Remove a broken screw from an outdoor electrical box

Short answer: grip any stub that stands proud with small locking pliers and back it out. If it snapped flush, center-punch the stub, start a 1/16 in [1.6 mm] left-hand bit spinning in reverse, and step the size up until the stub walks out. Damaged threads get chased with an 8-32 tap, or stepped up to a #10 screw on 10-32 threads through a #21 (0.159 in) hole. Epoxy and toothpick fills hold a plastic cover closed; in a metal box they are insulation where the bond should be. Budget twenty minutes for the drill-out, against half a morning of chiseling mortar to swap a box set in brick.

How do you get a broken screw out of an outdoor electrical box?

Work from the cheapest attempt to the biggest tool: grip any proud stub with locking pliers first, then drill 1/16 in with a left-hand bit in reverse, step up 1/32 in at a time, and finish with a screw extractor only if the stub still holds.

Cover screws on an outdoor box break for two reasons. Steel screws rust in wet threads until the head twists off, and stainless screws gall: stainless threads on a stainless boss seize, and the last quarter turn snaps the head clean. Either way, the stub is usually loose in the boss once nothing holds it loaded, which is why drilling it out works so often.

The left-hand bit does half the work for you. It cuts counterclockwise, so every revolution pushes torque the unscrewing direction, and a stub that a right-hand bit would jam often spins out between sizes.

While the cover is off, look at the rest of the hardware. Outdoor boxes fail at their fasteners first: a broken conduit hub nut and a loose box pulled off its mounting come from the same corrosion you are drilling out now.

What do you need on the bench before you start?

You need a center punch, a left-hand bit set, a screw extractor set, taps with their drills, cutting oil, small locking pliers, anti-seize, and a multimeter for the bond check at the end.

• Center punch and a small hammer - the punch mark is what steers the first bit onto the stub.
• Left-hand drill bits, 1/16 in through 1/4 in - reverse-cutting; the size steps carry the repair.
• Screw extractor set - a fluted cone that bites harder as it turns counterclockwise.
• Taps and their drills - 8-32 chased through a #29 (0.1360 in) drill, 10-32 cut through a #21 (0.159 in) drill, the tap-drill chart values for 75% thread engagement.
• Cutting oil - stainless stubs work-harden and dull a dry bit.
• Small locking pliers - for any stub standing proud of the boss.
• Anti-seize - for the new screw going back in.
• Multimeter - for the bond check once it is back together.

What drill size removes a broken 8-32 mounting screw?

Start at 1/16 in (0.0625 in) and step up in 1/32 in jumps; stop at #29 (0.1360 in), because a #8 screw leaves only 0.014 in of stub wall per side at that size.

Run the numbers once and you will see why the smallest bit goes first. A #8 screw measures 0.164 in across the threads, and the tap drill for 8-32 is #29 at 0.1360 in. The wall of the stub, per side, is:

0.164 − 0.1360 = 0.028 in, and 0.028 / 2 = 0.014 in per side

That 0.014 in is your whole margin. A bit that runs off center by the width of one thread wall chews the threads on one side and eats into the boss. Your 1/16 in starter leaves (0.164 − 0.0625) / 2 = 0.051 in of stub per side, about 3.6 times that margin, so the first hole forgives a little wander.

Keep the drill slow and oiled on a stainless stub; a hot bit dulls, and a dull bit is what walks. Many stubs spin out at 5/64 or 3/32 in, which is the cheap win you want.

What are the steps to drill the stub out and save the threads?

Power off and proven dead, grip if you can, punch on center, drill small in reverse, step up in jumps, chase or retap the hole, clean the chips, and reassemble with anti-seize and a bond check.

1. Open the cover, kill the feeding circuit, and confirm zero volts at the terminals with your meter before any steel touches the box.
2. Soak the stub and the boss threads with penetrating oil while you set up.
3. Grip any stub standing proud with the locking pliers and turn it out. If it moves, jump to step 7.
4. Center-punch the stub as close to dead center as you can judge. The mark steers the 1/16 in bit on metal, and on a soft plastic boss it keeps the bit from skating off into the plastic.
5. Drill through the stub with the 1/16 in left-hand bit in reverse, slow speed, light oil. The drill spins counterclockwise for a left-hand bit - that is the point of it.
6. Step up 1/32 in at a time. The stub usually spins out as a size bites; if it still holds at 3/32 in, seat the matching extractor in the hole and turn it out counterclockwise with steady pressure.
7. Chase the hole with the 8-32 tap to clean the bruised threads, running it in slow and backing out a half turn at a time to break the chip.
8. Vacuum the chips from the box, run the new stainless screw home with anti-seize to snug plus a quarter turn, and do the bond check below.

One rescue worth knowing: extractors are hardened steel, and one snapped off inside the stub will skate off ordinary bits. A carbide tile bit, the arrow-headed kind, cuts it out - that is the accepted way out of that hole.

The threads are stripped: drill and tap, or replace the box?

Step up one screw size: drill the hole to #21 (0.159 in), cut 10-32 threads, and drive a #10 stainless screw; the jump from 8-32 costs only 0.013 in of boss material around the hole.

Show the arithmetic: (0.190 − 0.164) / 2 = 0.013 in of wall given up. A metal boss keeps most of its material, and the 10-32 screw holds the cover exactly as the original did. In a plastic box the same tap job works - plastic holds cut threads well - but snug the screw rather than cranking it, because a #10 driven hard splits a plastic boss.

Epoxy and toothpick fills are for plastic covers only. A wood sliver or a 10-minute epoxy fill gives a screw bite in a stripped plastic hole and keeps a gasket lightly compressed, nothing more. In a metal box those fills break the bond: NEC 250.96(A) requires metal parts serving as equipment grounding conductors, enclosures included, to be bonded so they carry the fault current imposed on them, and epoxy is an insulator. Do not glue a weatherproof cover on with silicone either - it hides the missing fasteners and leaves the next person cutting it off.

Does the repaired box still ground and seal like it did?

Yes, when the screw turns into real threads: bond the metal box, prove the bond with a meter, and keep a box rated for its location - a weatherproof cover alone cannot turn an indoor box into an outdoor one.

The bond check takes a minute. Put one probe on the box body and one on the equipment grounding conductor; you should read the same resistance you see with the probes shorted together - your lead resistance, in practice a fraction of an ohm - rather than an open. The enclosure grounding and bonding guide walks the whole path, and enclosure grounding screw threads and lug fit covers the threads and lugs themselves.

Sealing is the box's own job. The NEC matches enclosure types to locations in Table 110.28, so an outdoor site needs a box of a type listed for wet conditions; the in-use cover closes the weather, and the box has to be built for it. Check the gasket while the cover is off - a flat, cracked edge is where water got in and rusted these threads in the first place.

When should you stop repairing and replace the box?

Replace it when the plastic is cracked or sun-brittle at the boss, when the metal has rusted through a wall, or when this is the second hole you are tapping; one retap is a repair, two is a failing box.

For a single-gang splice, a new metal weatherproof box set into the mortar ends the job. When the box carries equipment - a small drive, a power supply, low-voltage gear - go stainless and end the rust cycle: wall-mount stainless enclosures start at 6 x 4 x 4 in in 304, with 316L for salt air and chlorides. They are rated NEMA 4X / IP66 by the manufacturer; not UL listed. Standard boxes come empty in 304, and a box with your gland and mounting holes already cut is a custom enclosure - delivered in 10 business days, counted from your order. Measure the mounting boss spacing before you move equipment over, and the outdoor enclosure guide covers the rating choice for the site.

How do you keep the next screw from snapping?

Use stainless screws with anti-seize, swap any screw that shows rust at the first opening, and stop at snug plus a quarter turn instead of pulling the driver to the stop.

Stainless into stainless is the classic galling pair: the threads cold-weld under torque and the head snaps. Anti-seize at reassembly, or a coated stainless screw, keeps the threads separated. Rust is the other killer - a screw with scale on the shank is one season from breaking, so replace it while the cover is open. Seat the gasket evenly all the way round when you close up; a cover squeezed hard at one corner gets forced open next time, and forced screws break.

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