83 XJS. Video explains better than text:
But if you prefer reading: the front passenger seat rear rail-to-body mounting bolt (the one closer to the driver’s side) won’t thread in because there appears to be a nut on the other side that needs to be held steady while tightening the bolt. Problem is access. I pulled the carpet and that particular bolt is not mounted to a hollow “tower” like the other seat mounting bolt. I looked under the car and don’t see it hiding under there either.

But based on what few photos and videos I’ve found, it looks like this isn’t a nut and bolt setup - rather the female threads are fixed in place. Mine is clear moving around though so I’m not sure what to make of this. Any help is appreciated.

 

EDIT: Found a couple threads conveying the same issue. Not an easy process - just have to find a way to hold that bolt steay; I'll likely have to take out the bolt, apply super glue around the edges of the nut and then pull that back upwards using a dental pick and hope it bonds strong enough to stay put while I tighten the bolt down. Fun!

 

So its either a nut plate or a rivnut? Either way yeah those types of fastening methods can be a pain in "blind" applications. Good luck. is there a picture of the fastener?

 

For anyone curious, here’s a picture of the nut on the underside. Used my little wire camera to get a look.

 

Perhaps you can drill a small hole right next to it and use the drill bit to keep that from spinning as you tighten the bolt? Best thing is to fix it properly but i cant figure out what that entails. Sorry.

 

Well things got interesting, but maybe for the better. I ended up using a long allen wrench to wedge between the "wall" and the nut pictured above to keep it from turning. I feared I was cross-threading so I backed it out. Naturally, that's when the nut decided to fall into the depths of insulation and whatever else, so that's gone now. Good riddance, assuming it doesn't rattle around while I'm on the road.

But I think I have a better idea. Because it's a nut-and-bolt assembly and not a bolt-to-body assembly, I only need a bolt that is close to the diameter of the hole, and thread count doesn't matter a great deal. So I'm going to go to the hardware store and get a bolt near in length/diameter/thread count and then the party piece: a flange nut. With a flange nut, I should be able to set it inside a close-ended wrench and then stick the wrench in the little crevice without worry of the nut falling through the wrench opening. The nut only needs to be shallow enough to squeeze into the crevice. The rest writes itself.

Just fyi - the crevice I mention is 2-3 inches forward of the mounting hole, but it's a slim opening, so a standard wrench & socket absolutely would not work here. Awful design.

 

Kirby Palm's BOOK has a fix for this issue, pages 485-486. Well worth doing.

 

You're the man, thanks! Just ordered the bolts and clips he described.

 

Well this is interesting. As always the video explains more:
How is this u nut supposed to work around the permanent square nut, and what is the latter even for?

 

The clip nuts are to replace the loose front nuts that are hard to engage. You continue to use the fixed rear nuts and bolts. If you broke the rear nut loose, I can't help you.

 

Kirby states he used the long u-nuts on the holes nearest the console - this one qualifies unless I’m misreading this passage:

 

Notice the first sentence where he says "at the front". When he talks about the locations later, he's talking about the front mount near the door and the front mount next to the console. You use the shorter u nut on the front mounting hole near the door, and the longer one at the front mount next to the console.

 

His use of the plural “locations” in the last paragraph is what threw me. He must have been referencing the front console-side mount for both seats. I will attempt my flange nut method tomorrow.

 

Mine is an '87, which might be similar to an '83. I've removed and replaced the seats a couple of times and those nut plates are a pain. Each seat has one fixed nut (rear inside) and three of these floating nut plates. Maybe in those days they couldn't make things accurately enough, so one fixed location + 3 floating gives a bit of "wiggle room" to mount the seat. The bolts are too short to pick up the nut plate, so the nut plates need to be held in place for the bolt to screw into the nut. I use a telescopic magnet to locate the nut plate, and a trim removal tool under the nut plate to hold it up against the underside of the mounting bracket. I use a drift to centralize the nut plate and then screw in the bolt and washer. Usually I get it done in two or three tries. Don't forget the spacers under seat rails that sit in the holes in the carpet.

HTH, Dave

 

Well I fixed it, but very much in my own style.
Short version: flange nut worked, but couldn’t then remove the wrench, so decided to donate the wrench to “the cause”. The bolt is tight, and wrenches are cheaper than the time to find a classier solution.