Why Isn't My Allen Wrench Working? Causes and Fixes

Start with the fit

A hex key only works when it fills the socket snugly. A key that is slightly too small will wobble, spin and round out the corners of the screw. A key that is slightly too large will not go in at all, or it will go in only partway.

The most common mix-up is metric versus SAE (inch) sizes. Some sizes are very close. For example, a 5 mm key and a 3/16 in key look nearly identical, but neither fits the other's screw well. Try the next size up and the next size down. The right one slides in with almost no play.

• Test keys in the socket by hand before applying force.
• If a key rocks side to side, it is too small.
• If the key will not enter, check for dirt, paint or a different size system.
• Stop if you feel the key slipping, since more force will only strip the screw.

Clean and seat the key properly

Dirt, paint, thread locker and rust can fill a socket so the key only goes in part of the way. A partly seated key puts all the force on a small area, and the tip rounds off quickly.

Use a pick or a small brush to clear the socket. Then push the key in fully and keep it straight, in line with the screw. Hold it at about 90 degrees to the fastener unless you are using a ball-end key, which is designed to work at an angle.

Press in firmly while you turn. Steady inward pressure helps keep the tip from climbing out of the socket.

When the screw is stuck or over-tightened

If the right key fits and still will not turn, the screw is probably seized by rust, grime or thread locker. Applying more force with a cheater pipe is risky. It can twist the tip or round the socket.

Penetrating oil is the safer first step. Put a little into the recess, wait 10 to 15 minutes, and try again. Tapping the end of the key gently with a hammer, once it is seated, can help break the bond. A longer arm gives more leverage, so turn the key by the long end for loosening only if the fit is solid.

If you use any heat on a stubborn fastener, keep it away from fuel lines, plastic, and anything flammable. Wear safety glasses during this work, because a key that slips or snaps can send metal fragments or cause you to hit your hand.

• Clean the socket.
• Apply penetrating oil and wait.
• Seat the key fully and tap it lightly.
• Turn with steady, even force instead of jerking.

Worn keys, stripped sockets and star-drive screws

Keys wear out. If the tip is rounded, twisted or chewed up, it will slip even in a good socket. Compare the tip against a fresh key. If the corners look soft, replace the key rather than forcing it.

A stripped socket is harder to fix. A common home trick is to press a rubber band between the key and the socket for extra grip. For deeper damage, a screw extractor is the usual tool. Wear eye protection when using one.

Also check that the fastener really is a hex. Torx (star) screws look similar at a glance, but a hex key will not hold them properly. A hex-only set cannot turn them, so you need Torx keys for those screws.

Match the tool to the job

Sometimes the problem is the tool style. L-shaped keys give reach and are simple, T-handles are easier to turn with force, and folding sets suit lighter work. Ball ends help around obstructions but are generally not the best choice for final tightening or loosening a stuck screw, since the contact area is smaller.

Size range matters as well. Our best allen wrench set page lists which sets cover which sizes. For example, the ELEAD 41-piece set lists metric sizes from 0.7 to 10 mm, SAE sizes from 0.028 to 0.375 in, and Torx keys from T9 to T50 in one case. A set that tops out at 8 mm or 0.25 in will not fit larger bolts.

Pick a set that avoids the problem

If you keep running into mismatched sizes, a complete set with clear size marks makes the guessing stop. Our allen wrench set comparison covers color coding, laser-etched markings and slotted cases, which make it easy to find the right key and to notice when one is missing.

Look for chrome vanadium steel, which several of the ranked sets list, and chamfered tips that slide in smoothly. Check that both size systems and any Torx sizes you need are covered before you decide.

Frequently asked questions

Can I use a slightly smaller key if I have no exact match?

It is better not to. A loose key rounds the socket quickly. Find the exact size, or try a Torx key only if the screw is really a star drive.

Why does my key spin without gripping?

Either the key is too small, the socket is stripped, or the key tip is worn. Try a different key of the next size and inspect the tip for rounded corners.

Should I use a pipe on the key for more leverage?

Avoid it. Extra force can twist or snap the key and strip the screw. Use penetrating oil and a longer or T-handle key instead.

Do ball-end keys work for tight screws?

They are handy for reach at an angle, but the smaller contact area means you should use the straight end for the final tightening or a stuck screw.