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Discussion Starter #1
So somehow, two of the holes that the transmission crossmember bolts into got stripped, and unfortunately, they're both on the same side, so I can't just hope the remaining two bolts hold.

Is there enough material in the underside/nut to drill out and helicoil? If not, how would any of you with more sketchy repair experience handle this? Maybe I should drill and tap another hole in the crossmember and the car (not quite sure of the terminology of the underside surface of the car) between the two stripped holes?
 

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Can't tap sheet metal....
The OEM set up is probably welded in inserts.

Look up 'Nutserts', or 'threadserts' they may save your bacon.....
Like a giant pop-rivet.
They take a special tool install, but I have used just a nut, bolt, and washer to put them in.
Pretty sure they are available in 10mm, but 3/8" fine would work for you also.



Used them for my under-axle chassis braces.

 

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Can't tap sheet metal....
The OEM set up is probably welded in inserts.
Pretty much this. The factory setup is a couple square weld nuts welded on the back of a piece of sheet metal, and they are not big enough for helicoils. I've tried and the weld nuts on the backside of the sheet metal just split open

Depending on the size of the rivet nuts you go with you may be able to get them to work. The problem here is getting enough of the old weld nut gone for the rivet nut to grab.

It just so happens I removed the sheet metal panel on the driver side of my car so I can get you a picture later this evening to see what you're dealing with on the inside.
 

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Discussion Starter #4
Thanks for the info! Never heard of these before, but they look very useful. Looks like I might be drilling those nuts completely out and expanding the hole to 14mm, allowing an 8mm nutsert and retaining the original bolt sizes.

Is the sheet metal by itself strong enough to hold these? They'll be exposed to normal vibrations and accelerations, and each bolt would be holding around 25-30 lbs each in axial force. It would be a bad day if those inserts decided to shimmy themselves out.

I suppose an alternative would be to just drill the threads out, which would allow 5mm bolts, and for security drill an oversize hole between the two for a 10mm bolt.
 

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Rivet nuts are very sturdy when installed properly. Dont oversize the hole too much and make sure you get it pinched down tight enough and they'll be fine. The method sixpack uses is pretty much the same as what I've done for many years and never had one fail.

Here's the inside of the panel where the mounts are:



The holes were drilled out for 10mm helicoils and as you can see, there wasn't enough there for that to work.
 

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+1 for rivnuts
 
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