Driveshaft bolts sheared off!

[mitchlevy7]

I was getting on the interstate and giving it some gas and i lost power,the car redlined and it sounded like something went bad in the driveline/transmission. When i got it towed and took a look at it, the bolts that connected the tranny output shaft to the guibo were gone!

I got new bolts and put them in, and the car drives fine now.
but when i put the new bolts in I found a driveshaft bolt that was sheared in half. (only found one, figure the rest are somewhere on the highway at this point)



Has this ever happened to anyone?
As an engineering student, it almost makes sense that if the bolts were old enough and have gone through enough stress cycles that they would have a lower ultimate strength then a new bolt as a result of fatigue stress. However, these are driveshaft bolts, vital to the driveline, so wouldn't bmw make them with the strongest steel they can? At least something with a pretty high safety factor.
I am running an m50 with an intake and a g260, so I'm not giving it an unreasonable amount of torque, especially since people are using these same bolts in turbo s52s and such.

I've also thought about, What if a bolt was loose? However, the way the bolt broke looked like it was in pure shear, and I trust the mechanic who touched my driveshaft last, and I watched him put it in.

Discuss.

[LJ851]

Take the guibo off and look at the output flange, if the bolts were loose before failure it should be obvious.

[jlevie]

The wrong bolts could have been used, they could have been over torqued, or loose. What grade does the head stamp show the bolts to be?

[SlamedIAm]

When we did my guibo a month or so ago I had a similar situation. My bolt wasn't completely sheared but it had been worn down to about 1/3 of the diameter due to it being loose.

[mitchlevy7]

it makes sense that something like this would happen if the bolts get loose, but i trust the mechanic who installed the driveshaft. Also, they are locking nuts so they shouldnt come loose.

[audiquattrot]

had this issue as well on my guibo.

[LJ851]

it makes sense that something like this would happen if the bolts get loose, but i trust the mechanic who installed the driveshaft. Also, they are locking nuts so they shouldnt come loose.

As a professional mechanic (and a perfectionist) for the last 15+ years, i can assure you that everyone makes mistakes sometimes. The locking nuts could very well have stayed on until the end, but as long as the bolts weren't fully torqued (or over torqued) they would still be able to move back and forth and wear themselves thin enough to break.

[M-technik-3]

Stretch bolts, factory considers them one time use. Granted 10.8 should last longer and will but they are cheap.

[joncrz]

Stretch bolts, factory considers them one time use. Granted 10.8 should last longer and will but they are cheap.

Positive ? I am about to work in this area on mine and now I need to know where I can find these bolts.

[M-technik-3]

I was mistaken on my quote it was replace the nut after each use.

[joncrz]

I was mistaken on my quote it was replace the nut after each use.

what?



Also, OP where did you get the new bolts?