Seatbelt mount, floor

A particularly stubborn 17mm bolt, one of the two that hold the seatbelt rail in position, has decided to bring the thread mounting with it from the sill rail.

In order to access the floor the bolt has been cut. In hindsight the carpet should have been cut around it but can anyone help with a solution to remake this fixing?

Ta...

Can you cut I a new larger thread and use a slightly bigger bolt Endinburgh?

Or is there not enough room to do that?

Regards

MM

The fixing appears to be welded behind the sill panel normally but has now come adrift.

It may be a case of cutting out that section around it, re-welding an M10 nut on the rear then re-welding the plate ...

I had the same issue on Coupedummy's coupé years ago......... bolt seized and then sheared the nut inside the inner sill so ended up cutting the bolt and welded a new bolt counter sunk in to the same position, did this using TIG to reduce any chance of setting his car on fire with splatter.

As I recall, that's an M10 fine thread - 1.25mm pitch. I could be mistaken...

Edit here as it should have said "welded a new nut counter sunk", not bolt, in fact if my memory serves me correctly the nut used retains the belt to the shoulders anchor point, and as Neil has posted above its M10 x 1.25.

So while I don't happen to have a coupe near me particularly, having previously inspected the mounting, the recessed thread/nut sits behind a 10mm hole within a double skin, located within the skin nearest the outside of the vehicle.

This in turn is aligned within a slightly larger circular hole in the skin nearest the inside of the vehicle, probably 15mm or so in diameter.

To introduce a 10mm nut through this hole would mean reaming it out to approximately 19mm on the basis of the dummy one I possess - how "feasible" is this?

Or is it more practical to cut a 50mm section out of the sill, weld the nut on the back then weld the section back in.....

This just blue screened my head Simon so I'll read it again tomorrow when capable of a response

Discovered subsequently (by removing the large plastic grommet below the B pillar) that the fixing is pretty much a hexagonal nut, probably welded on - though there was talk of it being a compression fit

I'll post some pics tomorrow to clarify

Yes post some pics please Simon

Seatbelt rail mount (forward)


top of sill showing remaining bolt-end



same bolt with blutack around to prevent dropping inside


seatbelt rail mount (rear)



Pic 3 shows what I was attempting to describe previously

a) the inspection hole cover on the left which when removed allows one to feel what appears to be a hex nut
b) the thread
c) an outer hole surrounding the thread

Am I wrong in thinking there are two thicknesses of metal?


Question therefore - does one

a) drill a 10mm hole in a new piece of steel c.50mm square, weld a 10mm/1.25 pitch nut on the rear, and weld the piece onto the sill (having reamed out the mount so that the nut is recessed) or

b) cut out a section of sill around the mount, weld a nut on the rear, weld the section back in position


Hope this is clearer!



As you know I'm not a mechanic but I'm still interested in the process

That third pic looks like a bit of steel plate has been spot welded on behind to beef up the sill. Welding is going to be very messy on a currently running car. Could you not drill out the hole to a larger size and use a good quality M12 steel rivet nut? I know its a bodge but M12 is a large size for the application and it is a lot less destructive to install.

Oh! Are you saying that might not be a "factory" characteristic then?






hmm, this works on a similar principle to a plasterboard fixing, right? Expanding behind the material...

Presumably it would stay in position anytime the bolt was removed on any future occasion

No not at all, I would imagine the factory put it in because the steel typically used for sills is thin. Strong enough when folded and formed for the application of a sill but for a mounting such as this the thickness would run the risk of being "pulled" under stress.

They are rather like a pop rivet but don't let that make you think they are weak. Once installed the steel versions, especially at 12mm, are bloody strong! Yes, you can removed the bolt screwed into it just like a welded in nut.

They are used a lot in aircraft installs, I used them a lot back in the 80's in an industrial workshop environment and only ever saw one fail due to the surrounding material giving way - a steel rivnut mounted in an ally plate if IIRC.

That's really interesting, thanks.

If you ever need any techy info on pianos I'm your man

The rear retaining nut is part of a plate that's welded on the inside of the inner sill, you pictures shows the spot welds.
To repair this would be the same as Reuben's, drill out the old nut so it's big enough to insert a replacement and then weld the nut to the inner sill, if it's TIG welded you can penetrate the outer and inner plate giving you a stronger mount when finished, if MIG is the only option give it some amps and stitch weld in 1/4 sections so it doesn't distort or put too much heat inside the sill void, fire extinguisher near by is a must.

In effect would some of the nut be recessed and some "proud"?

A number of options, Simon. Personally I'd always be bloody careful about doing anything related to the seatbelts, you never know when you might need something that satisfies the quick tug of the MOT guy but not the 10g stop after impact...

1) on the last kit, the seatbelt nuts top and bottom were welded all round to the outside of the roll cage, with a hole behind the nut into which the stub end of the bolt could go.[1]
2) take a look at Paul's page here: not quite the same as yours but the way he's done pillar fittings is pretty much what you're trying to emulate, I think: https://paulssplit.blogspot.com/2011/08/fitting-seat-belt-anchor-points.html
3) *if* you can find a suitable compression nut (which I couldn't on a quick search, just 1.75mm standard pitch, but a phone call may help) and *if* the existing thing is also a compression nut then you might drill out the existing nut and access it through that inspection cover to fit the new nut; no welding required. Don't forget to fish out the old one or the rattle will drive you mad.
4) if you're welding a new nut in situ, it should be behind the metal that's currently there; you want it to have to pull through, not pull off...

Neil

[1] edit: but that roll bar was a bloody great lump of three inch pipe, much thicker than body/sill metal.

Cheers Neil. The place that will most likely carry out the welding (as none of us have any experience) has been very good to us generally and especially re our project car. One chap there in particular is a rare find, one who is not only skilled but interested - we have to trust him to come up with his version of the repair and tread diplomatically when making suggestions about potentially telling him "how to do his job". However it's reassuring that all the pointers in this thread are going in a similar direction.

I enjoyed Paul's antics with wires and magnets