YZ450F RELIABILITY reliability

That broken hub is from loose sprocket bolts, not Yamaha making weak hubs. I have always tightened my sprocket bolts in a criss-cross pattern after they were nice & snug & then check them again after I ride the bike.

To add one more thing along these lines. Once they have become stretched and you continue to tighten them they will continue to stretch until they give out. Once stretched they need to be replaced quick. Now if I have a bolt that becomes loose during a ride I would replace it, not tighten it. Once torq specs are followed and some use of loctite I have not had any come loose. Checking them is a good idea.

There is, simply, no reason for the sprocket bolts to ever loosen. The reasons that they would would include the nuts actually rotating off of them, which is exceptionally unlikely, and the sprocket moving on the hub, loosening the clamping pressure by wearing the two parts. This should not occur, either, and only would if the bolts were too loose to begin with and/or were a poor fit in the hub.

The idea that the bolts can be stretched by over torquing is just ludicrous. The bolts call for 30 ft/lb of torque, which produces 480 pounds of clamping force at each bolt. If you tripled that torque, and by some miracle the threads did not fail, the tension on the bolt would rise to 1440 pounds. A standard grade, Class 8.8 ISO bolt has a minimum yield strength of 92,000 psi, which on an 8 mm bolt means a tension of 7043 pounds would be required to pull the bolt beyond its elastic limit and make it permanently stretch. Even the lowest grade 2 SAE bolt has a minimum yield of 5700 psi, and and 8mm grade two would need over 4000 pounds of tension to permanently stretch.

When the bolts snap, it's because they were loose, and were sheared by repeated impacts, or because the chain was too tight, and the sprocket folded over and broke the bolt.

By the way, grade 10 (170,000 psi) sprocket bolts are available.

Gray you can throw out all the numbers in the world, the fact is that I know of several failures to include 2 myself. The lot of us didn't use torq settings, tighten them the old fashioned way, good and tight with plent-o-loctite(for the record good and tight was as tight as we could get them which was way over the recommended torq spec). Bolts kept coming loose and eventually caused a failure of the hub on a handful of bikes. All riders then started paying attention to factory torq settings for sprocket bolts using new bolts. This = no more failures or loose bolts. For the record no one changed their procedure for setting chain slack.

You can agree or not but them are the facts. Maybe you have some other explaination but the one your giving does not meet with the facts of failures I'm familiar with. This is what a handful of Yamaha owners I know did to stop and prevent further failures and it has been succesful..

Then can you explain why have I never had a problem with this, in spite of the fact that I have never used a torque wrench or Loc-Tite on my own rear sprockets?

Then can you explain why have I never had a problem with this, in spite of the fact that I have never used a torque wrench or Loc-Tite on my own rear sprockets?

Yes I can, because you are not tightening them as tight as my group was.

Now can you explain why you are so ready to throw out this idea when it has solved a problem for a group of us and it may solve a problem for others while causing no other damage???

As far as all the numbers go and theory and again this is just theory. This has only happened with Yamahas that I am aware of. Does Yamaha use a different bolt supply, process, manufacture than the others. While it is hard to believe that anyone could fatuige a bolt by over tightening. The threads on the nut could be fatuiged/stretched slightly allowing a minor amount of play. This alone would not cause a bolt to fail but could cause the play mentioned. Now with this play in the system you hit it with the chain coming tight under hard accelration out of a corner. The sprocket then comes tight on the bolt unleashing much more force than me over tightening the bolt. In day of riding this happens 100's times causing more and more stress each time.

Now the above is just theory as no one has ever questioned the method of this as you have. It does go beyond the simple static model or bolt tensile strength numbers and brings in real world application.

But we can go right back to the bottom line that by applying the simple idea of new bolts and correct torq settings no further failures have been seen in a group that had several.

I owned 2 Hondas before I bought my 05 YZF this past fall. All the valve problems I had (not to mention 2 other CRF owners I ride with) was just too much of a coincidence for me.

My 'new' 06 CRF had the rear sprocket bolts come loose shortly after I bought it. They did not look like it, but when a wrench was applied to them they were in fact loose. I retorqued them according to the manual, however it was too late, the damage had been done. Once those hub holes get even slightly elongated it's down hill from there. I had to replace the wheel shortly thereafter because they would come loose after every ride.

The other guys I ride with, have had Yamahas ( 03 and 04's) for years and they have not had any issues at all. So I switched and I guess we will see, so far I have been very pleased.:banghead:

I'm willing to discard the notion that the bolts were stretched based on experience, and because it flies in the face of known fact. I have had strain gauges on bolts like this and seen them stretch, then return to their original length numerous times. To stretch the bolt to the point that it is weakened, the elastic limit (the point at which the bolt will no longer return to its original length) must be exceeded. But there is one thing here that could involve the hub, so let me ask a couple of things:

In the cases where you had bolts that were over tightened, and would not thereafter stay tight, did you replace the bolts only, or the hub also?

The reason I ask is this. We have seen that there was a fairly small number of YZ rear hubs in which the drive end bearing pocket loosened up. This could still be rooted in excessive chain tension, but even then, it reveals the hub as the weak element of that chain of parts. Since the design of the hub is unchanged from previous years when this did not occur, this raises the possibility of a weakness of the material used in the hub due to some flaw in the process. If we can assume that is true, then what may actually have been happening is that the sprocket mounting tabs on the hub flange were being damaged by the over torque, and once brought to the point of yielding to crush, lost what integrity they originally had.

If you have corrected the loosening problem on a wheel where it previously existed, and not replaced the hub, then that would set aside the above possibility. In that case, I'd be interested in having a sample of the disused bolts, if they have not been discarded.

In the cases where you had bolts that were over tightened, and would not thereafter stay tight, did you replace the bolts only, or the hub also?

If you have corrected the loosening problem on a wheel where it previously existed, and not replaced the hub, then that would set aside the above possibility. In that case, I'd be interested in having a sample of the disused bolts, if they have not been discarded.

Yes there have been cases in our group where the bolts were replaced after becoming loose and the hub stayed intact. The fatigue on the hub was questioned since there is obviously stresses being placed on it beyond it's design when it is subjected to those sudden loads. I have even seen the holes apear to have been damaged somewhat, could have been optical tricks, but new bolts were put in and it held.

I have not had a failure or bolt I needed to replace in a few years so I'm unable to provide a sample.

I will say I don't understand all of the mechanics of the failures, my ideas on it are just theroy but more of a bottom line of how a group of us got the failures to stop.

Grayracer is right, you cannot "stretch" your sprocket bolts. Go & mic a new sprocket bolt & then one of the so called stretched ones...they will read the same length. Those hub failures were from, at one point in the bikes life, 1 or more sprocket bolts getting loose enough to elongate the hole in the hub. Also, if you were to put a higher grade bolt in a hub with elongated holes...those bolt/hubs would fail as well. BTW, I also have never used loctite or a torque wrench on my sprocket bolts & have never had 1 come loose.

Well then we will have to disagree because the senerio you paint isn't how things got broke on the bikes I am familiar with. I have outlined what I believe to be a possible way that they could have become fatigued and stretched you can choose to believe it or not. I have also outlined what things were done to correct the issues and that to has been 100% sucessful for those involved.

Yes in the end the hub is what gave out but only because the bolts continued to come loose with continuing loads placed on the hub, in my opinion because the metal fatigued and stretched. Metal can stretch and does, there are weak points on bolts and nuts, the threads. Those threads while still part of the bolt/nut do not have the same tensil strength as the entire bolt. I can't imagine that there is anyone reading this thread that has not striped threads on a bolt, bolts with rated tensil strength beyond what people say can't stretch by human hands.

Some bikes broke hubs. Others then were seen with loose sprocket bolts in the group the hub was inspected and owners advised to replace those bolts with new and torq to proper spec = no more problems.


I'd like to have 2 or 3 of the old bolts and nuts, if you haven't trashed them. PM if you have a couple.

One other thing that could be taking place here is that even though the bolts themselves cannot reasonably (not withstanding an unreasonably weak bolt) have been stretched beyond their AL, the torque force is born only by the total contact area of the threads, and these you could have stressed beyond their yield point and rendered them weaker than they originally were. You should have been able to feel that happening, however.

Grey I wish I had some of the bolts so I could send them to you and have them looked at.

As I have said I am unsure of the exact mechanics of what happened but there was some damage to the bolt, a theory was formed that there was some stretching, and a failure took place. After replacing bolts that would become loose and paying attention to torq settings their would be no further loose bolts or failures.

I have not doubt that the final failures were due to the loose bolts plus a heavy load on the drive train that was transfered to the weak point, the hub, that finally broke causing a major failure. The parts that failed on mine were not retained with the exception of one part of a rear sprocket I keep to remind me.

Those bolts that were in question would not stay tight after becoming loose no matter what procedure was used to tighten them. If new bolts were used and torq settings used they have stayed tight on bikes that didn't have a failure.

The dynamics of how this would all happen are pretty large and you have to take in all the facts to come to the conclusions we did.

In the end I know what worked for a group of us and hope that this helps some others.


however, my *next* bike... may be a different color.

And when are you getting your next bike...:busted:

I have had it with my 03 YZ450 I'm setting up a new bike that will out perform and out last ALL 450s....Not finished with it yet but here is a picture of what I have so far....

freddybike.jpg Notice Massive front brake, should be great

I'd love to hit a big double on that one!

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