How to Measure Metric Bolts
If you are working with metric bolts, you will quickly realize that you no longer know the part numbers and measurements. All the numbers you might need to know will now be given in millimeters instead of inches, or fractions of an inch, so if you find that you’re using metric bolts, you will likely have to measure them from scratch. Here is how you can obtain metric measurements if you find yourself in need of them during your next project.
Step 1 - Measure for Diameter
First you’ll want to measure the metric bolt from one side of the shank to the other. The easiest way to do this is to have a caliper that measures in metric units. Clamp the caliper around the bolt and adjust it until it fits and it should provide the exact measurement you need in millimeters.
Step 2 – Find the Pitch
The thread pitch of a bolt is the difference between threads measured in millimeters along the fastener. To calculate this manually, you can count the threads of the bolt on the entire shank and then divide the total length of the shaft by this number. There are devices that can help you with this a little more quickly, including a bolt gauge. On a bolt gauge, you will find a series of ridges with varying widths. Some of these will pertain to metric bolts and will be marked as such. You can test against these ridges, one by one, to find which fits tightly with the threads of your bolt. The number written beside this set of ridges will be the pitch you need.
The number you measure for your pitch also determines if the bolt is a fine or coarse type. As a general rule, bolts with a pitch of 1.5mm or below are considered fine.
Step 3 - Measure Length of Bolt
Since the length of the bolt is the last number in a metric measurement, you should measure for it last. Start from right below the head, including only the shank, and measure to the tip. This rule for bolt length holds true for most types, including cheese, hex, pan, socket, button, and low socket head types. Only measure the entire length if you are working with an oval or flat head variety.
Step 4 – Combine for the Final Part Measurement
These three numbers combined will give you the final measurement you will need, in the order that you’ve found them in these steps. So, for example, a bolt with a 5mm diameter, a 1.0mm pitch, and a 25mm length will be written as M5-1.0x25, with the “M” standing for metric.