Stripped captive nut (tranny motor mount), what are my options?

[CheeseHelmet]

For the past couple weeks I've been trying to figure out how I can repair the threads of one of the captive nuts that holds the transaxle motor mount in place (the bolt size is M10x1.25).

Here's the nut in question (red arrow):

Here are the solutions that I've come across so far:

1. Tapping the hole to a slightly larger imperial size (like 3/8-16NC)
   There's not enough thread left for this to work, I've tested this on the bench (running a 3/8-16NC tap through an M10x1.25 nut) and the resulting threads are very weak.
   
   
2. Drilling then tapping to M12x1.25 or M12x1.5
   Is the captive nut big enough that I can drill a hole for an M12 bolt? I fear that I'll make the hole but then have nothing left to tap. Also, this would require making the bolt hole in the motor mount larger which I'm not too keen on doing.
   
   
3. Installing a Helicoil
   This looks like the best option, mcmaster-carr sells an M10x1.25 helicoil kit for $43 and it looks like I wouldn't have to drill too big of a hole (iirc the diameter of the hole only needs to increase by 0.5mm). Please tell me that the nut is large enough for this option to work ;_;
   
   
4. Installing a Thread-Sert
   I can't find a kit for under $75, that's a lot of money for something that doesn't seem to be much better than a helicoil...
   
   
5. Drilling a hole in the bottom of the frame and passing a long-ass bolt through
   Is this crazy enough to work?

Please share your thoughts/ideas, I'd like to finally get this fixed.

 

[RABID_MP5]

What's directly below the captive nut? If you can get access I'd drill it out a bit and then put an M10x1.25 nut right below it. Then use a slightly longer bolt, if needed. You could put a LITTLE JBWeld on it to paste it to the bottom of the old nut under a little preload. ?? (just to simplify future removals)

 

[Guest20182]

What about getting a thread restore kit from AutoZone?

 

[CheeseHelmet]

What's directly below the captive nut? If you can get access I'd drill it out a bit and then put an M10x1.25 nut right below it. Then use a slightly longer bolt, if needed. You could put a LITTLE JBWeld on it to paste it to the bottom of the old nut under a little preload. ?? (just to simplify future removals)

I'd love to do this, but I don't think the nut is accessible. I'd have to get the car on ramps to check that out. If anyone knows please post!

What about getting a thread restore kit from AutoZone?

Are they any good? The autozone kit is $30, not that much cheaper than the helicoil one.

 

[Tom03es]

Wait- is the exterior of the nut stripped, or did you mess up the interior threads of the nut or the exterior threads of the captive bolt?

 

[CheeseHelmet]

Wait- is the exterior of the nut stripped, or did you mess up the interior threads of the nut or the exterior threads of the captive bolt?

There are 4 nuts that are mounted inside the frame. It's the nut's threads that are stripped, the bolt looks brand new.

 

[Guest20182]

Are they any good? The autozone kit is $30, not that much cheaper than the helicoil one.

Ask to rent one. I've had good luck with the one they rent, I can't remember the name, but it was a $95 deposit.

 

[P-Funk!]

Could you use a smaller dia bolt and nut through the captive fastener?

 

[CheeseHelmet]

Could you use a smaller dia bolt and nut through the captive fastener?

Yes, but the question still remains: how do I access the underside of that captive nut?

 

[slavrenz]

Oh man, this sound like a pickle. I guess absolutely worst case, maybe you could cut out that part of the frame, weld in a new nut, and then weld the frame back together?

My vote is for Helicoil.

 

[AusOrion]

Oh man, this sound like a pickle. I guess absolutely worst case, maybe you could cut out that part of the frame, weld in a new nut, and then weld the frame back together?

My vote is for Helicoil.

When my gearbox got done, the shop I took it to did exactly this.

They cut away the metal piece that was holding the nut captive, got a new nut and tightened the bolt and nut together.

 

[CheeseHelmet]

Oh man, this sound like a pickle. I guess absolutely worst case, maybe you could cut out that part of the frame, weld in a new nut, and then weld the frame back together?

My vote is for Helicoil.

I don't have the skills to do that, nor would I like to cut the frame apart

Do you think the helicoil will be able to hold ~65ft-lbs of torque?

 

[CheeseHelmet]

I don't want to get too excited but I just went outside to see just how captive that nut is, and I *think* I might be able to get at it from the bottom. I was able to shove my hand under the beam and it appears to be open at the bottom. I need to get the car up on ramps to confirm, but sadly it's raining here and no one's around to help ;_;

 

[RABID_MP5]

If you can get access, you might be able to shove a BOLT up the enlarged hole and secure from above with a nut. Whatever is easiest.

 

[CheeseHelmet]

If you can get access, you might be able to shove a BOLT up the enlarged hole and secure from above with a nut. Whatever is easiest.

If I can get access I'm going to go with your original suggestion (JB Weld a new nut to the bottom of the old one) since I *think* I'll be able to use the original bolt. However, JB Welding a new bolt up through the hole (making it a stud) is also a good idea. Thanks man

 

[CheeseHelmet]

I finally got a chance to look under the car, I was wrong

Looks like the helicoil is my only option for now.

 

[CheeseHelmet]

Good news! I installed a helicoil today and the mount is torqued down to 60ft-lbs. I could have gone way more though, those helicoils are damn strong.

I got the kit from McMaster-Carr, I wouldn't recommend anyone else get their kits. For $43 you don't even get a drill bit and it's not even helicoil-brand inserts. The bad tap lost a couple teeth while I was testing it and the helicoils themselves had little bits of metal in them that I had to clean out. If I have to do it again, I'll just get a $20 kit on amazon ;_;