Metallics JG8HC414 Bolts 5/16-18 X 1-1/4 G8 Hxbt St-zn (Jar 100)

It's 5/16-18, so coarse thread at 18 threads per inch. Shank outside diameter runs 0.3065 to 0.3125 inch. Any standard 5/16-18 coarse nut threads on, but pair a Grade 8 bolt with a matching hardened nut so the nut isn't the weak link.

The JG8HC414 is a Grade 8 cap screw, medium carbon steel, heat treated, with 150,000 psi minimum tensile and 130,000 psi minimum yield. Hardness runs Rockwell C33-39. Grade 5 tops out lower on all three, which is exactly why automotive and fleet work specs Grade 8 where the joint load exceeds what a Grade 5 can carry.

Head width across flats measures 0.489 to 0.500 inch, so a 1/2 inch wrench or socket is the fit. Head type is a standard hex.

The finish here is zinc over steel, which is a general purpose plating that handles indoor, dry, and lightly exposed conditions. It is not a marine or chemical service coating, so for constant wet exposure, salt spray, or coastal work, go to a stainless or hot dip galvanized fastener instead. Note that Grade 8 strength is not available in standard stainless, so a swap trades strength for corrosion resistance.

Both the ARRA and Buy American flags on this Grade 8 hex bolt are marked compliant. Country of origin is listed as multiple, so if your contract requires a single documented source country, get the certification paperwork for the specific lot before it goes on a federally funded project.

Metallics lists it as a hexagonal bolt and describes it as a cap screw made from medium carbon steel and heat treated. Practically, it has the closer tolerances and full thread run of a cap screw, so it works either through a clearance hole with a nut or threaded directly into a tapped hole.

Material standard is AISI 1021 through 1045 steel, a medium carbon range, then heat treated to reach Grade 8 properties and a Rockwell C33-39 hardness. That hardness is also why these should not be reused after being torqued to yield in a critical joint.