Repair an electrical box:
This article describes how to repair the mounting screw or screw opening at an electrical box used to mount a receptacle, switch, or other device. A stripped screw or screw opening at an electrical box is more than annoying, it's unsafe as the device will not be mounted safely and securely to the wall, ceiling or other location.
But if the problem is an over-stripped screw hole on the electrical box, we sure don't want to have to tear out the whole box: well we don't,. Here we describe what to do if the mounting screw itself is stripped and we explain how to tap the electrical box opening if the problem is that that component has become stripped or enlarged. Our page top photo shows two common screw locations in a metal junction box; Below we illustrate the screw and screw mounting opening or "ear" at a receptacle box.
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I started installing a box-extender on a receptacle in my kitchen because I'm tiling my backsplash and need to raise the outlet above the tile.
However, the top screw connecting the outlet and box wouldn't hold. I spent way too much time bent double under my cabinets trying to get it to bite, but when I finally gave up and pulled it out it was stripped at the tip (which was as far as it'd go in).
I'm sorry to bother you with triviality, but I'm new to home renovations and don't know what to do. Advice? - Julia 1/29/13
Reply: tips for replacing a stripped electrical receptacle or switch box mounting screw or screw opening
Don't feel bad, this stripped receptacle mounting screw problem has confronted countless electricians and electrical workers. It's always comforting to realize that for just about any problem we encounter, someone else has met it before, and there is almost always a known solution. In fact this problem has been around at least in the U.S. since we figure about 1918 when the first outlet box support device was patented. 
At separate articles on HOW TO ADD an ELECTRICAL OUTLET and about OLD HOUSE ELECTRICAL WIRING people have asked more than once how to fix a stripped screw or stripped mounting screw at an electrical box for an outlet (properly a receptacle), switch, or to suspend a light fixture or just to hold a box cover in place.
How to replace a stripped receptacle screw
For the case you describe, if the problem is just the screw itself is stripped, simply purchase a replacement screw or a handfull of them from your elecrical supplier. These screws are a standard thread and length, but longer versions are available at any hardware store.
How to fix a stripped receptacle or switch box mounting screw opening
But if the stripped problem is that the electrical box mounting hole for a receptacle, light fixture or other device has become stripped, you'll need to enlarge and tap the hole for the next size larger screw and tap threads for the new screw.
It's really easier and cheaper than you might think to fix a stripped mounting hole in an electrical box.
Taking care to move electrical wires out of the way of your drill bit, in a metal electrical box you can drill out the 6/32 screw opening to tap and accept an 8/32 screw.
This sounds like a bit of work, and there's the extra cost to purchase the right sized tapping drill bit (tap drill No. 29, body drill No. 19 for an 8/32 or 8/36 screw) along with the actual tapping tool.
On the other hand, if you can't borrow these little parts from someone, I'd argue that the cost of buying them is less than the trouble of tearing out and replacing the whole electrical box in a finished wall. Our photo (above left) shows an 8/32 tapping tool ready to cut threads into that stripped electrical receptacle or switch box mounting screw hole.
Can I just Use a Sheet Metal Screw or a Little Clip to Hold the Switch or Outlet Mounting Screw?
Watch out: Do not try using the little retainer that you find on the ear or back of an electrical receptacle or switch (photo at left) to substitute for a secure mount in the electrical box.
Shown in our photo (left) is a simple retainer intended to prevent loss of the mounting screw prior to installation of the receptacle or switch into the electrical box.
This little clip may be made of metal or just cardboard or plastic. But in any case it is not designed to be removed and then used over the electrical box mounting tab to hold the receptacle or switch in place. This little clip lacks the strength, is not intended for device mounting, and would be an improper and unsafe installation.
Watch out: don't use a pointed screw like a drywall screw (photo at left) or a sheet metal screw to secure a receptacle or switch to the box. Depending on wire positions in the wall or the length of the screw you could cause an ugly short circuit now or in the future.
I do not recommend using a sheet metal screw to "fix" this problem even though it's tempting. The sharp point on the sheet metal screw can pierce and short a wire inside the junction box. You or someone else later will be sorry.
If the electrical mounting screws are stripped, replace the screw, tap the mounting hole for the next larger screw size, or replace the box.
Patented Support Clip Secures Electrical Devices in a Junction Box
Since Larry Mears patented a support clip intended for supporting electrical fixtures in a junction box in 1987,  it may indeed also be possible to purchase a spring-metal clip-on adapter that slips over the stripped ear through which the original hole passed. We're looking for a retail outlet and accepted use of the Mears clip, and similar or related devices have been patented by Union Insulating Company and by TRW, Inc.
Similar support clip type devices, used more widely in automobiles and other components are basically a little clip made of spring steel that slips over the existing metal "ear" on the electrical box. The clip is stamped and cut to accept threads of a particular screw size.
I don't prefer this repair because I'm not sure this repair would be code-approved and I don't like adding little parts that get lost by the next repair person.
Metal Box Mender Repairs for Stripped Screw Holes in Electrical Boxes [DRAFT August 2015]
In the U.K. inventor David Sexton is selling "Metal Box Mender" a kit that "mends threadbare screw holes in electrical metal boxes without replacing the box and without damaging surrounding area of the box."
On first reading about this product I was concerned about the the use of sharp-tipped "mender" screws that could, after passing thorugh the polypropylene mender and pin assembly then puncture an electrical wire inside the electrical box. But Sexton's design now uses a metal wood screw with a flattened tip to reduce that hazard.
Dave was kind enough to send me some samples of the Metal Box Mender along with instructions and a little 35mm square metal electrical box listed for U.K. use. I took a look at the Metal Box Mender, its instructions, parts and components and have documented what I saw in the photographs and comments given here. Currently this product is sold only in the U.K. and on eBay and it has not been tested nor listed for use in the U.S.
The metal box mender is indeed intended only for metal electrical boxes. It is not sized nor designed to repair plastic or PVC electrical boxes whose screw threads have been stripped. However intelligent selection of an over-sized screw and possibly a tiny bit of drilling can permit repair and continued use of plastic electrical boxes using methods described beginning at ELECTRICAL OUTLET BOX SCREW REPAIR.
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At left we illustrate how the screw, passing through the upper surface of the metal box mender then passes through the metal ear of the electrical box and into the screw-holding base of the metal box mender screw receiver. As the screw is tightened the little black plastic pin falls out into the electrical box.
Watch out: in this illustration the parts are not being correctly used: for demonstration purposes I've left off the box cover or alternatively the electrical device such as an electrical receptacle or light switch that the screw would be holding in place. Otherwise you'd not have been able to see how these parts go together. But screwing down the bugle head screw as I'm doing here will damage the metal box mender causing that part to fail and resulting in a possible safety hazard: loose electrical wiring and components.
More about this problem is given later at METAL ELECTRICAL BOX MENDER SNAFUS
[Click to enlarge any image]
The basic instructions for the metal box mender are deceptively simple. Below I've included but expanded on the manufacturer's original instructions that can be read on the illustrations given just above. I found the company's website and printed instructions confusing, in part because I did not immediately understand that the product design involves five measurements, four instruction cards, two metal box mender sizes and two metal box mender screw sizes. If you wonder why there are four nearly-identical looking product cards in four different colors, that's the trouble.
Table of Metal [Electrical] Box Mender Sizes & Parts
I suspect that a product engineer will ultimately design a "one size fits all" solution that reduces product cost and price and simplifies the ordering and installation instructions.
Choose the Metal Box Mender Screw Receiver Size
The screw receiver, which is the main component of the metal box mender, is simply slipped over the stripped metal ear of the electrical box to be repaired. is 17mm in total height and consists of a thinner upper portion
Choose the Metal Box Mender Screw Length
When you add the additional thickness of a box cover or the steel mounting ears of an electrical receptacle of switch (about another 2 mm of metal ears or as much as 6mm if you are using a typical U.K. Crabtree brand plastic blank cover plate) to the 17mm screw receiver height, then there is no chance that the silver screw (or perhaps the longer gold coloured screw) will penetrate past the bottom of the plastic metal box mender. Thus there is little chance that the pointed screw would puncture a live wire inside the electrical box - a source of unpleasant (unsafe) events. such as sparks, shorts, or more.
The upper screw in my photo above is the short silver bugle headed wood screw provided by the Metal Box Mender manufacturer. The lower screw is a typical machine-thread, blunt-tipped and longer screw provided with electrical switches and receptacles and was the OEM screw used to secure these devices to the electrical box metal ears or tabs before you or I, with our power screwdriver, stripped the screw, screw hole, or both.
Watch out: if the repair screw is too short for all of the thicknesses involved then it will not screw securely into the "Mender" and / or may strip its hole in the mender resulting in dangerously-loose electrical components. This is exactly what happened when I used the short silver screw and the Crabtree #4001 blank cover plate atop the U.K.-sized electrical box with the Metal Box Mender. Turning the screw gently by hand I found it rotated endlessly and never got tight. It was too short for this application. The gold screw available from the manufacturer may have worked.
Watch out: if the repair screw is too long for all of the components being secured and if the screw has a sharp point (later screws I'm told will be blunt-ended) it can penetrate an electrical wire in the electrical box causing a dangerous short circuit or fire. The manufacturer warned to keep wires from behind the "Mender" in the electrical box, but in the real world of electrical wiring one cannot be absolutely certain that a wire was not pushed back into this forbidden zone when the device (switch, receptacle, spliced wires) is/are pushed back into the electrical box.
Below is the Crabtree E40001 blank plate or "blind" electrical box cover installed on the metal U.K. electrical box provided for our inspection. Using the silver bugle-headed wood screw provided by the manufacture I found that my screw stripped and spun in the Mender's screw receiver and I could not obtain a tight connection of the plastic blank plate to the electrical box. I suspect that a longer screw would have worked fine.
The silver Metal Box Mender screw provided by the manufacturer is about 15mm in length and is a bugle-headed wood-screw. OEM screws provided for U.S. & Canadian electrical devices such as electrical receptacles or switches are typically a machine-threaded screw of about 22mm in overall length. Other screw lengths vary according to application. For example the OEM screw for securing a blind cover plate or receptacle or switch cover is typically about 13mm in length.
Watch out: although the manufacturer recommends, and many electricians like to use a power screwdriver to speed the installation of electrical devices, you may find it a bit easy to strip the screw holding portion of the Mender. In fact that is how some metal electrical box screw-receiving ears got stripped in the first place. For a one-off electrical repair such as the typical application of the Metal Box Mender described here, I recommend turning your screws by hand and I recommend that you avoid over-tightening.
Earlier I explained that one of my illustration photos risked damage to the Mender by causing it to split. In my photo below I've backed the Mender screw out a bit but my green arrows point to faint markings that show where the bugle-head of the screw contacted the raised sizes of the top portion of the Mender, and my red arrow points to the spoit developing in this part, rendering it damaged and possibly unsafe.
Note: the photo below does not illustrate a procedure recommended by the manufacturer because I've omitted the metal tabs of an electrical receptacle or switch or a blind box cover, any of which would, using U.K. devices, be expected to avoid the splitting problem described here. However as I explain below there could be a splitting problem when using this repair part with U.S. & Canadian-sized electrical components.
At below left is an un-damaged, intact Metal Box Mender screw receiver.
Below you can see three interesting details in a damaged Metal Box Mender screw receiver:
The two small holes in the upper photo show us where the tip of the sharp-pointed wood screw missed the proper screw opening and tried to penetrate the Mender block at the wrong location. Using a manual screwdriver I could feel this trouble and I could move the screw for another try. If I'd been using a power screwdriver the screw would have entered the block at the wrong location (one of those two little holes) and I'm not sure how the device would perform.
The center screw opening shows a beveled edge - signs that the bugle portion of the bugle-headed wood screw entered and spread the screw receiver.
The split at the right center of the Mender shows that this device has "failed" or more fairly, I broke it. If you screw the bugle-headed wood screw fully into the Mender without the screw passing first through a box cover or switch or receptacle mounting tab the bugle-shaped screw head will pass into the Mender's plastic top segment causing it to split. The result is an insecure holding device that may be unsafe, risking electrical short or fire. I made this mistake. Don't do it.
Watch out: depending on the thickness of the metal tabs of the electrical switch or receptacle being secured using the Mender and depending on the width or diameter of the screw opening in the switch or receptacle tabs, the too-small bugle-headed wood screw provided by the manufacturer can pass right through the switch or receptacle tab. You may notice the problem and you might add a tiny metal washer and start over again. But the screw may have already split the Mender's retention head as shown in my photos above.
The OEM electrical device screw head profile shape depends on application: a U.S. or Canadian OEM electrical device screw (above left) typically has a 7-8 mm diameter screw head with a flat under-side on OEM equipment mounting electrical switches and a receptacles. The screw may have a sloped shoulder (similar to bugle headed screws) for shorter screws used to mount an electrical switch or receptacle plastic cover. (The ruler shown is divided in 16ths of an inch closest to the screw heads and in mm at the lower portion of the photo.)
The Metal Box Mender screw head (above right) is 6mm in diameter with a bugle head profile, wood screw threads (fine for soft plastic) and in my test versions a sharp point. The manufacturer informs me they will or have shifted to dull-point or rounded point screws for increased safety.
A millimeter or two may not seem important but when we combine the bugle-headed under-profile of the Mender repair screw with the smaller diameter, it's going to spread and go right through some electrical switch or receptacle screw openings, resulting in a loose and unsafe electrical device.
Below you can see my comparison of the sizes of typical screws used with and sold along with electrical receptacles and switches. At below left is the larger-headed machine threaded screw used in North America and inserted into the mounting ears of an electrical receptacle.
At below right is the smaller bugle-headed wood-screw provided by the Metal Box Mender manufacturer inserted into the same mounting ears. With tightening this screw sill push right through the opening in the electrical receptacle leaving the device poorly-secured.
The manufacturer advises that you can sand down the upper portion of the screw-receiving "Mender" if necessary. You'll need to do this in cases where the metal ear into which the device or cover mounting screws turn or pass is "high" in the box - as we illustrate below. You will see that the distance from the upper surface of the metal electrical box screw-receiving "ear" on U.S. electrical boxes is much closer to the upper edge of the electrical box than on typical U.K. electrical boxes.
This means that trying to use the Metal Box Mender on a U.S. or Canadian electrical box is going to give some trouble with the product's U.K. dimensions: if you don't sand the top of the Mender down to a quite thin dimension your wall plate or cover plate of the box is not going to sit flush and tight against the wall. Rather the plate will separate from the wall by about 5 mm or nearly 1/4".
Watch out: when you sand or file down the plastic top of the "Mender" to make your electrical parts fit properly you're reducing that overall 17mm screw receiver height to something less. In turn that means that the point of a longer screw may penetrate and pass out of the bottom of the "Mender" and thus it could damage an electrical wire causing a short or fire.
Below you can see the difference beween the ending position of the Mender on a U.K. electrical box and on a U.S. electrical box.
I had some trouble keeping the Mender in place regardless of whether the plastic pin was inserted or not. However if I could hold it in place until the repair screw had begun to enter the screw receiver the repair continued successfully. This problem may be avoided if the Mender pin is propoerly placed: the device may rotate on the pin but it should not move enough to lose screw opening alignments.
Metal Box Mender Assembly vs. Grounding Path on Older Electrical Work
Proper electrical wiring in most countries requires a physical grounding conductor wire between a grounding terminal screw on the electrical receptacle or switch and the electrical circuit's grounding conductor wire, usually joined by a crimp-splice or sometimes by a twist-on connector.
However amateur electrical wiring too often omits this connecting wire and instead relies on the coincidental ground path between the metal strap or body of the electrical receptacle or switch, through its mounting tabs, through its mounting screw to the metal ear on the metal electrical box that in turn may be grounded to the circuit ground or may be grounded through the metal jacket of older BX armoured cable.
Watch out: this is not a safe, reliable ground connector. However it may be the only one present on an older electrical circuit and it may have been "working" - somewhat. If you then repair a stripped metal electrical box mounting screw opening with a plastic Metal Box Mender and the Mender's wood screw, even that improper and rather iffy ground could be lost.
I suspect that the reason I could not get the plastic pin into place when installing the Metal Box Mender on a U.S. electrical box was a mis-alignment of the Mender screw opening with the opening on the metal tab of the electrical box. The key dimension here is not the distance from the screw opening (or the center of that opening) to the electrical box wall - the dimension that the manufacturer advises you to consider when choosing among the two Mender sizes.
Rather the critical distance is from the interior face of the Mender to the center of its screw opening. If that distance were made larger I speculate that this device would fit on both U.K. and U.S. - sized electrical boxes, though the Mender height sill needs to be addressed.
Note: my understanding is that the MBM or Metal Box Mender, sold directly from the producer and also sold in Ebay (£2.88 postage paid (VAT included) for Single packs of 2 Menders), has not been tested nor listed for use in the United States nor Canada. For North American, Austrailian, New Zealand, Canadian or other applications, look for a product listing and approval that applies in your country.
What if I Broke Off the Little Metal Ears Whose Holes Accept the Outlet or Switch Mounting Screw?
Watch out once more: don't try bending that metal ear on the electrical box or junction box - if you break it off then you're most likely going to have to tear out and replace the whole thing.
But before tearing out an electrical box, receptacle, or switch assembly and wiring because this horrible problem has happened to you, look for one of several patented outlet box cover products that include bendable mounting clips that are intended to secure the cover and in some applications the outlet in the box. Pimentel, Demetrio (Atlanta, GA) has patented a device that might work in this case. 
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