Metallics JSPMP99SS 1/4-20 x 1/2 in. Pan Head Phillips Machine Screws - 18-8 Stainless - Each

A #3 Phillips bit. The pan head on the JSPMP99SS takes a Phillips recess measuring 0.274 inch across with a gaging depth of 0.118 to 0.144 inch, which is why it steps up to a #3 driver rather than the #2 most techs keep in the pouch. Running a #2 in it will cam out and chew the recess.

They are built to ASME B18.6.3-2010, the dimensional standard for inch-series machine screws and pan heads, with SAE material standard called out. That gives a spec writer a citable head geometry and thread form for the submittal. Note it is a multi-country origin item and is not Buy American compliant, so it will not clear a federal or ARRA-funded job with domestic sourcing requirements.

The thread is 1/4-20 UNC at 20 threads per inch, so it engages any 1/4-20 tapped hole, strut channel nut, or hex nut without an adapter. Usable thread is the full 1/2 inch length, so count your material stack plus washers against that before ordering. Nuts and washers are not included.

They are 18-8 stainless, the common austenitic grade used where plated steel would rust, so they hold up in wet and outdoor locations and in enclosures that see washdown or condensation. Be aware of galvanic pairing: stainless in direct contact with aluminum or galvanized steel in a wet environment will accelerate corrosion of the softer metal.

Rated at 80,000 psi minimum tensile with a Rockwell BBS-B9S hardness. That is typical stainless machine screw territory, suitable for mounting boxes, straps, covers, and strut hardware. It is not a graded structural bolt, and Metallics notes that tensile values on stainless screws are approximations because there is no single performance standard for them. For structural connections, specify a graded carbon steel bolt instead.

They ship in a jar with a 100-count minimum, so the smallest purchase is a jar of 100. Metallics does offer other sizes and quantities on request if 1/4-20 x 1/2 inch is not the length you need.

This Metallics screw is a pan head, which seats on the surface and needs no countersink. That is what you want on sheet metal, enclosure flanges, and strut where the material is too thin to countersink. With a head height of 0.162 to 0.175 inch it sits proud of the surface, so if you need a flush face, specify a flat head and a countersunk hole instead.