Broken Unibody Seat Weldnut

[Nukeman7]

Weld in a patch from above or below?

If you want to get the best advice on how to resolve this problem, you should post photographs of both the interior topside where the bolt goes through the floorboard and of the exterior bottom side directly beneath the floor penetration. Once we have a visual understanding of the physical layout, we can advise you from an informed perspective. Otherwise, everyone is mostly shooting in the dark on the possible/best solution.

If you know for certain that you can drill through the bottom exterior directly to the interior opening, I would consider an alternate approach. Get an appropriate length/diameter HARDENED bolt and fender washer and weld it to the underside so that you effectively have a stud protruding up from the floorboard. Use a lock washer and nut to fasten the seat leg to the floorboard so you won't have the problem again.

 

[burbguy82]

I'm never removing another seat again to clean underneath.

I think this is a good idea. I assume this means that you have done this before? Had the bolt break that secured the seat to the frame.....yeah, I would stop what ever you are doing.

 

[D60]

I can't find where the original weldnuts are from below, but, I can almost certainly drill a hole all the way through whatever structures there are between the top of the seat and the very bottom of the car.

If I weld in a patch from above would it be advisable to weld in a patch on the adjacent bolt so that the seat is at least level?

If I did an interior patch I'd likely use 1/8" flat bar with a nut welded underneath.

You could probably get away with 14ga and there is a compromise at 12ga but it's kinda hard to find.

If you manage to pull a nut through 1/8" in a crash, I promise you you're already dead, guaranteed.

If concerned about level just shim the opposing side with a washer or two. Basically, any degree of "unlevel" will be minor.

If you choose to through-drill (not a bad idea), just be certain you won't hit anything like fuel or brake lines.

If you through-drill with a 1/4" bit you can then use a hole saw underneath to create access for a heavy duty, oversized washer, nut and socket. Then you can capture the internal layer of floor with no air gaps and concerns about sleeves.

Think about how transmission xmembers are often boxed but just provide holes for sockets to pass thru and reach a nut at the "upper" layer

But yeah, without pics my vision of what you're dealing with is largely a fiction in my tiny brain.

 

[Deontologist]

I forgot to take pictures but I took a look today, and it appears that I can drill a hole straight down from the seat and through a longitudinal cross-member without hitting any fuel or brake lines or wiring harnesses.

I like the idea of a bolt sleeve.

I have ordered some grade 8 structural washers from McMaster Carr

I will be using a grade 8 bolts too.

Now I just need to find some hollow steel rod that I can cut to size.

If you through-drill with a 1/4" bit you

Are you suggesting that I use a 1/4" diameter steel bolt?

 

[Deontologist]

I have drilled a through hole.

I have taken pictures from above and below.

There is absolutely no access directly to the floorboard where the original weldnut was from underneath the car. Not unless I take out the angle grinder and start cutting away structural monuments, from what I can see underneath the car. I can't even find where the broken weldnut fell. It is trapped, internally.

As you can see with the pictures, the through hole goes through what appears to be a cross-member.

There's about 4 inches between the top of the floor pan and the bottom of the cross-member.

My primary concern is: Fastener pull-through caused by crushing of the hollow section. The bolt has to travel through 4 inches just to go through the other side. The bolt is going through a hollow cross-member.

My instinct is to:

1) Use structural washers to distribute the load

2) Fill the hollow cavity with rigid pillar foam to reinforce the cross-member from crushing in case of a frontal crash.

I do not have access to weld anything internally.

I think that by using structural washers and a grade 8 or class 10.9 fastener, I will have sufficiently clamped the seat to the chassis.

I think that the structural foam is a good idea, as it reinforces the hollow section that the fastener has to pass through.

It also seems that BMW has used structural foam to reinforce cracked subframes. Not exactly the same idea as preventing a hollow cross-member from crushing, but, it seems that structural foam can be, indeed, structural.

 

[Jetronic]

So if you can't see the nut on the underside, then where is the nut at? In an enclosed area of the uni body frame?
Fire can be an issue when welding on an assembled car so be careful. If the nut is hidden, I'd cut a small window around the nut, and remove it and make the patch with a new nut, but like others say a larger plate will be best to distribute the load. Where is a photo of it?

it always is on it's own beam

 

[D60]

Easy. Drill that exterior hole to a touch over 13/16". Get a length of 1/2" Sch40 about 4" long <-- your measurement. Use a unibit and the body will drill like butter.

Cut the pipe so it's just barely shorter than the internal gap. The idea is to slightly preload or even BARELY crush the body when a bolt is snugged.

Use a 1/2" bolt of appropriate length. Drill inside seat track and floor to 1/2". Use loctite. Washers on the outside. Done.

NOTE if round washers won't fit outside no one said a washer must be round. You can cut it OR drill flat bar to make a long, narrow washer -- think wall toggle.

 

[Silverado12]

I would get the correct length and diameter bolt, a nylon locknut, a flat washer and a lock washer. There's a possibility that by using a lock washer alone it will eventually work loose but the lock washer along with a locknut (and flat washer to distribute some pressure at the frame) will give it a little tension so it won't rattle. The locknut will keep it from working loose and falling out. IMO we're overthinking this; I don't really see anything wrong with going through the two layers of metal with a long bolt. Just quit tightening if you see it start to crush.

 

[Silverado12]

Another thing is that you can get a threaded clip nut (not the light duty ones which only have a couple threads). Enlarge the hole in the floor so the clip nut fits. Those things are pretty strong and should hold one leg of a seat.Clip Nut