Motor mount bolts keep breaking

I've had my 2010 tc 250 since october but in the last month i just keep breaking motor mount bolts.Yesterday they both broke. They seem to all shear off right at the start of the threads. It happens no matter where ive ridden which has been all track. I've tried loosening up the swingarm pivot to get the motor to naturally sit where it wants and they still break. The tally is up to 6 broken bolts now. Any suggestions? Or anyone with the same problem?

Are you using 10.9 bolts?

proper torque per bolt size/ per manual? or strength tightening, all bolts have shear strengths not to be exceeded, everything has its limits. sounds like they are too tight, we have all had them become loose that is standard offroad check stuff. but shearing off is very unusual, note the hardware on our machines is good quality so standard torques should not be an issue

Can you post a picture of a failed bolt? It sounds as if you might be over tightening the bolt and nut combo, stretching the bolt and compromising its shear strength?

www.albatros-ul.dk/?download=Bolts and Tightening.pdf

Talk has been with the engine cases being soft that the bolts should break before damaging the cases from overtorque.

We never broke any but snugged them up twice and never move since..

Bolt failure is usually due to
a. Overload for a given bolt grade. The loading is common to all bikes so it's not likely it's being overloaded during normal use. Did you use the correct grade and torque?
b. Misalignment. Worth looking into. Especially since it could easily happen on motor mounts. The tooling might have been a little off that day.
c. lost preload or unsufficient preload. IMO, this is the most likely. Some are afraid to over-torque so they under-torque, which is just as bad. The material being bolted must be under sufficient preload to help support the bolt. Also, Carl mentioned soft cases. If they are soft to the point where they yield and the bolt loses preload, it could break.

Also, sometimes hardware that is under a lot of cycles or vibration (yes in this case) requires re-tightening a few times. I would double check your hardware grade and torque settings. Put a torque wrench on it as part of your pre-ride check to see if it's holding. Continue doing that before every ride. After a couple times, it should stop losing it's preload. It's common for us to do this on heavy equipment wheels lug nuts. They won't hold their pre-load when brand new but after being re-tightened several times, they eventually stick. If left alone, they break.

If it's still breaking, I would go up one grade and torque to the recommended preload for the higher grade. If it breaks again something else is going on.

I have had this problem on all of my Husqvarna bikes 08 TC 250,450, 09TC 450, I have not gotten my 2010 yet. Couple fixes torque spec. may be fine but does not last, 1) harder grade bolt, 2) check bolt every oil change for tightness, 3)check spacing between frame and engine if spacing is loose shim with a washer this helps a ton. 4) drill it out to a bigger size Yamaha Bolt that fits I do not remember the bolt but it works. If nervous drilling through a engine case and a I do not blame you do the later steps. Note I carry extra bolts. hope this helps. What skill level are you as a rider , I am A mx, AA off road ,skill I have noticed breaks these faster do to bigger hits etc.

+1. This speaks to pre-load. The material being clamped must solid, no gaps. It's hard to believe it would get out of the factory that way.