ASME B18.2.1Hexagon headExternal hex

ASME B18.2.1 1-8 × 8 Hexagon head

ASME B18.2.1 1-8 × 8 is a hex head cap screw with a 8 in nominal length, on a 1-8 UNC thread at 8 tpi. The inch-series hex head cap screw, supplied in UNC (coarse) thread as standard. This is the North American counterpart to DIN 933 and ISO 4017, but the head is held to inch dimensions rather than metric, so the two are not interchangeable across a wrench.

Dimensioned drawing — ASME B18.2.1 1-8 × 8k 0.66L 8⌀1.73⌀25.4
ASME B18.2.1 1-8 × 8 — Hexagon headNominal dimensions in inches · Not to scale
Nominal Ø1 in
Length8 in
Socket size s1.5 in
Head Ø dk1.732 in
Head height k0.6563 in
HeadHexagon head
DriveExternal hex

Dimensions

Standard
ASME B18.2.1
Type
Hex head cap screw
Size
1-8 × 8
Thread
1-8 (8 tpi)
Length L
8 in
Socket size s
1.5 in
Head diameter dk
1.732 in
Head height k
0.6563 in
Thread length b
2.5 in
Drive
External hex
Head
Hexagon head

Nominal values in inches as published in ASME B18.2.1. Estimated weight 203.218 g per piece in steel (204.513 g in stainless), calculated from the nominal geometry — real parts vary with tolerance and head chamfers.

Thread data

Full 1-8 thread page
Designation1-8 UNC
Threads per inch8 tpi
Nominal Ø1 in
Pitch Ø0.9189 in
Minor Ø ext0.8465 in
Tap drillØ 0.8748 in

Other lengthsASME B18.2.1 1-8

All 1-8 specifications
1-8 × 31-8 × 41-8 × 51-8 × 61-8 × 81-8 × 10

Choosing and fitting this size

ASME B18.2.1 1-8 × 8 is a hexagon head in 1-8, 8 in long under the head, on a 8 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, 1-8 takes a 0.8748 in tap drill, and the external thread's minor diameter is 0.8465 in — that is the section carrying the load, not the nominal 1-8 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.