Broken Crank/Flywheel Stud!!!!!!!!!!!

16 replies to this topic
  • Kylelind

Posted March 02, 2008 - 07:36 PM


I was just out installing my e-line external stator on my 2000 426 and when tightening the nut onto the crank I broke the stud. The directions I found said to tighten it until the wavy washer just barely went flat and it was not even there yet???? I later realized that I was putting way more than 35ft-lbs of torque on it, DUMB-ASS.

What are my best options for fixing this piece? Just the crank and bearings or more while I am in there. I also read that it is best to buy the whole crank assembly for $350 rather than the one part I broke for $150 and have it replaced and balanced and all of that.

If anyone has done this before please send me a list of parts needed and what all I should do while I have the engine apart. Thanks.

  • grayracer513

Posted March 02, 2008 - 09:18 PM


You could replace the left side of the crank, but with the age of your bike, you should also replace the rod, bearing, and crank pin, so by the time you figure labor, the complete crank is usually cheaper

While it's down, might as well replace the engine and trans bearings, check the bore ad piston, and drop a set of rings in it. Check the head, too.

  • Kylelind

Posted March 03, 2008 - 08:54 AM


Thanks for the input grayracer513. Any more suggestions everyone?? What portions of the build require more than my garage?? And how can I figure out all of the parts I will need to order them in one shot?

  • Kylelind

Posted March 03, 2008 - 11:36 AM


Besides easier starting, does the 03 exhaust cam mod improve anyhting? Should I replace it while I am in there?

  • grayracer513

Posted March 03, 2008 - 11:51 AM


Besides easier starting, does the 03 exhaust cam mod improve anyhting? Should I replace it while I am in there?

Broader, more linear power, and yes, you should do that if you can afford to.

  • mkporn

Posted March 03, 2008 - 01:59 PM


I had this happen to a buddy, believe it or not, I cut the stud off and being careful I hand drilled the center of it about 3/4" deep, threaded it and installed a stud with red lock tight. the stud will stick in to the keyway a bit so I used a cut off wheel on a dremrel tool and notched it for the key to fit.

didn't have to even pull the motor. I taped the entire area with duct tape before I did it.

This fix has well over 200hr on it with out so much as a hiccup. This bike does not get ridden kindly either.

Good luck, I can tell you this will work and won't cost more than a couple bucks....

  • Kylelind

Posted March 03, 2008 - 03:02 PM


How do you center the hole and get it on axis good enough to make the flywheel spin true? Especially on the external stator there is only about .020” of clearance between the flywheel and coil?? I am willing to try but am not sure it will work? Any suggestions?

  • Kylelind

Posted March 03, 2008 - 03:07 PM


Actually the taper does most of that would still have to be pretty darn close to axial though, the stud can flex a little

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  • Kylelind

Posted March 03, 2008 - 09:37 PM


Well, I have decided to do the stud replacement. It is an M10-1.25 and I am going to buy a grade 12.9 bolt that is 40mm long used with high strength threadlocker. I think that will be long enough if I thread it in about .5" which gives almost 1.5 times the diameter of the bolt. I think that is considered full strength. Thanks for the suggestion!!

  • LSmith

Posted March 04, 2008 - 04:51 AM


Do you know if the crank has ever been replaced before? The bike is 8 years old and even though Yamaha's are very reliable I would seriously reconsider. If something were to let go because of fatigue, you would be looking at a much larger repair bill. Something as simple as the cam chain sproket (they wear fairly quickly unless you replace the cam chain regularly) could allow the chain to slip, jump time and bend a vavle or two. Then you could add a new head and valves to the bill. Just my opinion.....

  • mkporn

Posted March 04, 2008 - 09:22 AM


It's not to tough, and actually all you are looking for is for it to be as close as possible to center and straight.

cut the end flush so you have a good surface to work with. Get a metric stud....they are high grade and will work the best.

Center punch the crank shaft end and drill with a pilot 1/8" bit. Then follow ith the larger bit. The pilot will allow you to work a bit slower and with less force, allowing better accuracy. The lager bit will center up and follow the pilot better.

Tape the hole and install the stud with lock tight.

This does not require a very large stud. the retaining force of the stud does not need to be that great. The shaft is tapered so centering is no problem. The nut holding the flywheel is very low stress. think what it was like to pull the flywheel in the first place.

  • Kylelind

Posted March 05, 2008 - 07:50 AM


So, I started drilling last night and was having a really hard time getting the drill to stay put when drilling the pilot hole. Even though I center punched the shaft, the drill would walk to the side. Now I am trying to make a fixture to hold it all centered and straight.

My problem is that the external flywheel centers on this stud so it needs to be very well centered. We will see what happens.

  • USED YZ426F

Posted March 05, 2008 - 08:21 AM


Double check that your center punch has a definite point on it. If it doesn't, either sharpen it up on a grinder or find another center punch. If it does, strike it harder with the hammer and make a definite dimple for the drill bit to start in. Sharp drill bits will also bite better and not be prone to wandering.
A smaller pilot hole may also help. If you are starting with a 1/8" bit, try a 3/32" or even a 1/16" bit to drill your first hole.
You can always drill a larger hole once you get the pilot drilled as close to center as possible.

  • grayracer513

Posted March 05, 2008 - 09:06 AM


My problem is that the external flywheel centers on this stud so it needs to be very well centered. We will see what happens.

There is no way you are ever going to get this fixed by hand well enough to run the external stator if it in fact depends on the crank thread to center up. You engine turns 11,000 RPM, after all.

A repair like this is a band-aid fix for a bike with a stock flywheel, and works in that case because the stock flywheel is supported and centered by the taper on the crank. All the threads do is hold it on, and for that, being slightly off center or out of square would not matter much.

  • Kylelind

Posted March 05, 2008 - 02:46 PM


I am having a fixture made that centers off of the taper so it should be close. I am going to give it a try and see how centered I can get it. I will let you all know how it goes. Thanks for the input.

  • Kylelind

Posted March 05, 2008 - 06:43 PM


So, I drilled it today after getting the fixture. The fixture worked great and the hole looked very good. The problem was that when I hand tapped it with a bottoming tap it went in a little crooked...oops.

So on to the next step of the fix. Now I am going to have the external flywheel machined slightly to center off of the stock flywheel center flange. Then all i need is tension from the bolt. Good to go hopefully. I will update again after I get the machining done.

If anyone ever reads this and needs the drilling fixture let me know.

  • Kylelind

Posted March 06, 2008 - 09:41 PM


After making a recess in the external flywheel and a thicker spacer to fit into the recess then locate off of the stock stator, it is GREAT. Very concentric and true, even better than it was originally. Now we will see if it all holds together. Thanks for all of the help!

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