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I'm trying to install a new ceiling light replacing an old fan. The light fixture is around 8 lbs in weight. The issue I'm running into is that I cannot find the right screws to use with this electrical box (unfortunately, we can't find the old screws). I tried SAE screws - 1/4" was too small and 5/16" was too large. I then tried metric screws and M7 screws seems to fit (M6 was too small, and M8 too large) but there is resistance when I try to screw it in. I suspect that the thread pitch doesn't match.

This photo is of the electrical box that I'm working with - enter image description here

This photo is with a M4 screw for size reference of the mounting holes - enter image description here

I have a few questions and would greatly appreciate if someone can answer -

  1. This has to be a standard box but I can't find anything similar in HomeDepot or Lowe's. Do you know what this box is and if M7 are really the screws to use with it? My concern is that M7 screws are not that easy to find and the only ones I could find have a bolt head, which make me think that I might be using the wrong screws.

  2. Can I use a smaller screw (say 1/4") and just use a wing nut to make sure the light fixture remains fastened to the box? The light is relatively light so I'm not as worried about the weight but I do not know if the vibrations from the washer/dryer for example will unscrew the nut overtime.

  3. Should I use a M7 screw tap and try to "fix" the threads on this box? I'm keeping this as the last option as I might be destroying otherwise perfectly fine/standard mounting hole

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  • What about something like 2BA?
    – Solar Mike
    Commented Apr 16 at 12:08
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    If this is in the US, the screws will not be metric. At a certain point, where do you stop fooling around with possibly stripped screw holes and simply replace the box?
    – Huesmann
    Commented Apr 16 at 12:17
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    Have you tried having the screw go into the little holes at the top of the box above the big holes? The lower holes seem like they are there to locate the cover/fixture, not to support it, and every fixture I've installed has really long screws.
    – gbronner
    Commented Apr 16 at 12:41

6 Answers 6

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It looks to me like the threads in the mounting holes have been wallowed out somehow, perhaps the previous "electrician" used the wrong size machine screws.

Off the top of my head I forget exactly what the thread pitch for those are supposed to be, probably 8-32 or 10-32. It wouldn't be metric though.

But if the threads are damaged beyond repair, it also looks like it would be pretty easy to just replace the whole junction box. Remove the four screws inside the box and the whole thing will probably just pull out. Detach the cable and pull it out, and then reverse the process using a new octagonal junction box.

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Maybe the box ears are damaged or stripped.

Buy a light box cross bar. Attach it to the box with nuts and bolts and lock washers that only have to match each other, not the box. The cross bar has correctly threaded holes and a useful ground terminal for your light.

enter image description here

Another way

[Deleted an alternative to attach directly to a joist that might be behind the box. Thanks to @Astrogator pointing out, given there was a fan there, the four screws are most likely tell-tale of a metal cross bar attaching the box to two adjacent joists. It's not worth exploring them, just use a cross bar.]

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    OP says a fan was previously mounted to this box, and usually fans are recommended to be hanged on (i.e. attached to) joists. So, there's a good chance of finding one behind (above) this box.
    – Astrogator
    Commented Apr 16 at 19:38
  • @Astrogator I think you just pointed out a big flaw in my answer. It could be a ceiling fan box hanging from a metal cross bar. With that in mind, now the four screws look like machine screws that came with the box and it's a bad bet to touch them. I'm going to change the answer. Good call.
    – jay613
    Commented Apr 16 at 20:12
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Replacing the box is the best answer, but if that's too fiddly then find some Panel Thread Clips which might also be known as "Spire Thread Clips" or "Fairing Clips" or "threaded clips"

enter image description here

You'll need a suitable screw for each one. Clip each over the visible tabs, so the holes line up, and then screw in the fitting like normal. Your new screwheads will have to fit inside any recesses too.

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If you don't want to replace the whole box, then there is an alternative approach. Find the next size up of screw that doesn't quite fit, then use a tap to thread the hole to that size.

Do make sure that whatever size screw you use will still fit the fan.

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big box stores usually sell specific electrical fixture bolts (as well as painted faceplate bolts) at the end of the hardware aisle. You can get a small bag of those to try before you give up on the current box.

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It sounds like you're trying to thread into the metal ears. This is wrong. 1/4" is much to large for mounting a fixture. It looks like there are threaded holes in the back of the box, under the ears. I believe the screws should go through the ears and thread into the back holes. You'll need long screws, probably 6-32, 8-32 or 10-32.

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