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.
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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