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I'm trying to convert old "razor only" receptacles to GFCI protected ones in bathrooms. Since the old outlets are large 2-gang 4" boxes, I had to get GFCI conversion kits (manufacturer's specs) with a 2-gang, 1 centered receptacle support plate and wall plate:

GFCI conversion kit

The problem: the holes where the new GFCI receptacle should screw in (the small circles in the support plate at the right in the image above) are untapped.

It is extremely difficult to screw in the receptacle's screws into those holes. The hole size seems about right, matching the shaft diameter of the screw without the threads. A 7/64" drill bit slides smoothly in the hole.

The receptacle's screws are tapered at the tips and they can get the threading started, but it gets stuck quickly. I tried using the old receptacle's screws to continue the threading and it is so difficult that the screw head broke. I also found another switch with what appear to be self-tapping screws of the same thickness and threading, and they can't even get started.

I unfortunately don't have taps. I also thought of drilling the holes a bit larger, but the next drill bit size seems to be 1/8" which looks too big.

Am I missing something? I would think this must be a standardized part that is meant for simple installation without so much difficulty or additional steps such as thread tapping. For example, this how-to guide.

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3 Answers 3

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Remember you can just fit a GFCI upstream of here

All GFCI devices have the ability to protect downline locations. So if you know where this outlet location is fed from, then you can simply fit the GFCI device there instead, and leave the receptacle as you found it or use a plain receptacle.

I mention that because a lot of people don't think it's GFCI protected unless there's a GFCI receptacle right here. That's not true.

This is the only decent solution I have seen

... to the GFCI/Decora into a 4" box problem. I'm sorry that ABB didn't tap it, and this does seem like a weird oversight for such a prestigious company, but this thing is such a win otherwise.

  • The problem with other shallow plates is they provide nowhere for the Decora cover plate screws to go, which go just outboard of the receptacle mounting screws. So you wind up with Decora cover plates missing, or floppy-doppy, and then they cut you or get broken. No thanks.

  • Domed 4" covers (which are their own cover-plate) force you to mutilate the GFCI - the ears that catch the cover screws are too tall to fit under the dome, so you must break them off. This means you will never be able to use the GFCI anywhere else.

  • Deeper "mud rings" only work if you're lucky and the mud ring itself allows for those Decora screw positions. Some do not.

So learn to tap

You need a #6-32 tap, and a tap holder to match, and these are just a few bucks.

When tapping, whenever you run into a spike of torque resistance, just "back off" 1/2 turn. That lets the chips clear out of the cut.

Taps come in gradual, normal and short (bottoming). Don't use bottoming. Normal is fine.

If you're in school, see if the school has a "maker space". They probably have a set of taps.

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  • I think this is the best and most complete answer. Ultimately tapping the holes is what is needed. Thank you for mentioning the required tap. Right now with some difficulty, I successfully managed to force the screws of the receptacle into the plate. 1. Lubricant helped tremendously (WD-40). 2. The screws did not go in perfectly perpendicularly which probably made it more difficult. Due to the difficulty with screwing, I first secured the receptacle onto the support plate and connected the wires later. A huge thank you to everyone for your comments!
    – adatum
    Sep 27, 2019 at 7:53
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You may just need a better support plate, like this one from Home Depot:

enter image description here

"Mounting holes are pre-tapped to accept devices faster"

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  • Maybe so. The one I got is the only conversion kit I've found (I'm not in the US). Is it typical to need to tap these plates? P.S. I added a link to the manufacturer's specs.
    – adatum
    Sep 27, 2019 at 2:16
  • The conversion kit is really nothing special - just the stuff you need to mount a Decora style (that's the norm for GFCI dual receptacles) device in a 4" box. If you have a big box store (Home Depot/Lowe's), a decent traditional hardware store or an electrical supply house you should have no problem finding something like what I listed "pre-tapped".
    – manassehkatz-Moving 2 Codidact
    Sep 27, 2019 at 2:25
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    I would have fully expected the plate to be supplied with self-tapping screws. That's not that unusual. It helps to know how to tap; after the tap starts to get stiff you should back off 1/2 turn then resume.
    – Harper - Reinstate Ukraine
    Sep 27, 2019 at 3:33
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    @adatum -- have you tried an electrical supply house? a single gang Decora cover for a 4" square box is a rather standard part AFAIK, or you can use a 4" square single gang mudring with a standard 1 gang Decora cover....
    – ThreePhaseEel
    Sep 27, 2019 at 3:44
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    @manassehkatz No, not t "just" that. It's got holes in all the right places to address the problem with Decora cover plate screws. Most solutions just ignore that, and expect you to mutilate your devices, or have floppy cover plates. The "better support plate from Home Depot" pictured above is a fail, and will give floppy cover plates. The domed 4" square covers require mutilating. The ABB kit is the first thing I've seen that actually addresses the problem instead of ignoring it. A mud ring will also work if it's tall enough, but that's just dumb luck.
    – Harper - Reinstate Ukraine
    Sep 27, 2019 at 4:04
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It sounds like, from the huge amount of discussion in the comments, that you have already expended enough energy on this to have been able to go to the store and buy the necessary tap. The tap you need will likely cost just a few US dollars.

The tap size needed for US style electrical fixture mounting screws is #6 - 32 NC.

You can even use the small tap by clamping its top square tang into a small pair of vice grip pliers in the case you want to avoid buying the special tapping tool holder. You use the tap very similar to the way you have been trying to use screws to make threads in the holes of the cover plate. Screw the tap in place about 3/4 to 1 turn and then reverse it back 1/4 turn before proceeding in the forward direction again.

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  • Thanks! I agree that ultimately tapping is the correct way to go. Special thanks for mentioning the needed tap size.
    – adatum
    Sep 27, 2019 at 7:55

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