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I want to order screws like the one I have. The diameter of the threaded portion is 2mm. The length of the threaded portion is 8mm.

The screws are listed as Mn (where n is 1.6,2,2.5...) on various sites. The length parameter is also specified.

I have a feeling that the n represent the diameter of the threaded portion however even after having searched a lot I found no document that explicitly states this. Some documents state diameter=M2. So M2 relates to the diameter. But 2 could be in mm, 1/10 of an inch or it could be like shoe size where the number is not a dimension in itself but simply represents one of the many standard sizes.

Side note: I wonder why something as basic as this is not documented clearly. Specs generally go into excruciating detail which includes the naming convention. Where should I have looked for the meaning of this convention? I have gone through ISO-7045 but even it does not state what the n in Mn means.

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    So far as I know, the metric thread designations are not associated with American businessman Henry F Phillips, inventor of the crosshead screwdriver named after him.
    – RedGrittyBrick
    Apr 24, 2017 at 9:40
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    If you were searching for information based on the head type, that's why you couldn't find anything. It's unrelated.
    – jscs
    Apr 24, 2017 at 12:21
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    You're looking in the wrong place because Philips has little to do with it. This is all documented on the Screw and related Wikipedia pages, e.g., en.wikipedia.org/wiki/ISO_metric_screw_thread
    – whatsisname
    Apr 24, 2017 at 17:33

2 Answers 2

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"M" screws are all metric, by standard. The number following the M is the outer diameter of the screw threads, in millimeters, so an M4 screw's threads have an outer diameter of 4mm.

See Wikipedia for more information.

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I assume you're talking about a machine screw, with orderly threads like a bolt, rather than a metal or wood screw, with a coarser rougher thread that is usually not consistent.

There are a lot of parameters on a machine screw. The key is to divide and conquer!

Diameter and thread pitch

These are a matched set (in imperial units) so take them together. Diameter is the optimum unthreaded diameter of the bolt, i.e. The size of the hole the bolt is meant to fit. This will be in mm (M4, M12) or fractions of an inch (1/2) or rod numbers (#4, #6, #8, #10) for sizes below 1/4".

Thread pitch is the fineness of the thread, measured in millimeters between threads (1.25) or threads per inch (20).

They come in generally standard pairs, like M4-0.70, M12-1.25, #8-32 or 1/2-20.

Bolt length

This is measured from the surface that is clamped. A 50mm bolt length does not include the head unless it's a countersunk type.

Bolt head type

You have countersunk bolts... Carriage and elevator types... Domed head... Squared-off head... Square, hex or torx head where the outside of the head is the drive mechanism. And several varieties of each.

Drive type

Slotted, Phillips, socket head (which uses a hex key to turn), Torx, Robertson, you name it. In most cases you can afford to use a different head type if you can't find an exact match. In fact, it's usually a win to get away from the inferior slotted and Phillips types.

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