05 V70 2.5T: engine mount issues

[RoVolvo]

210k miles. Old mounts seemed bad - car was vibrating terribly. Replaced the mounts with the Hutchinson kit on FCP Euro. They did not include all of the right bolts and nuts, so I re-used some from the mounts I removed.

At first car felt fine, but then it started vibrating again. I dropped it off at mechanic for an exhaust repair and they said they found the mount to be loose and corrected it. I drove a bit and it felt very smooth… then it started vibrating again. I got home and the rear mount was loose again. I get my torque wrench, torque it to spec. Today I drove a mile before it started vibrating again.

I completely uninstalled it, examined it, and found it wholly unremarkable. I just cannot understand why it’s coming loose. I’m using proper torque spec for bolts. Threads seemed fine. What could possibly be causing/enabling this? What are the proper boots/nuts I should use? Idk what else to do at this point. I don’t even understand how this is possible.

[BlackBart]

Do they have lock washers on them?

[jonesg]

try loctite. or spot weld it.

[RoVolvo]

Do they have lock washers on them?

No, definitely not. Should they? The ones I removed did not