[TighteningBoltz]

If the bolt is indeed an M12, then I would find a flange head M12 bolt long enough to protrude slightly past the end of the nut. I’d get it from the auto parts store before home depot.

Bolt length is measured from under the head to the end of the bolt (not the overall length). Most automotive fasteners are marked 10.9. This is a metric classification of bolt strength similar in strength to an SAE grade 8.

The website indicates a torque spec of 75 lbf-ft (102 Nm) although typical installation torque for an M12 is 84 lbf-ft (114 Nm). This lower torque is probably due to the lower strength of the aluminum threads which you already found the limit to.


You don’t have to go crazy with torque accuracy. Feel how the bolt tightens. The torque should build evenly with as the angle turned. If torque stops building and you’re still turning the wrench, stop, you have gone too far, replace the bolt and try again.

The 10.9 designation is an indication of the max axial stress of the bolt material (10 means 1000 N/mm2).
That means each square millimeter of cross-section can hold a minimum of 1000 N (225 lbf)
The second number (.9) refers to the yield stress of the material being 90% of the max.

The stress area of a M12 x 1.75 is 87 mm2.

That means the ultimate tensile strength of the bolt is 87,000 N or 19,700 pounds!

 

[TighteningBoltz]

For that particular bolt's use case, shear will be more important than ultimate tensile strength.

Plus, with a 10.9 bolt and aluminum threads, if the torque stops building and the wrench keeps turning, chances are pretty good it's the aluminum that is damaged, and not the bolt in my experience.

Hope that helps,
Mark

I agree, this joint is loaded in shear. The bolt, however, is not (at least not in the simple form as a pin would be). When tightened, the bolt elastically stretches and creates tension force (preload). This in turn creates compression force (clampload) on the bracket and bushing. The friction between the joint surfaces (bushing and bracket) that is created by the clampload withstands the shear forces applied to the mount.

The threads stripping is really the result of a flawed design. A failure test of the bolted joint should always result in a tension failure of the bolt. There is not enough thread engagement and/or material strength in the female threads.

 

[TighteningBoltz]

It's like I'm re-reading one of my textbooks, lol!! Between the post, and the user name, I am going to assume that you may have properly torqued a bolt or two, rofl.

Haha, yes a profession and a passion. I chose my username because it seemed like the topic I’d be the most likely help to others with here as I know nothing about tuning or racing.