Metallics JG8HC416 Bolts 5/16-18 x 3-1/2 G8 HXBT St-zn (Jar 50)

The JG8HC416 is a Grade 8 cap screw rated 150,000 psi minimum tensile and 130,000 psi minimum yield, hardness Rockwell C33-39. Grade 5 in this diameter is rated 120,000 psi tensile, so the Grade 8 gives you meaningfully more clamp load capacity in the same 5/16 shank. That is why it gets specified on automotive and fleet assemblies where a Grade 5 is marginal.

It is a fine thread. The JG8HC416 is 5/16-18, with 18 threads per inch, so it will not thread into a standard 5/16-18 coarse nut... to be exact, the thread count on record is 18 TPI and the shank outside diameter runs 0.3065 to 0.3125 inch. Match your nuts and tapped holes to that 18 TPI pitch before installing.

The hex head measures 0.489 to 0.500 inch across the flats, which is a 1/2 inch socket or wrench.

Fifty bolts per jar, and the jar is the order minimum, so 50 count is the smallest quantity you buy. Metallics notes other sizes and quantities are available by request.

The finish is zinc over a steel body, which is a mild corrosion barrier suited to dry indoor and sheltered use. In standing water, road salt, or coastal exposure the plating sacrifices quickly and the medium carbon steel underneath will rust. For sustained wet or exterior service, use a hot dip galvanized or stainless fastener instead and accept the lower strength grade that comes with stainless.

The record flags the JG8HC416 as both ARRA compliant and Buy American compliant, with country of origin China. If your contract carries a strict domestic-melt or domestic-manufacture clause rather than an ARRA-style provision, verify the origin against your spec language before submitting it.

Medium carbon steel in the AISI 1021 to 1045 range, heat treated to reach the Grade 8 hardness of Rockwell C33-39. Because it is a quenched and tempered carbon steel, avoid welding on it or re-heating it, both of which destroy the heat treat and the strength rating.