NPSpipe60° flankASME B1.20.1

3-8 NPS Thread Dimensions

3-8 NPS is a National Pipe Straight thread. There are 8 threads per inch on a 3.5 in major diameter, giving a metric-equivalent pitch of 3.175 mm. The size label is a nominal bore rather than a diameter.

Major Ø3.5 in

Threads per inch8

Pitch (metric)3.175 mm

Pitch Ø3.4189 in

Tap drill3.375 in

Thread profile

millimetres unless stated

3-8 NPS — basic profile60° flank angle · Not to scale

Basic information

Designation: 3-8 NPS
Thread system: National Pipe Straight
Series: pipe
Standard: ASME B1.20.1
Basic profile: Unified profile, 0.8P thread height
Flank angle: 60°

Dimensions

Major diameter: 3.5 in / 88.9 mm
Pitch (metric equivalent): 3.175 mm
Threads per inch: 8
Pitch diameter: 3.4189 in / 86.84 mm
Minor diameter (external): 3.3189 in / 84.3 mm
Minor diameter (internal): 3.3646 in / 85.46 mm
Thread depth: 0.1 in / 2.54 mm
Recommended tap drill: Ø 3.375 in / 85.725 mm

How these values are derived

tabulated standard values

This is a pipe thread, so the published dimensions are tabulated values from ASME B1.20.1 rather than figures derived from a formula: a pipe size is a nominal bore, and the relationship between that bore and the thread diameter is historical, not geometric. The thread is parallel, so these diameters hold along the whole length — the seal comes from a gasket or an O-ring, not from the thread.

Compatible parts0 specification pages

No specification in this database uses 3-8 NPS. That is expected: the catalogue covers screws, bolts, nuts, washers and rivets, and pipe threads belong to fittings, which are a separate family of parts. The dimensions above are still the standard ones for the thread itself.

Neighbouring sizes in National Pipe Straight.

1-1/4-11.5 NPS 1-1/2-11.5 NPS 2-11.5 NPS 2-1/2-8 NPS