ASME B18.3Socket head (cylindrical)Hexagon socket

ASME B18.3 4-40 × 0.25 Socket head (cylindrical)

ASME B18.3 4-40 × 0.25 is a socket head cap screw with a 0.25 in nominal length, on a 4-40 UNC thread at 40 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.

Dimensioned drawing — ASME B18.3 4-40 × 0.25k 0.11L 0.25⌀2.84
ASME B18.3 4-40 × 0.25 — Socket head (cylindrical)Nominal dimensions in inches · Not to scale
Nominal Ø0.1118 in
Length0.25 in
Socket size s0.094 in
Head Ø dk0.183 in
Head height k0.112 in
HeadSocket head (cylindrical)
DriveHexagon socket

Dimensions

Standard
ASME B18.3
Type
Socket head cap screw
Size
4-40 × 0.25
Thread
4-40 (40 tpi)
Length L
0.25 in
Socket size s
0.094 in
Head diameter dk
0.183 in
Head height k
0.112 in
Thread length b
0.75 in
Drive
Hexagon socket
Head
Socket head (cylindrical)

Nominal values in inches as published in ASME B18.3. Estimated weight 0.696 g per piece in steel (0.7 g in stainless), calculated from the nominal geometry — real parts vary with tolerance and head chamfers.

Thread data

Full 4-40 thread page
Designation#4-40 UNC
Threads per inch40 tpi
Nominal Ø0.112 in
Pitch Ø0.0957 in
Minor Ø ext0.0815 in
Tap drillØ 0.0870 in

Other lengthsASME B18.3 4-40

All 4-40 specifications
4-40 × 0.254-40 × 0.3754-40 × 0.54-40 × 0.6254-40 × 0.754-40 × 1

Choosing and fitting this size

ASME B18.3 4-40 × 0.25 is a socket head (cylindrical) in 4-40, 0.25 in long under the head, on a 40 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, 4-40 takes a 0.0870 in tap drill, and the external thread's minor diameter is 0.0815 in — that is the section carrying the load, not the nominal 4-40 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.