Seat Rails Nut Stripped?

Technical MGB discussion
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Peter Cresswell
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Re: Seat Rails Nut Stripped?

Post by Peter Cresswell » Wed Jul 15, 2020 9:33 pm

From you description, it sounds as though the nut is still welded intact in the crossmember, and you can remove the bolt to examine it. If it looks ok then the thread in the nut is the problem, if it looks very worn then a new bolt might solve the problem especially if you run a tap down the thread before putting the new bolt in. Another option might be to cut a UNC thread instead of the UNF and use a UNC bolt instead. They use the same spanner size but the coarser thread might provide enough purchase to hold the seat securely.
If you can't get the bolt out then the welded nut will almost certainly have broken free in which case the only option is to cut a hole through the floor from underneath to get to the nut and hold it whilst the bolt is removed. Use a circular hole cutter of a size that a socket can pass through and of a size you can get a grommet for. Use a nut and bolt replacement, fit the grommet and apply plenty of underseal over the grommet and surrounding floor to seal it. If you need to take the seat out in the future you can remove the grommet to get to the nut. As an alternative you could fit a bolt from underneath and secure this with a nut on top of the crossmember before fitting the seat, then fit the wooden spacer with the hole adjusted to accommodate the nut, and then use a second nut to secure the seat. This would be a more permanent solution, but not quite as neat!
Pete
1969 MGB Roadster
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nathan_f
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Re: Seat Rails Nut Stripped?

Post by nathan_f » Thu Jul 16, 2020 7:20 am

I was able to get the bolt out with the seat still attached to the rails, and it looks like the threads on the bolt are fine. Maybe a bit worn but definitely not stripped. I didn't pull the seat rail so I wasn't able to pull up the carpet to look down and see if I could see the condition of the threads in the welded nut. It's definitely biting on something, as it screws down in place, but just doesn't stay there. I may try to pull the seat and lift the carpet tonight to see if I can get a visual on the nut.

I don't have a thread tap, so I'm thinking of just going the route of a self-tapping bolt/machine screw slightly larger than the existing bolt.

Ian Fozzard
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Re: Seat Rails Nut Stripped?

Post by Ian Fozzard » Thu Jul 16, 2020 8:23 am

If there are no wooden strips in place, or metal spacers, the bolt will be going further in than intended I think?
You need to check the whole length of the bolt threads for damage, and also look at the tip of the bolt to see if it is bottoming out on the chassis section.
If the welded nut is damaged I'm not sure you will get a secure fix by the method you suggest. It would be worth your while getting an inexpensive tap and die set - come in handy for all sorts of jobs on an MGB.

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

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Tom Brearley
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Re: Seat Rails Nut Stripped?

Post by Tom Brearley » Thu Jul 16, 2020 2:21 pm

Ian's right, but a cheapo way to make a tap is to cut a slot into the head of a bolt with a hacksaw. The sharp edges then clean up the thread of the nut. :idea:
1973 MGB GT (Exeter) - currently shaking down after 3 years off the road

Ian Fozzard
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Re: Seat Rails Nut Stripped?

Post by Ian Fozzard » Thu Jul 16, 2020 3:38 pm

Tom,
that often works for cleaning up threads if you use a bolt of the same size, but I think for tapping threads for the next size up, you would need to use a drill followed by a proper tap.

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

nathan_f
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Re: Seat Rails Nut Stripped?

Post by nathan_f » Sat Jul 18, 2020 11:33 am

I have a die set that should be delivered today... I pulled the seat out (the wooden slats were in fact in place still, by the way) and can't tell if the nut is still in. I can see threads, but I'm not sure if they're just where the floor plate is threaded. If I twist the bolt in, it will hold but it's loose in the hole and pops with the leverage of the seat rail., so I'm thinking that I'm just tightening in to the floor plate without the bolt. I'm going to drill it out and tap it slightly larger, so we'll see how that goes. Worst case I'll probably end up trying the rivet nut solution. I'm really uncomfortable drilling a hole in the cross member to access that seat nut. Why did they hide it under there?!
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Ian Fozzard
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Re: Seat Rails Nut Stripped?

Post by Ian Fozzard » Sat Jul 18, 2020 1:26 pm

Nathan. - a quick thought to try before anything else!!!
If the bolt is a sloppy fit make certain that you haven't got a wrong sized bolt in there. Someone may have fitted a metric bolt for instance?

Check with the other bolts fitted to the rails

Ian-F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

nathan_f
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Re: Seat Rails Nut Stripped?

Post by nathan_f » Sat Jul 18, 2020 2:49 pm

Just checked that to make sure I'm not crazy! Same with the rest, they either don't bite the threads or wobble about. I think the threads are stripped. The threading appears too deep to be just the steel from the floor board.

Dave Wheatley
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Re: Seat Rails Nut Stripped?

Post by Dave Wheatley » Sat Jul 18, 2020 5:52 pm

Could be 6mm bolts into the 1/4 unf nuts. They'll just about fit, but will be very sloppy. The thread in the nut might still be OK
1978 MGB GT
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Ian Fozzard
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Re: Seat Rails Nut Stripped?

Post by Ian Fozzard » Sat Jul 18, 2020 7:17 pm

It should be possible to determine whether metric or unf by looking at the markings on the bolt heads (unless they are very cheap replacements).
They should be unf of course on the MGB.

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

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