screws
This doesn't really relate to a problem I have (the screws work just fine), it's more of a curiosity question. Some of the screws I've used recently have a small triangle on their head, as if for lining up something:
I was wondering what this is for, but couldn't really find anything.
What I did find was that bolts commonly have markings that relate to their strength, and screws often have manufacturer markings on their head, but this looks like neither to me.
I would say the optimal screw hole shape that will allow the maximum amount of torque to be applied, would be no hole.
Removing material from the head of the screw; no matter what shape, will weaken the screw slightly. The only way to maintain the full strength of the screw; and therefore achieve maximum torque, is to not remove any material. So a hex head screw (not to be confused with a hex drive screw), would allow for maximum torque.
I think I would see if I could remove the rusting screw. It boils down to, it is never going to be less likely to break off than it is right now. If you're lucky, you could coat it with silicone, seal out the air, and it won't get any worse, but that's just the head - the shaft may continue to deteriorate.
I'd try to get it out with just the torque I can work up with a screwdriver with a Torx bit, if I couldn't get it out by hand, I'd leave it.
I'd also try to get some penetrating oil on it, although it may be tough to get it past the head and down to where it counts. Also tap down on the head a little with a hammer to vibrate it a little, this often helps break a stuck screw free.
Best Answer
After reading up a bit on screws of various types I noticed this small snippet in the German Wikipedia:
It indeed seems to be a Spax screw and a bit worn. The cutting part of the thread is barely recognizable as such and the point is not quite pointed anymore. But other features are still there, such as the part of the head that creates its own counter-sink.
So I guess it came from some old wooden parts and I salvaged the screws. Or whoever. No idea where they came from, they were in my random screws drawers in the basement and just happened to be of the right size for what I needed.
When knowing what to search for one can also find images with that marking:
And it seems that some special-purpose screws still have the old marking:
Well, learned something new about screws, that not all wood screws are equal, and that I didn't even have to pre-drill holes for what I've been doing recently.