Metallics JG8HC418 Bolts 5/16-18 x 2-3/4 G8 HXBT St-zn (Jar 50)

It is coarse thread. The JG8HC418 runs 18 threads per inch, which is the standard 5/16-18 UNC pitch, with a thread outside diameter of 0.3065 to 0.3125 inch. It will not thread into a 5/16-24 fine thread nut or tapped hole.

Strength. The JG8HC418 is a Grade 8 cap screw made from medium carbon steel, heat treated to Rockwell C33-39, with 150,000 psi minimum tensile and 130,000 psi minimum yield. That is the reason it gets specified in automotive and fleet work where a Grade 5 does not carry enough tensile strength. If the joint was engineered around a Grade 5, do not substitute up without checking the clamp load the assembly expects.

No. The JG8HC418 is not Buy American compliant and is not ARRA compliant, with country of origin listed as multiple. On a federally funded project with domestic sourcing requirements you will need a domestically produced Grade 8 hex cap screw instead.

The hex head measures 0.489 to 0.500 inch across the flats, so a 1/2 inch wrench or socket is the fit. Head type is a standard hex, no washer face flange or internal drive.

The JG8HC418 is steel with a zinc finish, which is a light-duty corrosion coating suited to dry or sheltered service and general shop and fleet use. For sustained weather exposure, washdown, or coastal work, specify a hot dip galvanized or stainless fastener instead. Note also that zinc plating on Grade 8 does not change the mechanical rating.

It ships in a jar of 50, and 50 each is the order minimum, so the jar is the buying unit. Metallics lists other sizes and quantities in the G8HC and JG8HC series by request.

AISI 1021 through 1045 medium carbon steel, heat treated to Rockwell C33-39. That is the normal material band for a Grade 8 cap screw, and it is what gets the bolt to its 150,000 psi minimum tensile. It is not alloy or stainless, so it is not a weldable or corrosion-resistant choice.