Finding The Right Size Hex Key

A guide to finding the right size of hex bolt and key for your project.

Working with hex bolts can sometimes seem like more trouble than it's worth, especially when you can't seem to find the right size key to fit the bolt.

Hex keys (also known as Allen Wrenches) come in a variety of sizes, measured in two different scales. For the metric system, there are hex keys that range from as small as 1 or 2 millimeters to as large as a centimeter. For the English or standard system, there is an equally large range of keys... unfortunately, the two systems don't really share the sizes of the keys, so if you've got the wrong system's bolt then there's a good chance that you won't be able to find a key that fits properly.

In order to find the correct size for an unknown bolt, start with the metric set of hex keys. The reason for going metric is that it is much more common than the standard set, so the odds are greater that your unknown bolt is gauged in metric instead of standard. Once you have your metric set, take a look at the hole in the top of the bolt and use that size to make an estimate as to which key should fit. If the key is larger than the hole, work your way down until you find the right one. If the key is smaller, work your way up. Should you find that you need a key that's right in between two sizes, then you might consider switching to standard to find the right size.

Of course, there is another (and slightly better) way to go about finding the right size hex key. Pack in with your hex keys either a caliper (preferably one that measures in both metric and standard) or a small 6-inch ruler with both inch and centimeter markings. Measure the size of the hole on the bolt (a caliper, especially a digital one, will make this much easier to get an accurate measurement), getting a measurement in both inches and millimeters. If the measurement comes out even with the millimeter size of one of your metric hex keys, then you know that you have a metric bolt and you need to use that specific key. Should it come out even with some fraction of an inch (which might take a little bit of mathematics on your part), then you know that you need to use your standard set and the appropriate hex key for that size.

Taking measurements can greatly reduce the amount of time that it takes for you to find the right hex key, though it does mean that you need to have a caliper or ruler handy. It's possible (if a bit more time consuming) to find them by simply narrowing down the possibilities one by one, and after a while you'll start to get the hang of which sizes are more common than others (as well as being able to identify individual sizes from a glance.) Either way, hex keys and bolts can be exceedingly useful if you simply take the time to learn how to use them.

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