Metallics JSS51634 Screw 5/16-18 x 3/4 SKT S-Scr Cup St (Jar 100)

Cup point. The hollow, ring-edged tip bites into the shaft and resists rotation, which is what you want for locking a pulley, collar, sheave, or coupling onto a plain shaft. If you need to clamp without marring the shaft, or you're seating against a flat or a drilled spot, a flat point or cone point screw is the better choice.

Hex key. It's a socket set screw, driven internally with an Allen wrench, with no external head, so it seats flush or below the surface of the collar it threads into. A 5/16-18 socket set screw uses a 5/32 inch hex key.

100 screws per jar. Metallics packs it as a jar rather than a loose box, which is what you want on a service truck or in a shop crib where a spilled box of set screws disappears into the floor.

Steel, per the Metallics JSS51634 spec. For wet, washdown, or exterior service where corrosion is the concern, specify a stainless 5/16-18 cup point socket set screw instead.

Metallics part number JSS51634. That single MPN covers the 5/16-18 x 3/4 steel cup point socket set screw in the 100-count jar, so there are no finish or pack options to specify on the order.

At 3/4 inch, the screw is longer than the wall thickness of most standard collars and hubs, which means it will protrude into the bore well past the inside face. That's fine when the screw has to reach a shaft across a clearance gap or a deep boss, but on a thin-wall collar it bottoms on the shaft before it seats and can be hard to recess. Measure from the tapped face to the bore and pick a length close to that if you need the screw to sit flush.