WiseAutoTools.com Summary: We cover few ways to determine the size of a bolt. This is needed to select the correct size thread bolt repair kit. Different size kits have different size tooling, so they are not interchangeable. We also debunk the common mistake of using the head of the bolt to determine the size of the bolt threads. A very common mistake. The simplest way to determine the size is to go to a parts store and use their thread size sample display (AKA identifier or gauge) or use their nuts and bolts selection to match your size. Not all parts stores have the nut and bolt size gauges to determine the size of nuts and bolts. Read on to learn more about what those measurements actually mean and how you can measure them yourself if a nut and bolt thread size identifier are not available.
To determine the size of a bolt we basically want to measure three things. Diameter, thread length and thread pitch.
Measure the diameter with a digital micrometer. Metric measurements of diameter are typically slightly under and rounded up. So in this case the diameter is considered 12mm. Being slightly under-sized allows the installation into a 12mm hole. If it was exactly the same size with no tolerance, it would not fit or would be easy to cross thread.
Measuring the thread length is self explanatory. It's important to consider if the threads in the hole start down a ways or are basically flush with the surface. For example, head bolt threads in cars commonly have threads that start down in the hole several inches. Surface mount, or regular small Time Sert kits are not suitable for that application because the tools are only designed for threads that start near the surface. For that reason, head bolt threads are repaired using Time Sert head bolt kits.
Measuring the thread pitch. The thread pitch is simply a measurement of how coarse (or fine) the threads are. In the example above, the thread pitch is being measured from peak to peak, the distance is 2.00 millimeters.
The size of this bolt is M12x2.0. The head of the bolt is 13mm where the socket fits. If someone thought the head of the bolt determined the size of the bolt they would be mistaken. It's a common misconception.
For even more information, see our main page with many more articles and videos on how to repair stripped threads.
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