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Japan has a JIS metric screw thread standard that largely follows the ISO, but with some differences in pitch and head sizes. The coarse pitch is the commonly used default pitch for a given diameter. In addition, one or two smaller fine pitches are defined, for use in applications where the height of the normal coarse pitch would be unsuitable (e.g. threads in thin-walled pipes). The terms coarse and fine have (in this context) no relation to the manufacturing quality of the thread. The screw length is the third and final number you find in metric callouts. It’s measured differently depending on the type of the screw head. A metric ISO screw thread is designated by the letter M followed by the value of the nominal diameter D (the maximum thread diameter) and the pitch P, both expressed in millimetres and separated by a dash or sometimes the multiplication sign, × (e.g. M8-1.25 or M8×1.25). If the pitch is the normally used "coarse" pitch listed in ISO 261 or ISO 262, it can be omitted (e.g. M8). [4] :17 If you’re used to using the imperial screw sizes, reading screw sizes in mm might be confusing to you.
Typically, screw callouts contain three numbers whether for imperial or metric ones. Metric callouts usually contain:
Metric and Imperial Screw Sizes
Below are some common spanner (wrench) sizes for metric screw threads. Hexagonal (generally abbreviated to "hex") head widths (width across flats, spanner size) are for DIN 934 hex nuts and hex head bolts. Other (usually smaller) sizes may occur to reduce weight or cost, including the small series flange bolts defined in ISO 4162 which typically have hexagonal head sizes corresponding to the smaller 1st choice thread size (eg. M6 small series flange bolts have 8mm hexagonal heads, as would normally be found on M5 bolts). [9] It is always best to get the right type of screw for the job, so we have written a project specifically about this. Have a look here for our project on the different kinds of screw you can buy. These are the basics to know when looking at screw sizes but to find out more about the thread of screws and other items you can look at Wikipedia. There’s a lot to learn if you’re interested! Metric Screw Sizes Explained In our metric screw size chart, you’ll find all the information you need to buy the right size of screws for your project. Our chart includes: The relationship between the height H and the pitch P is found using the following equation where θ is half the included angle of the thread, in this case 30°: [3] H = 1 2 tan θ ⋅ P = 3 2 ⋅ P ≈ 0.866025 ⋅ P {\displaystyle H={\frac {1}{2\tan \theta }}\cdot P={\frac {\sqrt {3}}{2}}\cdot P\approx 0.866025\cdot P}
If it won’t budge and needs to be drilled out, start by taking an awl or centre punch and using it with a hammer to make a small indentation in the visible end of the grub screw - this will help reduce the risk of the drill bit from slipping or wandering while you drill through the metal Unlike imperial callouts, the metric screw size callouts always start with the letter M followed by a number. This number is the screw diameter.Insert the correct kind of drive for the type of grub screw in question, and make sure it is securely seated Most manufacturers put both the metric and imperial size on the box of screws which is very helpful, however when purchasing online, many retailers do not. This is largely because the title of the product becomes too long and cumbersome, so something has to go. Whether you deal in old or new money, as it were, you still need to know what you are getting. This is the difference between the two and what you need to look for: The length is given next and it should be remembered that the length given for a screw is the length that is buried in the wood or other material, it does not include the head of a raised, or domed headed screws. If you are struggling to find the best way to get your screw into whatever you are screwing, then have a look at our project that explains all about screwdrivers.
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