I wouldn't want to make the call for you, but there is some reading material available to help you decide. There are probably better sources available than the ones I've listed, but the general suggestion is that there used to be many different thread types (for various reasons) and now most manufacturers are on the ISO engineering standards. As an example, fine pitches were/are used where thread height is too great to use - as with thin-walled pipe.
Metric screws
The diameter of a ISO preferred series screw is specified in millimetres (mm) prefixed by the capital letter M, as in "M6" for a 6 mm diameter screw.
The pitch of metric threads varies according to the diameter, but not absolutely regularly. Some examples: a M3 thread has a 0.5 mm pitch, M4: 0.7 mm, M6: 1 mm, M10-12: 1.5 mm, M14-16: 2 mm, M18-22: 2.5 mm.
The diameter of a metric screw is the outer diameter of the thread. The tapped hole (or nut) into which the screw fits, has an internal diameter which is the size of the screw minus the pitch of the thread. Thus, an M6 screw, which has a pitch of 1 mm, is made by threading a 6 mm shaft, and the nut or threaded hole is made by tapping threads in a 5 mm hole.
Metric screw threads are also available in "fine pitch" versions, sometimes several pitches for one diameter (example: M18/fine in 1, 1.5 and 2 mm pitches). The fine thread series is deprecated and not recommended for use in new designs. The fine metric threads were once found in equipment made in the Far East , but that has changed with the standardisation of the ISO preferred thread series.
Note that M12 x 1.75 is the ISO default:
web page 2 web page 3If you poke around, you should be able to find an engineering table that would compare the holding strength of various thread sizes. Here's a list of ISO standards - if you can identify the correct one, you'll find your answer, but you'll probably have to pay for a copy:
web page 4 If I had to choose between bolt strength and thread pitch, I'd choose bolt strength. I'd also use lockwashers AND nylock nuts (I have them on my A-arm bolts).
Frank
