Solved: Stripped Transmission Mount Bolt

[alvocado]

I'm looking for some advice regarding a stripped transmission mount bolt and possible fixes. My transmission has been out twice since new: the first time in January when a shop rebuilt the trans and the second time in July when I pulled it for a rear main seal. When I had it out, I noticed that one of the bolts that attaches the mount to the rear of the transmission wouldn't torque down. I suspect the shop who did the rebuild used an impact to remove the bolts and install them, likely over torquing it and stripping the threads.

I replaced the mount a couple of weeks ago after hearing a clunk and even with the new mount, the clunk persists. I can replicate it by rocking the truck and it's coming from the side of the mount that isn't torqued down.

I don't think chasing the threads is going to help since the bolt spins and never tightens down.

I considered exploring a longer bolt with matching specs and seeing if I could get a nut on the top but there is a metal dome on the top of the transmission tail over each bolt hole that might be an issue.

I appreciate any input.

 

[Charlie207]

Are you 100% sure it's the transmission mount, and not an engine mount?

If it is the transmission mount, a blind hole, that's stripped.... you can maybe heli-coil it?

 

[alvocado]

Are you 100% sure it's the transmission mount, and not an engine mount?

If it is the transmission mount, a blind hole, that's stripped.... you can maybe heli-coil it?

100% sure. Engine mounts are two weeks old and when I have someone rock the truck side to side with me underneath, I can hear and feel the mount clunking.

 

[ReaperHWK]

Helicoil would be the proper fix for this.

 

[alvocado]

Two votes for Helicoil. I'm pretty sure this is a through hole with the bolt stopping before protruding through the top.

I've never installed a Helicoil so would this involve drilling the hole larger then installing a helicoil that matches the bolt size?

 

[Fless]

A helicoil requires drilling the hole larger, then tapping it. The helicoil screws into that, with its own external and internal threads, which then are the same thread size and pitch as the original hole.

Time Sert is also a solution.

https://www.partzilla.com/blog/heli-coil-vs-time-sert-thread-repair-kits

 

[alvocado]

A helicoil requires drilling the hole larger, then tapping it. The helicoil screws into that, with its own external and internal threads, which then are the same thread size and pitch as the original hole.

Time Sert is also a solution.

https://www.partzilla.com/blog/heli-coil-vs-time-sert-thread-repair-kits

Steve, do you like one over the other based on your experience?

 

[Fless]

Steve, do you like one over the other based on your experience?

I've done one Helicoil without issue but no Time Serts, so there's that.

 

[alvocado]

I have a Heli-Coil kit on order. I couldn't find much info on how to spec the insert length and the kit only comes with one option, the 1.5D length. Will post back after install.

I know I'm going to have clearance issues getting the cross member out since the passenger side exhaust blocks the cross member bolts. Aside from dropping the exhaust at the down pipe, has anyone come up with a trick to access those bolts? I was able to swap the mount without removing the cross member by lifting the transmission up enough to clear the mount bolts. The cross member will need to come out to get a drill and tap in there.

 

[alvocado]

Following up and happy to report the Heli-coil install solved the issue. It's a little unnerving drilling out a hole but the coil inserts are pure genius.

I would do one thing differently the next time around. I drilled the hole to a depth for the insert plus a one-thread recess. I should have drilled the hole to at least the bolt length as once the bolt passed the Heli-Coil, it was difficult to tighten because the bolt was grinding its way through the remaining old threads. Fortunately it didn't take much to get it into aluminum but I clearly made an installation error. The Heli-coil locks the bolt so the end threads are expendable.

PSA for anyone swapping or removing a transmission mount: R&R the bolts by hand. The aluminum threads are so soft on the transmission tail and easy to damage.