How to Choose the Right Allen Wrench Size for a Set Screw

Start with the screw, not the tool

A set screw is a headless screw with a hex socket. It presses against a shaft, spindle or stem to hold a part in place. The size of the hex socket is tied to the screw's diameter, so a bigger screw has a bigger socket.

That means the first step is to learn the screw's diameter and whether it is SAE (inch) or metric. Metric screws are labeled with an M, such as M4. Inch screws use a gauge number, such as #8, or a fraction, such as 3/8 in. A metric key in an inch socket may feel close, but it will not grip fully.

Common set screw sizes and the key they take

The pairings below follow the standard sizing for socket set screws. Treat them as a starting point, since some makers use a slightly different socket. Always confirm with a test fit before you turn.

• For an 8-32 set screw, use 5/64 in. Some web charts list 9/64 in for a #8 screw, but that is the key for a #8 socket head cap screw, which has a larger socket.
• For an M4 set screw, use 2 mm.
• For a 3/8 in set screw, use 3/16 in. Make sure 3/8 is the screw's diameter and not a length.

Faucet and door handle set screws

Faucet handles and door knobs often hide a small set screw under a cap, on the side of the base or under the lever. These screws are small, and the exact size varies by brand and age. The most common keys are in the small range, around 3/32 in, 1/8 in or 2.5 mm, but do not rely on a guess.

Try the key you think is right, then try one size down and one size up. The correct key slides in fully and does not rock. If it rocks, it is too small. If it will not enter, it is too large or the wrong system.

Pop off any decorative cap with a thin flat tool, taking care not to scratch the finish. For door hardware, a long-arm key helps when the screw sits in a recess, and our [[best-allen-wrench-set]] page shows which sets list long arms. Pay attention to the direction the screw faces, because it can sit on the underside of a lever.

How to test fit and turn the screw

A good fit matters more than anything else here. A loose key rounds out the socket, and a rounded set screw can be hard to remove.

Follow these steps:

• Clean any dust, paint or debris out of the socket with a pick or a puff of air.
• Insert the key all the way to the bottom of the socket.
• Check for play. Wiggle the key lightly. A correct size has almost no movement.
• Hold the key in line with the screw, then turn counterclockwise to loosen.
• Use steady pressure. Do not hammer on the key or add a pipe to the handle for more leverage.

What if the socket is stripped or the screw is stuck

If the key spins without gripping, stop. Try the next size up in the same system only if it fits without forcing. A slightly rounded socket may still accept a key that is a hair smaller than the original, but pushing a key in with a hammer can break the tip.

Stuck screws can sometimes be helped by a drop of penetrating oil and a few minutes of waiting. Keep oil away from brake parts and anything that must stay dry. If the screw still will not move, a screw extractor made for small fasteners is the next step.

Safety note: wear safety glasses when working with stuck screws and small tools, because a snapped key tip can fly. If the screw is on an appliance or a fixture near wiring, switch off the power first.

Which Allen wrench set to keep on hand

Set screws come in many sizes, so a full set saves guessing. Look for a set that covers both SAE and metric, and check that the smallest size is small enough for your screws. For the sizes above, you want at least 1/16 in through 3/16 in and 1.5 mm through 5 mm. Our best Allen wrench set comparison lists the size range for every set, so you can check coverage before you choose.

For example, the Amazon Basics folding hex key set lists metric 1.5 to 8 mm and SAE from 5/64 in to 1/4 in. That covers the 5/64 in key for an 8-32 set screw and the 2 mm key for M4. If you have very small screws, the ELEAD 41-piece set lists sizes from 0.7 mm and 0.028 in.

Long arms and ball ends help in recessed spots such as door handles. Ball ends let you turn a screw at an angle, but for final tightening, use the straight end for full contact. If you also work on bolts and nuts around the same projects, our best combination wrench set page covers the open and box wrenches for that side of the job.

Frequently asked questions

Can I use a metric key on an inch set screw if it fits loosely?

It is best not to. A near-fit key contacts only the corners of the socket, which can round it out. Use the correct system, or test several keys until one fits with almost no play.

How do I tell if a set screw is metric or inch?

Check the equipment's markings or manual first. If you have no information, try a key from each system. Only the right one will seat fully and feel snug. A thread gauge gives a sure answer if you have one.

Why does my set screw key keep slipping?

Usually the socket is packed with debris, the key is slightly too small, or the socket is worn. Clean it out, try the next size, and use a key with sharp, undamaged tips.

Should I tighten a set screw as hard as I can?

No. Snug it firmly with normal hand pressure. Overtightening can strip the socket, mark the shaft or crack a delicate part such as a faucet stem.

Do I need a Torx key for a set screw?

Almost never. Set screws use a hex socket. Torx keys are for star-drive screws, so a hex-only set is enough for set screws.