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I changed a deadbolt lock on my front door, and in the pile of old parts, there was a weird screw with 8-32 threads near the head and wood threads at the end. I'm just curious:

  • what is the name for this type of screw?
  • what is it used for?

thanks

enter image description here

FJZ
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  • Looks like a specially made double threaded screw. Be a good ideal if you need it, do not lose it. Probably have to go to the manufacturer to get a replacement. – crip659 Nov 14 '21 at 16:37
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    So where was it fitted in this lock? Telling us that or even adding an image may help. I have an idea but without you showing where it was fitted I can’t be sure. – Solar Mike Nov 14 '21 at 16:40
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    I've seen those. My guess is that the same screws can be used to attach door latch hardware to both wooden and metal doors, for example the strike plates, and are included as some kind of cost-cutting move by the manufacturers. – gnicko Nov 14 '21 at 16:40
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    It's a variant on a "self-tapping" screw. There's a specific term for it but I can't recall it. – Hot Licks Nov 15 '21 at 01:10
  • @HotLicks I think they call these a 'lag screw' – Aaron Lavers Nov 15 '21 at 04:46
  • @AaronLavers This does not look like a lag screw to me; those typically don't have threads all the way up the body (and are also always much, much bigger than this tiny little thing), so that screwing the screw all the way in pulls the two members tighter together. With threads all the way in you can't pull them closer together. – TylerH Nov 15 '21 at 14:27
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    @gnicko It's unlikely specialty hardware would be used as a cost cutting measure. It's probably going to be cheaper to include both wood and sheet metal screws than have their own screw made that's rarely ever used in other applications. Quantity does wonders for price reduction for hardware. – Doresoom Nov 15 '21 at 14:37
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    @AaronLavers - It's not a lag screw – Hot Licks Nov 15 '21 at 16:40
  • @Doresoom - I've seen weirder things. I didn't say it was a _good_ move. Should have used the sarcasm font I guess.... – gnicko Nov 15 '21 at 17:35
  • Are there any brand or manufacturer markings on the pile of parts? Are there threads in the metal parts this screw went through? – Freiheit Nov 15 '21 at 21:28
  • I recently purchased some locksets from Schlage (a popular lock manufacturer in the USA) and they included one pair of standard wood screws, and one pair of these. One pair for the strike plate, and one pair for the bolt flange. But I don't know which was intended for which :) – bitsmack Nov 16 '21 at 05:03

3 Answers3

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That makes a lot of sense to me. The 8-32 threads would hold securely in the metal frame of the lock mechanism itself, while the wood screw portion is just small enough to slide through the metal part and then be able to bite into the door frame.

If you used a simple wood screw then it would just slide through the metal part but not actually "hold" to it, so it would not be as secure unless tightened really well. This combination screw is not, in my opinion, a cost-cutting measure, but rather increases holding power, and therefore security. Which is the point of a lock.

In a little (Google...) research, I found a lot of double-ended screws without a head. Those appear to be either "one screw to work either way you need it" (cost-cutting by supplying one part instead of two) or for connecting two dissimilar items - e.g., a cabinet knob (machine screw) to a cabinet door (wood screw). But my hunch for this example, with a head and used with a lock, is a different functional purpose.

manassehkatz-Moving 2 Codidact
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    Thanks for the comments. I think it held the latch faceplate to the side of the door, and the hybrid threads would make sense in that they would work with a wood or metal door. And it looks like it may be called a machine/wood combination screw, for example at https://discountdoorhardware.ca/product/flat-head-12-24-combination-screws-satin-nickel-50-pack/. – FJZ Nov 14 '21 at 17:10
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    The dual-threaded rods (wood screw on one end, machine thread on the other) are indeed for attaching dissimilar things. You put 2 nuts on the machine thread, jam them against each other and use those to drive the wood screw. Un-jam the nuts with 2 wrenches, then attach your device with nuts. – Harper - Reinstate Monica Nov 15 '21 at 02:29
  • "*That makes a lot of sense to me.*" Sorry, *what* makes a lot of sense to you? 'That' is a reference to an undefined subject, since it's the first thing you say in your answer... (and OP doesn't offer a guess as to something which you could be referencing) – TylerH Nov 15 '21 at 14:27
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    @TylerH the existence /design of the screw makes sense to me. – manassehkatz-Moving 2 Codidact Nov 15 '21 at 14:31
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    I assume this screw goes on the *inside* faceplate, wouldn't it? At least every lock I've ever seen would not have accessible screws on the *outside* if installed properly, that'd be an obvious vulnerability. – Darrel Hoffman Nov 16 '21 at 15:36
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First, you don't want to have screw threads on the latch assembly (the outer of the two things you are screwing together). Otherwise it may bottom out on the latch assembly before it is screwed all the way into the wood!

In my observation, this is a "universal"/compromise screw that is designed to be usable in a wooden door... but also usable in a fabricated metal door which has tapped locations for the latch to mount.

If anything I'd call it weaker, as the machine thread will have extremely poor purchase in a softwood door. I would much rather use a proper wood screw that is wood its whole way. On a metal door these are as good as any brass machine screw of the same length of machine thread.

Harper - Reinstate Monica
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  • Is it possible that the machine thread portion was meant to engage with part of the lock hardware? – JimmyJames Nov 15 '21 at 16:01
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    @JimmyJames No - if it did that (say the machine threads loaded up on the hardware somewhere) then the screw force would be between those loaded threads and the head of the screw. Nothing could hold the hardware to the door frame in this case - the hardware would just be held to itself and you'd still need a fastener to hold the whole thing to the door. Harper is right - this is a universal cost-saving convenience screw - the manufacturer puts one into the bag and it works no matter what kind of door you have. – J... Nov 15 '21 at 18:11
  • @J... Good point, I didn't consider that. – JimmyJames Nov 15 '21 at 18:35
  • @JimmyJames Same deal. It won't hold as well. – gnicko Nov 15 '21 at 20:58
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Figured I'd move my comment to an answer:

The same screws are intended to be used to attach the strike plate hardware to both wooden and metal door frames.

I'm not sure why the manufacturer(s) are doing this. Perhaps so that they can include only one pair of screws instead of two or more pairs for use with different materials. It may be a cost-cutting decision or just something intended to make installation easier.

gnicko
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