Installing a lockset today (el cheapo Kwikset entry w/deadbolt), I noticed they included two of those weird screws in the hardware package. I worked out that they were intended to hold the passage lock strike to the jamb.
They're regular flat-head wood screws, except that the threads just under the head change from coarse to fine, almost like a machine screw.
Does anyone know the purpose, or the rationale of using these screws? I've often seem them in lock installations, but nowhere else. Are those fine threads supposed to grip the wood better?
They're finely spaced but not more shallow than the other threads, right?
I haven't seen them but from your description, I think the screw is meant to screw in quickly most of the way, and then slowly but with more staying power for the last bit.
But does that really solve any problem. AFAIK, there is no risk of a lock strick screw coming out head first. The problem, as when someone kicked in my door the first summer I lived here, is that the wood behind the jamb isn't thick enough sideways, or the screws aren't long enough to spread the load when the door is kicked. The heads of my strike screws didn't move to the side at all. but the door jamb cracked, probably more than a half inch, and then sprang back all but a quarter inch.
And when I got home, 8PM on a summer Sunday evening, the door was ajar. But I never found anything missing, not the new car radio still in the box on the basement workbench, not the fancy home burglar alarm, still in boxes on the bedroom floor, and not even the cookies on the kitchen table.
All I can imagine is that the dog next door, which I hated because its barking woke me up in the morning and wouldn't let me get to sleep at night, barked when he heard the noise and scared away the burglar. :)
Those are called combination machine-wood screws...
The lock manufacturer supplies them in the lock kit so that you can install the strike plate included with the lock with the same screws on both wood and metal door frames... It saves the lock manufacturer money by omitting inclusion of two pairs of screws one of which would go unused and be discarded...
Yeah, you had my head spinning trying to figure out how a small amount of finer threads near the head could result in the screw going in slower near the end. It would still have the other 80% of the coarser threads deep in wood pulling it along too, no? I couldn't figure out what they were for either, but the metal explanation sounds logical.
My only question about that is they do the same thing with the screws you use to put down roof tiles and there is no metal involved. I always assumed the 2 different screw pitches tended to wedge the threads and make them harder to back out. The other thing that happens is when you are pulling through laminations and the screw delaminated the top layer the slower thread at the head will pull the lamination back together right at the end.
Some doors or jambs are already cut for 8-32 threads. With the "hybrid" screws, one screw either does wood screw usage, or goes into threaded metal doors, or metal jambs.
It's a very inexpensive and quick thing, but have you considered removing the screws of your strike plate and replacing them with
3" ones that reach well through the casing and into the crip? On some of my exterior doors, the strike plate for the deadbolt has an inner reinforcing plate with long screws provided for just this purpose. Once installed, the adjustable strike plate is screwed on over top of it.
It is logical only if the fine thread portion is larger diameter than the course thread part. I have never seen screws as described but I _could_ open the new lockset I bought a month ago....nah, that would only result in posting a fact, why screw up speculation? :)
I'm not the one you asked, but in my installation there was no need for such substitution. The lock kit came with extra-long screws to use to secure the deadbolt strike. (No adjustable 2-piece strike in this case, though.)
The machine-threaded part of the screw appears to be exactly the same root and overall diameter as the wood-screw part. hard to see how the wood-screw threads wouldn't mess up the machine threads in the jamb.
You probably have these installed somewhere in your house.
So I think I'll go with my instincts next time and just throw those away and replace them with regular wood screws. I don't think those machine threads hold as well as wood threads would (in wood) (if a woodchuck could ...)
I've done that on a lot of my doors in the past. Also a few long screws on the hinges.
I've used long heavy duty strike plates that combine the latch and the dead bolt holes into one piece. I've seen the wrap around piece of sheetmetal for on the door to strengthen the latch and deadbolt but never used one.
I've used these in machine thread applications. Yes, I'd also wonder that the wood screw threads would disturb the machine threads. However, it's worked out and not been a problem. The screws go in, and appear to engage the machine threads properly.
If you're going to use long screws for security, you should also shim between the casing and the frame so the screws don't screw up the casing. ;-) It'll make the assembly stronger, too.
. . . or use common sense and an oversized pilot hole to prevent racking. I never warped a casing when I drilled an oversized "slip" hole for the screw and then didn't overtighten it.
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