WR450 Serial Numbers, Flywheel torque, key failure

That's another thing. If you bolt has a lubricant on the threads or is stone cold clean has a lot to do with the torque on the bole. Think about it, if the bolt turns slicker the torque will be greater. I have heard no talk here of the proper way to torque the nut?

Ok 2 more posts And im better than just a TTmember! Ok this is one of them! :)

F=Pressure x area. Area remains the same, Pressure increased due to greater torque = greater force on bolt.

Motorod the torque spec of the nut will not change from 47ftlb with or without lube it will remain 47ftlb as this is the clickpoint.but presure applied to the flywheel will increase due to the advanced positioning of the nut along a lubriated thread. ie less resistance/friction, the nut travels further along the thread,clearace is reduced between mateing furfaces.

In reply to:"That's another thing. If you bolt has a lubricant on the threads or is stone cold clean has a lot to do with the torque on the bole. Think about it, if the bolt turns slicker the torque will be greater. I have heard no talk here of the proper way to torque the nut? "

Your are correct. I mis-spoke, I should have said the force will be greater using a lube. Thanks

Motorod would the yz flywheel bolt up to the wr?

any reports of this happening to a yz?

...force will be greater using a lube.

That's what she said! Ahem..uh, sorry. :)

We need to bring all of the post on the woodruff key together for documenation purposes. I am going to make two new post, one for those who have not had any problems and one for those that have. The information will be specific and limitted only to the woodruff key. I am going to ask the moderator to keep these two posts together and to stay active on the first page.

Just something to consider...

Torque is the most often used means to determine the amount of stretch a bolt or shaft undergoes to obtain the optimum holding force for whatever mechanism it is used for - because its easy. However, for certain kinds of critical equipment, torque is not used, measurement of bolt stretch is used because it is more accurate - torque readings can vary in accuracy depending on thread lubrication, corrosion, machined thread finish, washer types, etc. It may be that over-torqueing of the shaft nut is causing some flywheel taper fit distortion (I think Dutch is right). Yamaha would certainly recommend the key being replaced because it could have been stress-damaged by a possible poor-fitting flywheel. Lapping, or at least spot-checking would likely be necessary for any bike that had spun a flywheel because of the potential for shaft-fit damage. The poster who lapped his flywheel had a spin-failure as I recall (let me know if I'm wrong).

The real test of reliability (in my mind) is whether any of the retorqued bikes have experienced any failures.

Let’s take another look at this dilemma. Where I work we manufacture fuel and brake line systems and torque to seal is critical. Under torque and over torque can cause brake failures or fires. Here are some things we must consider.

Significant under torque on tubes/nuts or a flywheel/shaft is a failure mode for sure. Overt torque to a moderate level on either system should not be a problem, neither material low carbon thin tubing, or much thicker more robust masses such as the tapered shaft and the flywheel mating surface are not going to distort or stretch so much as to create any other problems with the flywheel/shaft relationship. Over torque to a very high level, and threads can slip/strip, the shaft could stretch causing misalignment of the tapered surfaces, though I feel this would require very high torque values. If threads on the nut or shaft are messed up enough two things can happen. If you are using a torque wrench (clicker type) you can under torque it if the bolt to shaft interference is greater that your torque value. If interference is less, the torque wrench will drive if home to the desired value, no more.

If you are using an air driven wrench any interference greater than your torque value you will under torque. Clicker wrenches and air wrenches set to the correct value cannot over torque regardless of how easily the nut/shaft threads can turn. It’s true that as you wrench up to torque you are pulling hopefully mating surfaces together. At some point the surfaces will not get any tighter as you continually increase torque, you will strip threads and or stretch the straight portion of the shaft. I plan to lap mine to fit the tapers if it isn’t a good surface to surface fit. I will torque to 47 minimum, and if I can find information on the shaft and nut that show they could be torqued higher with out damaging the threads I will torque higher by about 25 % of the difference.


For a taper-fit to hold, very little torque is needed. As an amateur machinist, I know that inserting a morse-taper tool-holder into a sleeve - and having it hold a drill chuck tightly (for example) only requires hand pressure. So, if the taper is correct on both the crankshaft and flywheel, and there are no burrs or other anomalies to prevent proper contact, the flywheel should "bond" to the shaft with very little torque on the shaft nut.(probable reasons for higher torque is to handle thermal expansion and the effects of sudden speed and rotational changes found in an engine) If you were to see a chart on bolt stretch vs. torque for a small diameter bolt such as 1/2 inch you would be amazed at how much design stretch in built into fasteners. It is not inconceivable that a shaft nut tightened to higher torque (as someone mentioned, via an air gun) causes the excessive stretch on a portion of the shaft that would cause improper fit. Imagine the tapered shaft as a progressive series of ever-larger diameter "bolts". The larger diameter area would resist stretch more than the smaller diameter - and thus the possible distortion of the taper. This may not be what has occurred, but it is a real possiblity to explore - by keeping track of re-torqued bike success or failure. The distortion may have even occured in other earlier models to some degree, but the combination of incorrect torque, changed flywheel design, and backfire/starter issues may have brought a formerly inconsequential effect to this problem level (for some bikes).

But then again, it may all be flatulence. Hey, I'm in the middle of doing my income taxes and I don't know what's going on anymore!


I agree that good fitting tapered surfaces will hold together well. But many of these also hold as a result of end loads (torque), as when pushing a drill bit into the surface to be drilled. Also to hold well they must fit well together as you mentioned. That is why I will spot mine and correct the fit if it is needed.

Once the two tapered surfaces are line to line it's very difficult to distort those surfaces any more since in our case the masses that we have in the shaft and flywheel will force the threads to distort and/or the unsupported shaft to stretch. Therefore the threaded shaft which is smaller than the smallest taper diameter will stretch especially since it has nothing constraining it, or as mentioned the threads will distort. Therefore some extra torque is ok as long as it does not weaken the threads or unsupported shaft.

In the aerospace industry there is a designation for some nuts and bolts, called "Safety Critical Fasteners". This term is used on things like fuel fittings, engine mounting bolts, landing gear fasteners, etc. Anything where a failure can put a pilot in serious trouble. This term might apply to some of the fasteners on our bikes. Fly wheel nuts that were torqued beyond spec and other nuts and bolts that are removed and torqued regularly would fit this designation. It is really false economy to reuse these fasteners, especially when you consider the possible costs of a failure.

There is one other aspect to reusing fasteners that is interesting in this application. Bolt breakage due to "stretch fatigue" is not the only failure mode.

A few years ago there was an investigation into a helicopter crash. When the NTSB looked at the rotor, they found that some fasteners had backed out, the rotor came apart and somebody got hurt. The root cause of the crash was identified as something like "reuse of safety critical fasteners".

Seems that when the bolts / nuts are fastened and torqued for the first time, the mating surfaces sort of bed into each other. And the back off torque (force required to disassemble) is usually about 95% of the original torquing value. When they torqued the bolts / nuts through a second and third cycle, back off torque dropped to 80% then 65%, but the 5th cycle the backoff torque was under 50% of the original torque value. There was a continual deformation of mating surfaces.

This is something we all need to be aware of - motorcycles are a lot like aircraft, one small detail gone wrong can kill you.

i got my bike jan.29th.vin # 0914. starts kinda hard but runs like a champ once it's started. so i don't know if it has a sheared key or not. didn't check the torque

At my work we also use "Safety Critical Fasteners". I however have not heard of the re-Torque lessons learned. I think I will post this to our Safety guy at work though. Thanks!

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now