Failing engine A-Mount (bad design by the automotive manufacturer?)

Question

I currently own a Tata Nexon Petrol car (in India) where I have seen (on facebook, Telegram, WhatsApp owner groups and youtube) that the A-Mount on the engine has suffered multiple failures for some of the owners (though significantly less numbers). This A-mount uses three M10, 10.9 metric grade studs.

The image of the mount:

For some owners, they broke at as early as 5000kms, and for some after 50,000kms, and for some at 70,000kms. Due to the reason the number of failures is less or we can say the company is unethical, they have not recalled these vehicles for any resolution to this issue since the year 2017. Every other car engine in this segment of engines with similar displacement sizes seems to have been using at least an M12, 10.9 grade on the mounts. Sharing the pictures of the broken mount, It actually breaks due to the failure of these M10 studs:

I am worried that it will fail on my vehicle as well at some point in the future. Is there any solution so that I can add strength to the A-mount joint myself? Like I was thinking of doing one of the following but wanted to ask what you think should be done or if is there any other better option:

1. Replace those OEM 10.9 grade studs with 12.9 grade studs but those are hard to find.
2. Thread tap the center stud for a bigger M12, 10.9 grade bolt which will eventually strengthen the joint.

Answer 1

I think there are two problems (at least) in this case, as follows.

First, it is safe to acknowledge that the failure rate of these engine mount studs is not 100% (because there is no recall!). every surviving stud assembly in the field is an existence proof that the motor mount can be made to function as it should, and that there is a hidden variable in the motor mount which in some cases ruins the longevity of the joint, and your photos illustrate what the hidden variable(s) is/are.

First, some of the studs clearly show fatigue failures meaning that the combination of tensile stress (from tightening the nut on the stud) plus the cyclic stress (from the engine vibration) is sufficient to walk a fatigue crack across the stud cross-section in some but not all cases.

Second, the photo of a boogered-up stud (bent, and with its threads scuffed and smooshed smooth!!) indicates that the bolted joint was loose for an extended period of time before the joint failed.

Both of these things point to improper torqueing of the nut-and-stud joint at the factory, which would cause overstressing and fatigue if overtorqued or loosening up and scuffing of the joint with use if undertorqued.

Here are my recommendations.

1. remove all the stud/nut assemblies from all the engine mounts in your car and replace with factory-new items.

2. reassemble the joints with lock washers and/or thread-locking compound to prevent the parts from unscrewing themselves during use

3. determine from the service manual what the factory-recommended torque setting is for those assemblies, get you a good-quality torque wrench with which to tighten them to that setting, and tighten to that specification- and finally,

4. check that torque setting several times a year when the car is being serviced.

Answer 2

Hmm, that's a rather ugly design, vertical loads in the gearbox mount will cause a prying action on the studs. As such your idea of bigger or better studs should help. However the endurance limit of a 12.9 is no better than a 10.9, unless you use a higher torque on the 12.9 which risks stripping the thread in the gearbox. So the proper thing to do is to helicoil the gearbox, use a 12.9 and a higher torque.

I expect the load case causing 5000 km breakages is idiot starts. A solution to that other than the obvious is to put a torque reaction arm between the top of the engine and the firewall.