Thread Pitch Chart
Thread pitch defines the distance between adjacent thread crests and determines how a bolt and nut engage. In imperial systems, pitch is expressed as threads per inch (TPI); in metric systems, it is given as the distance in millimeters between crests. Selecting the correct thread pitch is required for achieving proper bolt preload and preventing cross-threading during flange assembly.
Unified Inch Thread Pitch Chart (ASME B1.1)
The Unified National thread system includes UNC (coarse) and UNF (fine) series. UNC is standard for stud bolts in piping; UNF is used where vibration resistance or fine adjustment is needed.
| Bolt Size | UNC TPI | UNC Pitch (in) | UNF TPI | UNF Pitch (in) |
|---|---|---|---|---|
| 1/4” | 20 | 0.0500 | 28 | 0.0357 |
| 5/16” | 18 | 0.0556 | 24 | 0.0417 |
| 3/8” | 16 | 0.0625 | 24 | 0.0417 |
| 7/16” | 14 | 0.0714 | 20 | 0.0500 |
| 1/2” | 13 | 0.0769 | 20 | 0.0500 |
| 9/16” | 12 | 0.0833 | 18 | 0.0556 |
| 5/8” | 11 | 0.0909 | 18 | 0.0556 |
| 3/4” | 10 | 0.1000 | 16 | 0.0625 |
| 7/8” | 9 | 0.1111 | 14 | 0.0714 |
| 1” | 8 | 0.1250 | 12 | 0.0833 |
| 1-1/8” | 7 | 0.1429 | 12 | 0.0833 |
| 1-1/4” | 7 | 0.1429 | 12 | 0.0833 |
| 1-3/8” | 6 | 0.1667 | 12 | 0.0833 |
| 1-1/2” | 6 | 0.1667 | 12 | 0.0833 |
| 1-3/4” | 5 | 0.2000 | - | - |
| 2” | 4.5 | 0.2222 | - | - |
| 2-1/4” | 4.5 | 0.2222 | - | - |
| 2-1/2” | 4 | 0.2500 | - | - |
| 2-3/4” | 4 | 0.2500 | - | - |
| 3” | 4 | 0.2500 | - | - |
| 3-1/2” | 4 | 0.2500 | - | - |
| 4” | 4 | 0.2500 | - | - |
UNF threads are not defined for sizes above 1-1/2” in standard practice for piping bolting.
Metric Thread Pitch Chart (ISO 261)
Metric threads use the designation M followed by the nominal diameter and pitch (e.g., M16 x 2.0). Coarse pitch is the default; fine pitch must be specified explicitly.
| Thread Size | Coarse Pitch (mm) | Fine Pitch (mm) |
|---|---|---|
| M6 | 1.00 | 0.75 |
| M8 | 1.25 | 1.00 |
| M10 | 1.50 | 1.25 |
| M12 | 1.75 | 1.25 |
| M14 | 2.00 | 1.50 |
| M16 | 2.00 | 1.50 |
| M18 | 2.50 | 1.50 |
| M20 | 2.50 | 1.50 |
| M22 | 2.50 | 1.50 |
| M24 | 3.00 | 2.00 |
| M27 | 3.00 | 2.00 |
| M30 | 3.50 | 2.00 |
| M33 | 3.50 | 2.00 |
| M36 | 4.00 | 3.00 |
| M42 | 4.50 | 3.00 |
| M48 | 5.00 | 3.00 |
| M56 | 5.50 | 4.00 |
| M64 | 6.00 | 4.00 |
| M72 | 6.00 | 4.00 |
| M80 | 6.00 | 4.00 |
| M90 | 6.00 | 4.00 |
| M100 | 6.00 | 4.00 |
8UN Series (Piping Industry Standard)
For ASTM A193 B7 stud bolts used in ASME B16.5 flanged joints, sizes 1” and above use 8-thread (8UN) series rather than standard UNC. This provides a uniform pitch across large bolt diameters and simplifies nut interchangeability.
| Bolt Size | 8UN TPI | Pitch (in) |
|---|---|---|
| 1” | 8 | 0.1250 |
| 1-1/8” | 8 | 0.1250 |
| 1-1/4” | 8 | 0.1250 |
| 1-3/8” | 8 | 0.1250 |
| 1-1/2” | 8 | 0.1250 |
| 1-5/8” | 8 | 0.1250 |
| 1-3/4” | 8 | 0.1250 |
| 2” | 8 | 0.1250 |
| 2-1/4” | 8 | 0.1250 |
| 2-1/2” | 8 | 0.1250 |
| 2-3/4” | 8 | 0.1250 |
| 3” | 8 | 0.1250 |
How to Identify Thread Pitch
Measure thread pitch using a thread pitch gauge (a set of serrated blades that match thread profiles). For imperial threads, count the number of crests within one inch. For metric threads, measure the crest-to-crest distance in millimeters using calipers.
Mismatched thread pitch between a stud and nut causes cross-threading, which strips thread flanks, reduces thread engagement, and compromises joint integrity. Flanged connections using cross-threaded bolts must be rejected during inspection.
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