ASME B18.3 5/16-18 × 2.5 Socket head (cylindrical)
ASME B18.3 5/16-18 × 2.5 is a socket head cap screw with a 2.5 in nominal length, on a 5/16-18 UNC thread at 18 tpi. The inch-series socket head cap screw, the US counterpart to DIN 912. Threads run UNC with UNF available on request, and the socket sizes follow inch tooling — a 5/16 AF hex key, not an 8 mm.
Dimensions
- Standard
- ASME B18.3
- Type
- Socket head cap screw
- Size
- 5/16-18 × 2.5
- Thread
- 5/16-18 (18 tpi)
- Length L
- 2.5 in
- Socket size s
- 0.25 in
- Head diameter dk
- 0.469 in
- Head height k
- 0.312 in
- Thread length b
- 1.125 in
- Drive
- Hexagon socket
- Head
- Socket head (cylindrical)
Nominal values in inches as published in ASME B18.3. Estimated weight 31.6 g per piece in steel (31.801 g in stainless), calculated from the nominal geometry — real parts vary with tolerance and head chamfers.
Thread data
Full 5/16-18 thread pageOther lengthsASME B18.3 5/16-18
All 5/16-18 specificationsChoosing and fitting this size
ASME B18.3 5/16-18 × 2.5 is a socket head (cylindrical) in 5/16-18, 2.5 in long under the head, on a 18 tpi UNC thread. Mating threads should be a matching class nut or a tapped hole of the same nominal size.
Inch fasteners are graded under SAE J429, not ISO 898-1. Grade 2 is unmarked low carbon, Grade 5 carries three radial lines on the head and Grade 8 carries six. Grade 8 reaches roughly 1034 MPa tensile — close to metric class 10.9 — but the grades and thread forms are separate systems and the two should not be mixed in one joint. Never put a Grade 5 nut on a Grade 8 bolt: the thread strips first and the failure moves into the nut.
For a tapped hole, 5/16-18 takes a 0.2571 in tap drill, and the external thread's minor diameter is 0.2445 in — that is the section carrying the load, not the nominal 5/16-18 figure. Inch tap drills come from the thread tables rather than the metric rule of thumb (major diameter minus pitch), because TPI does not map onto that formula. Engagement of at least one times the nominal diameter is what lets the bolt fail before the thread strips.