Why are there metric fine (and extra fine) threads?
Metric fine fasteners are typically stronger in tensile loads than standard metric (coarse) fasteners, as the thread depth is not so great and therefore there is less likelihood of necking and shearing failures. Note that any higher strength will only be achievable if the appropriate nuts are used (see below). Higher grade fasteners are more expensive to manufacture. Note that there are sometimes multiple fine pitches at the same diameter – if it isn’t a standard metric (coarse) thread, you should be careful to identify the precise thread pitch. You can do this by measuring over several threads, if necessary.
