Open-End, Box-End, and Combination Wrenches

The largest part of your tool collection will probably consist of wrenches. Wrenches are used to tighten and loosen nuts and bolts. As you’re probably aware, wrenches come in a variety of sizes, from very small to very large. The size of a wrench is determined by the width of the opening at its end

Note that auto repair technicians use both SAE and metric size wrenches. SAE wrenches are sized according to the English system of measurement; that is, SAE wrenches are sized in inches. Metric wrenches are designed according to the metric system, so metric wrenches are sized in millimeters. SAE wrenches are commonly used on older vehicles, while metric wrenches are used on most newer vehicles and imported cars.

Wrenches not only come in different sizes—they also come in different styles. The three most common styles are the open-end wrench, the box-end wrench, and the combination wrench.

An open-end wrench has a U-shaped opening on its end (A). An open-end wrench can easily be placed onto a bolt or nut. Because the wrench can be positioned easily, the open-end wrench is commonly used to quickly loosen or tighten bolts or nuts. However, an open-end wrench should only be used when the bolt or nut isn’t fastened very tightly.

Because an open-end wrench only contacts the sides of a nut or bolt, the wrench can easily slip. Therefore, it’s better to use a box-end wrench to loosen very tight bolts or nuts (B). The end of a box-end wrench fits completely around a nut or bolt head, providing better contact. Box-end wrenches also have thin heads, so they’re useful in tight places where there’s limited space around the nut or bolt head.

The combination wrench combines the best features of the open-end wrench and the box-end wrench in one tool (C). In most cases, both ends of the combination wrench are the same size. Therefore, you can use the box end to loosen bolts that are tight, and the open end to quickly remove them once they’re loosened.

Box-end wrenches are made in two different styles: the 6-point and the 12-point. The number of points refers to the shape of the inside of the box end. A 6-point wrench will provide better support to a bolt head than a 12-point wrench. Therefore, 6-point wrenches should be used on bolts that are very tight. When you use a 6-point wrench on tight bolts, the chance of the wrench slipping off the bolt head is greatly reduced.

However, one drawback of the 6-point wrench is that its head is thicker than that of a 12-point wrench. Also, a 6-point wrench can only be placed on a bolt in six positions. For this reason, the 12-point type is the better choice to use in tight places. Its head is thinner, and it can be placed on the bolt head in more positions. Often, in tight places, you can’t get enough “swing” on a six-point wrench to move it to the next turning position. However, with a 12-point wrench, the wrench swing required to turn to the next position is only half that required by the six-point wrench.

(D) shows a typical adjustable wrench. The opening at the end of an adjustable wrench can be adjusted to fit almost any size of bolt or nut. This allows one wrench to be used for a variety of tasks. However, adjustable wrenches don’t grip as well as other types of wrenches. They have a tendency to slip, and can wear off the corners of nut and bolt heads. For this reason, adjustable wrenches should only be used to remove bolts or nuts that are already loose. They can also be used in emergencies when the correct size of wrench isn’t available. However, adjustable wrenches shouldn’t be used when the correct size open-end or box-end wrench is available.