Metallics DS178M 14 x 1-1/2 in. 3/8 Indented Hex Head Slotted Sharp Point Tapping Screws - Steel Zinc - Each

Either works. The DS178M has an indented hex head with a slot, so you can run it with a 3/8 inch socket or nut driver or back it out with a flat blade. Head width measures 0.367 to 0.375 in. across the flats and 0.409 across the corners, so a 3/8 inch driver seats properly. The slot is 0.064 to 0.075 in. wide and 0.083 to 0.101 in. deep, which suits a medium flat tip rather than a fine cabinet blade. Most installers run the hex and keep the slot for removal in tight spots.

Type A is the right call when you already have a hole or the material is thin enough to pierce. The DS178M is a Type A tapping screw with a gimlet (sharp) point and 10 threads per inch, a coarse thread meant for thinner sheet and a pre-punched or pilot hole in heavier gauge. It does not drill its own hole, so if you are fastening into heavier steel with no pilot you want a self-drilling (Type TEK) screw instead.

Head dimensions follow ASME B18.6.3-2010 and the thread form follows ASME B18.6.4. The steel is AISI 1016-1024 heat treated to Rockwell C28-38, with a minimum torsional strength of 125 lb-in. One caveat from the manufacturer's notes: dimensions for the #9 diameter are independent of ASME B18.6.4.

Metallics lists the DS178M as both ARRA compliant and Buy American compliant. Country of origin is recorded as multiple, so if a submittal requires a specific mill origin on the delivered lot, that is confirmed per shipment rather than by the catalog spec.

The DS178M ships as a box of 1,000 screws, and 1,000 is the order minimum. Box weight is 19.6 lbs. Shop stock quantity for an MRO crib, not a handful for one panel.

The DS178M is zinc plated carbon steel, which is a dry indoor to lightly protected finish. It handles normal interior electrical, plumbing and HVAC work, which is the application Metallics lists for the series. For wet, coastal, or exterior exposure, step up to stainless or a hot dip galvanized fastener instead of relying on zinc plating.

There is an integral washer under the hex head, with an outside diameter of 0.480 to 0.520 in. and a thickness of 0.030 to 0.050 in. Head height runs 0.172 to 0.19 in. That washer OD of roughly a half inch spreads load over a punched or slightly oversize hole in sheet metal, so you usually do not need to add a separate flat washer.

At 1-1/2 in. long with a #14 diameter, the DS178M suits stacked sheet metal, strut, hanger iron, or fastening through wood blocking and plywood into sheet. Allow enough thread past the far side of the joint for the gimlet point to engage and pull up. For thinner single sheet applications a shorter length in the same family makes more sense, and Metallics offers other diameters and lengths in the series on request.