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Captive Nut Replacement

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Palmer

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The 3 just got back from the bodyshop. Looks fantastic, but I've got a little problem. I forgot to have them weld on a new captive nut for the brake dimmer switch. The old one had frozen up and been ground off with the idea of replacement, but now the fender is back on. Of course, I could remove the fender and weld on a new captive nut, but does anyone know of some sort of sheet metal anchor that I could use as a short cut? Thanks.
 
Depending on the size you need, a Rivnut might work.
 
There are nutserts available on the market. The best one for thin metal applications looks similar to a stove pipe hat. The barrel has knurling along it which allows it to grip the panel and keep it from spinning when in place. I suppose McMaster Carr would stock them. They aren't cheap as you also need an installation tool. But they do a great job.
 
DougF said:
There are nutserts available on the market. The best one for thin metal applications looks similar to a stove pipe hat. The barrel has knurling along it which allows it to grip the panel and keep it from spinning when in place. I suppose McMaster Carr would stock them. They aren't cheap as you also need an installation tool. But they do a great job.
Click to expand...

(same thing as a Rivnut) Usually sold as a kit including the installation tool and various size nuts. Basically, it installs like a rivit. I think the kit I have I bought at Carlisle... or Macungie... or a local NAPA... or maybe even Harbor Freight... It was cheap enough that I bought it without a specific need at the time.
 
Drill out the old screw remains and retap for the proper thread pitch (1/4-28). Simple.. If not you can rivit the piece as one of the above posters mentioned.
 
martx-5 was so kind to send me some of these for my rear fenders and they work great. https://www.mcmaster.com/#90594a040/=ld1cm If you would like I would be happy to send you a couple for your dimmer. Just pm me with your address
Greg
 
IanF said:
DougF said:
There are nutserts available on the market. The best one for thin metal applications looks similar to a stove pipe hat. The barrel has knurling along it which allows it to grip the panel and keep it from spinning when in place. I suppose McMaster Carr would stock them. They aren't cheap as you also need an installation tool. But they do a great job.
Click to expand...

(same thing as a Rivnut) Usually sold as a kit including the installation tool and various size nuts.
Click to expand...
Actually, I think DougF was talking about a PEM nut instead of a Rivnut. Instead of being soft and collapsible like a Rivnut, PEM nuts are rigid and have knurls that displace the softer base metal they are pulled into. We used to use them extensively at work.
https://www.mcmaster.com/#pem-style-nuts/=l2r4i

But they are much trickier to install properly, and require access to both sides.

I'd try drilling and tapping first; then if that fails, install a Helicoil.

I'm also guessing we are talking about a headlight dimmer switch, which on a '3 is mounted to the kick panel with bolts into captive nuts inside the front fenders. The only brake light control on a '3 isn't mounted to sheet metal at all.
 
I'd go with a Rivnut or equivalent, assuming there is clearance behind that panel.. I used them to mount door mirrors on my first spitfire. Very strong, and can be mounted blind. You need to drill about a 1/4"-3/8" hole for them but other than that I'd think they'd take the strain of the bracket.

With the old nut ground off, I don't see how tapping/helicoil would be an option.
 
TR4nut said:
With the old nut ground off, I don't see how tapping/helicoil would be an option.
Click to expand...
Good point. I <span style="font-weight: bold">assumed</span> he meant the bolt head was ground off, but we all know what that leads to. If the nut is gone, then of course it can't be repaired.

However, I think you can get some limited access to the area the nut is (was) in, by removing the splash panel inside the wheel well. You <span style="font-weight: bold">might </span>be able to get in there to place a nut, or even pop-rivet in a captive nut ala Mark Macy's article in the TRA newsletter. Those nuts do take a fair amount of force when the driver steps on the dimmer switch (again I'm assuming that's what we are talking about), and I'm not sure how long a Rivnut would hold up.
 
If removing the splash panel gives reasonable access, that sounds like a very good way to go - even something like a fender washer and nut arrangement would hold very well at that point. Might need to bribe a helper for the operation, though.
 
19_again said:
I don't have a 3, but I'm very curious about what a "brake dimmer switch" is?
Click to expand...

It's a sign that I'm a complete idiot! Of course, I meant the <span style="text-decoration: underline"> </span> headlight <span style="text-decoration: underline"> </span> dimmer switch.

Thanks for all the great ideas. I probably can reach behind the splash panel to get some access. That will certainly make things easier.
 
It didn't even occur to me that you were referring to a headlight dimmer, after all we ARE talking LBC here, aren't we? I thought maybe it somehow tied into those brighter tail lights you see on some imports that are evident when the fog lights are on. While on the subject and not trying to hijack this, but is there a method to actually brighten a tailight when you have fogs on, after market fogs of course. The added visibility surely wouldn't hurt.
 
19_again said:
While on the subject and not trying to hijack this, but is there a method to actually brighten a tailight when you have fogs on, after market fogs of course. The added visibility surely wouldn't hurt.
Click to expand...
I can think of several ways to accomplish this. Probably the easiest would be to add a diode and resistor, to light the brake filaments at partial brightness when the fogs are on and the brake lights aren't. Of course you need to be careful to keep them dim enough that full brightness is still a distinct change.

Another thought might be a DC-DC converter, to bump the taillight voltage up a bit when the fogs are on. That would shorten the life of the bulbs of course, but better than being hit.
 
TR4nut said:
If removing the splash panel gives reasonable access, that sounds like a very good way to go - even something like a fender washer and nut arrangement would hold very well at that point. Might need to bribe a helper for the operation, though.
Click to expand...

yeah, if you have access to the back side, the question is moot. Bolt, washer, nut - done. Might need a 2nd person to hold the nut or bolt. I was assuming no access and don't see why 1 Rivnut out of 4 shouldn't be strong enough. Depends on the application... On our Volvos, the front sway bar mount is held on by a bolt into a captive nut welded inside the frame rail. Obviously, a Rivnut won't work here (maybe the other would... will investigate...) Usually, the fix in this case is to cut a access hole in the frame, weld on a nut nut, then weld it all closed again... would not want to run into this after the unibody sub-frame had been freshly painted...

edit: looked at PEM nuts on McMaster... I think a rivnut would be better for what he needs (low-stress application).
 
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