is painfully obvious why Yamaha is changing the crank to a spline for the CB gear instead of this key. The keyway on the crank and the gear look fine. I hope this is the cause of the noise. The key is 5mm x 5mm x 10.85mm. My dealer had a 5x5x9 key in stock, that apparently had the same part number. I called McMaster, and they had a 5x5 keystock 12" long, so I can custom cut my own. I'll pick it up tomorrow and let you know how it works. With any luck, this repair will cost less than $3.00.

Hopefully the YZ426F will be purring (and roaring) again by Christmas Day.

? ? You didnt happen to put ATF in it did you. I know thats kind of a rage with 2 smokes but it wont work on thumpers. Any extra additives ? Did you flush the motor with anything ? Its just odd that it started just due to an oil change.



Same oil as always, no additives, no flush, nothing different. This is my third YZF, so it seems my luck has run out. I will pull the clutch cover today and see what I can find.


Thanks to this great forum, and guys like Hick, I was able to get an accurate diagnosis and quick solution to this problem. It was a mashed CB gear key, and it's hard to believe that it could cause so much noise.

Because the shops are closed, and I wanted to fix it right away, I made a replacement key for the CB gear. The keystock that I bought was a couple thousandths oversize, so I sanded it down to make it a tight fit, tighter than stock. I also cut it a bit longer than stock, to get maximum contact with the keyways.

To make a long story short, the repair went very well and was fairly easy.

When I fired it up, it ran perfectly just like it always has. It looks like it's going to be a Merry Christmas after all!

Thanks for posting the fix to this problem. If my 2000 426 ever makes that noise, I know what to look for first. I bought two of those keys a month ago expecting at some time in the future to needing to replace it. Do you recommend doing any grinding or sanding on the replacement key to avoid this problem down the road?

It's good to hear that you got out of it for a few pennies, Scott.


Was the nut loose and if so, was the lock washer bent over (presumably to prevent the nut from loosening)?

Boit, the replacement key from the dealer (old stock) was not very tight, and was shorter than the original. But if you order one, it may be identical to stock. How long are the ones you bought?

I decided to make one to get a tighter fit, and to get it done quickly. It was easy to make, and I could make one for anyone that needs it.

Holeshot, the clutch and crank nuts may have been a bit loose, I'm not sure, but the washers were bent over. During reassembly, I just torqued them to spec and rebent the washers.

Scott: I have the stock replacement keys so I assume these are identical to the orginal which is still in place.

Originally posted by Scott F:

I also cut it a bit longer than stock, to get maximum contact with the keyways.


Just make sure that the key isn’t too long. If it protrudes from under the CB drive gear it will interfere with the proper tightening of the main gear retaining nut.

Merry Christmas!

Yeah Hick, I took that into consideration. It ended up recessed a bit. I would rather it be flush. Even if it protuded a little, it could be tapped in flush, or maybe tightening the nut would push it in flush. But you are right, you wouldn't want to tighten the nut against the key and not against the gear.

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