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Questions relating to threaded fasteners and their use.

29 votes
2 answers
147k views

What do the numbers in screw sizes M3, M4 etc. mean?

I noticed that in some (many?) countries, screw sizes are listed as "M+number", e.g. M3, M4, M5 and so on. What does this size designation mean?
2 votes
1 answer
896 views

Are black/dark-gray-colored screws typically weaker?

I am no expert on screws. When I go buy some, I'm typically handed screws that have a silvery color. I have never asked about the metal composition. … Does cam-out remove more of their material than regular screws? …
38 votes

What do the numbers in screw sizes M3, M4 etc. mean?

Mn for a number n means that the diameter of the screw, projected onto the transverse plane, is n milimeters - including the threading. The "M" stands for "Metric" (as opposed to British-imperial unit …
ThreePhaseEel's user avatar
32 votes
10 answers
15k views

What is a NON-self-tapping screw? What does self-tapping mean, really?

How do I tell apart self-tapping from non-self-tapping screws? Note: I'm not asking about "self-drilling" screws - the ones whose tips look like the end of a metal drill bit. … Those are markedly different than what I know as "just" screws. …
0 votes
3 answers
955 views

How do I measure my screws to match how stores label them?

When I buy screws, the container often has measurement labels, e.g. : 50/4 or 20/2.5 and such. … However, when I try to actually measure these screws - it doesn't quite fit. My 50/4 screws are around 43 or 44 mm long, including the head; and without the head it's about 40mm. …
-1 votes
1 answer
813 views

What kind of screw should I use to affix plywood to a hollow metal door?

I need to affix a plywood frame (depth ~1.25 cm or so) to an office entrance door. The door is metallic, but not a solid metal block: it's hollow. The metal face of the door is about 2mm. This is the …
0 votes

What kind of screw should I use to affix plywood to a hollow metal door?

I ended up using 4mm-width, 30mm length wood screws, i.e. screws with a conical head and a cross shaped recess (for Philips/Posidriv screwdrivers). …
einpoklum's user avatar
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