How to Use an Allen Wrench

May 02, 2023

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It's a simple tool that only grows more useful in experienced hands.

At first glance the Allen wrench doesn't seem to be much of a tool. It's little more than a small, L-shaped piece of metal. But don't be fooled by its humble appearance, the Allen wrench is an effective hand tool that can produce an incredible amount of torque. In fact, no toolkit is complete without a set of Allen wrenches.

Allen wrenches, which are also called hex-key wrenches or hex keys, have a hexagonal cross section to fit various machine screws and set screws that have hexagonal-shaped recessed holes. Hex fasteners are common in all sorts mechanical equipment and devices, including bicycles, motorcycle engines, and assemble-it-yourself furniture (aka Ikea).

On motorcycles, hex fasteners allow an engine cover to be mounted flush to the engine case without protruding bolt heads. Most motors and machines, including anything with belts and pulleys, have tiny setscrews that only accept Allen wrenches. Most importantly Allen wrenches have six driving points of contact, so they’re easier to use and much less likely to strip or slip (known as cam out) than Phillips or slotted screwdrivers.

Most DIY furniture kits come with an Allen wrench that fits the included hex screws. But that little, unmarked wrench is easy to lose or misplace, so it's best to have a set of Allen wrenches on hand to complete the assembly.

💡To speed up the furniture assembly process, cut off the short L-leg of the wrench with a grinder or Dremel tool, then chuck the remaining long leg into a cordless drill. Now use the speed of the drill to quickly tighten all the hex screws.

Allen wrenches come in both SAE fractional-inch sizes and in metric sizes. The wrenches can be bought individually, but it's always best to buy them in sets, which are more economical and include all the most popular sizes. And since metric-size hex screws are more common than ever, be sure to buy two sets of Allen wrenches, one SAE and one metric.

One challenge of owning two entire sets of Allen wrenches is how to store them so that they’re neatly organized and readily accessible. Fortunately, Allen wrenches come in several different styles to suit most users’ needs. Some wrenches come attached to a metal split ring, which keeps all the wrenches linked together, but makes it awkward to use an individual wrench in tight spaces, unless you first detach the wrench from the ring.

Folding Allen wrenches come in a convenient, compact housing that's similar to a pocketknife. The individual wrenches fold out and the housing acts as a handle for increased torque. However, because the wrenches are permanently attached, you’re limited in some tight-space situations.

I prefer using Allen wrenches that are individually packed in some kind of snap-closed storage case. With this set, it is easier to lose an individual wrench, but it's a fair trade-off to have full and easy access to each end of all the wrenches. If you do lose a wrench, you can always buy a replacement. And if you’re going to buy one of these sets, be sure to get one that includes both SAE and metric sizes in one storage case.

Finally there's one more type of Allen wrench worth mentioning: T-handle Allen wrenches. These are popular with mechanics and technicians that work with hex screws all the time. Each wrench is affixed to a large handle that provides both comfort and enhanced power for really amping up the torque.

Joe is a former carpenter and cabinetmaker who writes extensively about remodeling, woodworking, and tool techniques. He has written eight books and is a contributing editor to Popular Mechanics. He also appears on the Today's Homeowner TV show, and co-hosts the weekly Today's Homeowner Radio Show. Joe writes from his home in Roxbury, Connecticut.

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