Welcome to our Site for MG, Triumph and Austin-Healey Car Information.


MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - seat captive nut broke

Last year I installed new floor boards that had seat captive nuts already installed. The floor boards otherwise seemed to be good quality and fit very nicely.

Finally finishing the interior and installing the seats when one of the front captive nuts broke loose. Of course it is located on top of the box.

If I cut a small access hole in the rear of the box below the seat bolt will this have a serious effect on structural integrity.

Steven Rechter

After reconsidering, I am going to drill a hole through the bottom of the cross-member box and get a long grade 8 bolt, nut and washer.
Steven Rechter

I have done that before - drilled right through and used a long bolt and nut, with a penny washer to spread the load. When I put the new floors in my present car I reinforced the captive nuts with a couple more blobs of weld because I knew they are prone to snapping off. Hindsight is always 20/20 vision isn't it!
Mike Howlett

Long bolt will do but you won't be able to fully tighten, but then again do you need to? Going all the way through the box section, as suggested, then enlarging the box section hole to allow a sleeve to pass though, would allow you to fully tighten the seat bolt.
A nutcert, aka threaded blind rivet would be another choice.
Allan Reeling

Forgot to fit my nuts before welding the floor in place! I cut a slot where each nut would go, the width of a nut long and slightly more than the thickness of a nut wide. Welded the nut to a small strip of metal the width of the nut in one direction and slightly more in the other. Screwed a bolt into the nut, and fed the nut with its strip through the slot, turned it 90 degrees, then welded it in position. That was 22 years ago and the seats have been in and out a couple of times for diaphragm replacements.
PaulH Solihull

Allan, thanks for the suggestion of installing a sleeve on the bolt. I didn't even consider that the box would compress.

Mike, I guess this all falls under the "live and learn" category. I don't expect to ever do a floor pan again but if I can save just one person from my misfortune I will pass along your suggestion.

Paul, yours was a very creative solution.

But what about my first thought of cutting a 1 inch diameter hole through the back of the box. It would be large enough to thread a bolt under the pan and insert a box wrench to tighten.

Afterward the hole would be sealed with a rubber or plastic plug.
Steven Rechter

Hi Paul, do I understand you right. The slot was in the floor pan, I can see that if you weld the nut and bracket up and make the slot to suit you can feed it in and see it through the hole. Then you could put in the bolt tighten up to lock it where it needs to be then trim the bracket and weld it into the slot. I imaigne some mole grips help while you are getting the bolt in if this is how you did it.
This is great solution to a scary potential problem. Re cutting a hole to access from below. This is done on aircraft for lightening. The punches used swage a flange at the same time. I imagine the radius has been calculated to work with 1/2 hard dural. Mild steel would probably be OK with a less cerebral approach.
Stan Best

I used Nutserts on our our MGB after discovering that the new floor boards didn't come with them the welded nut cages on them (realized that after the floors were welded in of course). Decided on the Nutserts as the easiest approach and they have worked well for the past ten years. Cheers - Dave
David DuBois

OK Paul, I think i get it now.In fact the strip is pretty much dropped into the floor pan filling the slot, but it has the nut pre welded in place. I guess you then held them in place using the bolt plus that they are an interference fit, and out with the welder agsin.
Stan Best

Steven, your solution is the simplest & I can't see why it wouldn't work OK. I would cut out the nut with a hole saw about 1&1/2" to 2''dia,reweld or replace the nut & then weld the the disc back in place.You can strap a strip of metal across the the disc using the new anchor nut to stop it falling through the hole while you weld it back in place.I repaired one many years ago this way only I used a "penny "washer instead. Barrie E
B Egerton

Barry, thanks for the insight.

David, did you use 1/4-20 nutserts? I can't seem to find any that are 1/4-28.
Steven Rechter

Stan - that's pretty much it. By making the slot rectangular, and the strip rectangular, one you have fed the nut and strip through the slot and turned the bolt upright then turning the strip 90 degrees virtually covers the underside of the slot, as well as giving you a decent area to weld the strip to the pan, without welding the bolt to the nut!
PaulH Solihull

This thread was discussed between 16/06/2012 and 18/06/2012

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now