Screw identification techniques/tools?

With all the work-from-home I’ve been trying to get my desk set up more properly with some multi-monitor mounts. I have a supply of similar/identical mounts from old business LCDs(so they have proper spring-loaded height adjustment and VESA mounting, not just little plastic bits) which seemed like a good starting point.

They all have a similar design, with a pair of slide rails held inside the outer frame by machine screws. My plan is to replace the screws with slightly longer equivalents and add a couple of pieces of L-bracket with holes drilled in the correct place to create a ‘stack’ of 2 or more of the mounts held together by screws that go through the L-bracket, through the original frame; and into the slide rails.

However, I’ve hit a little bit of a snag: I’m not entirely sure how to correctly ID the size and thread pitch of the screws the vendor used so that I can obtain equivalent-but-longer replacements. Normally I’d head down to the hardware store and ask them; but I’m trying to minimize household exposure right now.

I’ve got calipers and decent vision, so if there are any “just use these rules of thumb idiot” tips I can use I’d appreciate them; if there is some sort of ‘test plate’ with a bunch of labelled holes tapped in common sizes and thread pitches that I should add to my toolbox; would also love to know about it.

Many thanks if anyone has input(both on the screw matter specifically and on the question of whether I should really be going down a different path here).

If your LCDs were manufactured outside of the US, the screws are probably metric and it’s not terribly complicated.

The common sizes are

M4 (4mm Diameter, 0.7mm Pitch)
M5 (5mm Diameter, 0.8mm Pitch)
M6 (6mm Diameter, 1mm Pitch)
M8 (8mm Diameter, 1.25mm Pitch)

M10 and M12 are generally found in course pitch (1.5 and 1.75mm) except on Japanese motorcycles.

If you are dealing with US made stuff you’ll probably be using either UNF (Unified Fine Thread) or UNC (Unified Course Thread). I struggle to identify these 'cos it’s all 11/53rds of a mile and such.

But if you know what your dealing with (UNC, UNF, Metric, Whitworth etc.) you can get screw thread gauges like this…


I was in a similar situation a few (20+, lol) years back and was able to pick up a set of thread gauges in American, Imperial and Metric from a snap-on truck. They’ve been occasionally indispensable.

Today you can get a full set from Amazon for a fraction of the price I paid back then.

There’s two philosophies of thread gauges; either get ones with great big friendly handles, or get the nearly microscopic ones you can’t use when you’re drunk and keep the whole set in a padded altoids tin with your feeler gauges to save toolbox space. It’s like cooking pans, don’t go middle of the road.


And I thought this thread was going to be dirty.

Ditto on thread gauges.


I have one of these, it works for 90% of such jobs, most hardware stores have them under one name or another for under $10:

(And if you want to use equipment on hand, your caliper should give you diameter, and you can count threads/inch or threads/centimeter, which is enough with the right table, but I don’t have a link bookmarked. This is what I used to do.)


Thanks for all your assistance; I appreciate it.

Truth be told I’ve been finding myself really nervous trying to get into this project, without obvious reason, so I’m glad to have some assistance on this question; I’m hoping that it will help.

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