OT - Tapping A 10MM Weld Nut To 12MM - Doable? (Hitch Install)

[DerbyDad03]

I was attempting to install a hidden hitch on my van today. Before I reached the specified torque given in the instructions for one of the 10mm bolts, the weld nut on the vehicle stripped. The bolt still threads in, but never tightens fully. It's not the bolt, it's the weld nut.

There is no access to the weld nut from under the vehicle. There is nothing strong/thick enough in the area to drill through for a nut and bolt.

I'm thinking of tapping the 10mm weld nut to 12mm and using a 12mm bolt for that one hole.

As described in the image below, there are two 12 mm bolts that hold the hitch on the vehicle's frame up from the bottom (one on each side) and four

10mm bolts that go through the face of the hitch into the holes that also hold the bumper bar into the frame. The facia then covers the bumber bar and hitch, leaving only the receiver showing.

The upper right 10mm weld nut is now stripped.

All suggestions welcome. Thanks!

[hubops]

Crap .. everything was going so well - until it stripped. :-(

1. Are you actually pulling a loaded trailer ? I installed a hitch on our Jetta - for bicycle carrier only !. .. was surprised to see a very low torque number 26 ft. lbs. ? for the two bolts that held at the rear exhaust support. The other supports - frame and trunk floor were solid.

Sometimes, the internet sellers have better-than-average online support - it's their bread & butter ..

Good luck. John T.

[DerbyDad03]

Yes, I'll be towing a trailer. Not as often (or as heavy) as I used now that the kids are gone, but I still tow. I have a 5 x 8 enclosed trailer. Towed 1500 lbs and 1250 lbs a few months ago.

The torque is 76 ft-lbs for the 12mm bolts, 59 ft-lbs for the 10mm. I know I was getting close, but I don't actually know when the weld nut stripped.

I'm not going to try for 59 on the other bolts, but I still want to use all four. As noted earlier, those 4 bolts also hold the bumper bar onto the frame.

[Clare Snyder]

I'd try a Heli-Coil type insert .. Takes virtually the same drill size so take your pick, I guess. .406" (10.3mm for helicoil, 13/32 for

12mm - virtually the same)

[Clare Snyder]

Or just pop it out to 7/16" and use an SAE Grade 10 bolt - smaller hole than 12mm (.39 instead of .406)

[J. Clarke]

I'd go for the Heli-Coil too. You have determined experimentally that the weld nut was not hardened to spec. Putting in a helicoil with a

10mm thread should let you tighten to spec without stripping the hole.

Oh, and if you haven't done it recently you might want to check the calibration on your torque wrench.

[Dr. Deb]

J. Clarke's helicoil suggestion is the way I would go. Its a bit more work, but you will have hardened threads, vs what you have now.

[DerbyDad03]

Thanks. I'll order a kit.

I actually started thinking about a Heli-Coil late last night and planned to look into them today. I'd heard of them but never used them. Guess it's time to learn.

[DerbyDad03]

re: "I'd try a Heli-Coil type insert"

There are a bunch of "Heli-Coil type" kits available that are way cheaper than the actual Heli-Coil kit. Am I paying extra for the Heli-Coil name? Are the cheaper kits (Highking, E-Z Lok, etc) serviceable for a (hopefully) one time use?

Maybe the inserts in the cheaper kits aren't as hard...

[DerbyDad03]

Needs some more advice on the Heli-Coil.

How does the installation tang on the insert impact what I'm trying to do?

Do I need to order a tang removal tool to ensure that the bolt is free to travel as far as it has to?

Keep in mind that I'm dealing with a weld nut, which is pretty shallow (not many threads) and open ended. I haven't measured the bolt, but I'm assuming it extends all the way through the weld nut.

[J. Clarke]

Just to give credit where it is due, Clare came up with the helicoil suggestion. I just added my 2 cents worth.

[dpb]

...

No reason to think to ever remove it is there?

Put the bolt in, tighten as it will, then just spot weld the head.

[DerbyDad03]

2 issues:

1 - You can't reach the bolt head with a welder once installed. The bumper bar hangs from the same set of bolts. The hitch is sandwiched between the bumper bar and the frame of the vehicle. The bolt head ends up deep inside the corner triangle seen in the image below. It's hard enough to get a socket on it since you have to feed it through 2 holes to reach the "keyhole" that is shown in the second image.

Bumper from the top:

Here's a picture of a guy about to hang the bumper bar on the exact bolt where I stripped the weld nut. That "point

2 - Maybe I'm missing something. Even if the bumper bar wasn't involved, what would be the point the spot welding the head to the hitch? Sure, it wouldn't turn, but what about the outward & downward force on the weakened threads while towing. Isn't the point of possible failure those weakened threads?

If that is not an issue, I might as well just put some lock-tite on the stripped one and mainly rely on the other 5 bolts.

[knuttle]

and bolt of the appropriate size?

[DerbyDad03]

There is no access to the area behind the weld nuts.

As far as I can tell the order of the build was to weld the nuts to the part of the frame that runs across the back of the vehicle and then weld that part of the frame to the square tubes that run towards the front of the vehicle. (The tubes that you would hang a hitch from if you were mounting one of the exposed kind) The weld nuts are essentially inside those tubes.

What they did seems to make sense because those weld nuts are really for the bumper bar that is hidden behind the fascia. If the vehicle was hit from the rear, the curved bumper bar would transfer the force to the square tubes which are pretty darn solid. That would hopefully keep the vehicle that rear ended you out of the passenger compartment.

[Ed Pawlowski]

You can tap the nut larger but it still won't work. No matter how many times you run the 12MM die over the 10mm bolts they will still be loose. I have no idea why that happens but I guess it is a metric thing.

Glad I could help.

[dpb]

On 9/12/2020 11:18 AM, DerbyDad03 wrote: ...

You already said able to snug it up reasonable; just anything to keep it from backing out will adequate at that point.

Ayup...that was the other suggestion I was going to make. Permanent type.

--

[Clare Snyder]

This is still the SIMPLEST fix - you might call it "gerry rigging" or "Bastardizing" due to use of a non-metric thread - but it is the cheapest, simplest solution - error - use "grade 8" SAE bolt no such thing as a "grade 10" bolt

[Joe Gwinn]

For one insert, or even four, there is no savings by the time you get to done.

Weld nuts can have pretty thin walls. Is there enough meat for helicoils to work?

Grade 8 bolts may rust too quickly.

Joe Gwinn

[DerbyDad03]

But there is a 10.9. I'm sure you know that.