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How to Determine the Wrench Size for a Given Bolt Size

by Dan Ferrell

Knowing what wrench size to use when removing or installing bolts on your vehicle or any piece of equipment saves you a lot of time as you work. Sometimes, bolt manufacturers give you a clue about the correct wrench size to use with their bolts right on the package. Many times, however, you do not have this information handy. Either way, you have some practical methods at your disposal to determine the wrench size for a given bolt. This will save you time when making repairs on your car or any piece of equipment.

Finding a Wrench for Standard-and Metric-Sized Bolts

Read the size number given on the bolts' package, if you just purchased them. For example, it may read: 1/2 - 10 UNC - 2A x 1 3/4. A package containing metric-sized bolts will have an "M" before the first number.

Use a pocket calculator, if necessary.

Multiply the first number which is the bolt size in inches or millimeters, by 1.5 using a pocket calculator, if necessary. For example, for the numbers in Step 1, 1/2 x 1.5 = .75 or 3/4, which is the wrench size needed for this standard-sized bolt.

A vernier caliper is very useful when measuring bolt sizes.

Measure the distance across the flats of the bolt's head using a vernier caliper if the bolt is installed in your vehicle or on a piece of equipment.

Measure the outside diameter of the bolt's threads using a vernier caliper, if the bolt is installed in your vehicle or piece of equipment and the bolt's head is not accessible to the vernier caliper. Follow the instructions in Step 2 to find the wrench size to use for this bolt.

Try different wrench sizes until you find the one that fits snugly around the bolt's head, if you do not have immediate access to a vernier caliper or a calculator. Eliminate as much play between the wrench and the bolt's head as possible to prevent damage to the bolt as you tighten or remove it.

Tip

  • Always carry a plastic vernier caliper and a pocket calculator when working under your car. This will save you time and frustration when trying to determine the correct wrenches to use as you remove or install components. With practice, you will often be able to determine the correct wrench size to use just by looking at the bolt's head.

Items you will need

About the Author

Since 2003 Dan Ferrell has contributed general and consumer-oriented news to television and the Web. His work has appeared in Texas, New Mexico and Miami and on various websites. Ferrell is a certified automation and control technician from the Advanced Technology Center in El Paso, Texas.

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