12 years ago
The minor diameter of the fine thread (under 2") appears to be about .115
(mebbe less, due to inadequate penetration of caliper edge), and .090 on the
coarse. Ergo, more "grab" with the coarse.
I'm guessing the following:
Coarse thread is for sheet rock on metal studs -- more grab on studs.
Altho framing screws, which afaik are only for metal stud to metal stud,
also come in coarse and fine thread, so this may muddy this particular
logic. Mebbe for different gauges of metal studs? I've seen some much
heavier than others.
Coarse thread for particle board et al.
Fine thread for hardwood.
fyi, there exists a 3/4" and 1 1/8 SR xcrew, very hard to find.
The 3/4 is VERY handy, when screwing from the back side of a good face, and
the 1 1/8 occasionally comes in handy.
A good hardware store, proly one that sells SR screws by the pound, can
order a 25# box. If he hesitates, tell him that the 3/4" will FLY out of
the store when people grok their utility.