How to Tell the Size of a Threaded Bolt

by Dan Ferrell

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The most common method to fasten components to engines, appliances, furniture and many other types of equipment is to use bolts. They come in different sizes and configurations, depending on the specific part they need to hold in place. Therefore, it is important to replace a worn out, damaged or lost bolt with the same type used by the manufacturer. However, before you go out to buy a replacement, you need to know exactly what bolt size and type you need.

Step 1

Determine the bolt strength. On a standard (US) bolt, lines or slash marks on the head indicate grade or strength. The more lines it has, the stronger the bolt. Metric bolts may have numbers on their heads for the same purpose. This code tells you how much torque (tightening) the bolt can withstand, and the vehicle service manual for your particular make and model will indicate the exact torque to apply. If you buy a bolt of lesser strength, it might break when you try to fasten it.

Step 2

Measure the outside diameter of the bolt threads using a vernier caliper. The thread diameter is what is referred to as the bolt's size on the product package. Use inches for standard bolts and millimeters for metric bolts.

Step 3

Measure the distance across the flat sides of the bolt head using the vernier caliper. This measurement will give the size of wrench you would need to unfasten or tighten this bolt. Use inches for standard bolts and millimeters for metric bolts.

Step 4

Measure the distance between the bottom of the bolt head and the opposite end of the bolt using the vernier caliper. This will give you the bolt length. Use inches for standard bolts and millimeters for metric bolts.

Step 5

Measure the bolt thread pitch using a thread pitch gauge, which you can buy in most tool or auto parts stores. This will give you the number of threads per inch, if you are measuring a standard bolt or the distance between each thread for metric bolts.

Step 6

Determine the type of thread, if you are measuring a standard bolt. You will find two types of threads: Coarse thread (Unified National Coarse, or UNC) and fine thread (Unified National Fine, or UNF). Coarse threads have a wider distance between each thread than fine threads.

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