Flange Bolt Size Chart
Complete ASME B16.5 bolt chart: bolt count, stud diameter and thread series, heavy-hex nut across-flats and wrench size — for every NPS 1/2" to 24" and Class 150 to 2500.
Quick Answer
The most common flange bolting in process plants: a 2" Class 150 flange uses4 studs of 5/8"-11 UNC with 1-1/16" (27 mm) heavy-hex nuts; a 4" Class 300 flange uses 8 studs of 3/4"-10 UNCwith 1-1/4" (32 mm) nuts; a 6" Class 600 flange uses12 studs of 1-1/8"-8 UN with 1-13/16" (46 mm) nuts. Stud size = ASME B16.5 bolt-hole diameter minus 1/8" (studs ≤ 2-1/4") or 1/4" (larger studs).
Class 150 Flange Bolting
Covers NPS 1/2" - 24".Stud sizes derived from B16.5 bolt-hole diameters.
| NPS | Bolt Circle (mm) | Bolts | Stud Size | TPI | At (in²) | Nut (heavy hex) | Socket / Wrench |
|---|---|---|---|---|---|---|---|
| 1/2" | 60.3 | 4 × Ø15.9 | 1/2"-13 UNC | 13 | 0.142 | 0.875" (22 mm) | 0.875" (22 mm) |
| 3/4" | 69.9 | 4 × Ø15.9 | 1/2"-13 UNC | 13 | 0.142 | 0.875" (22 mm) | 0.875" (22 mm) |
| 1" | 79.4 | 4 × Ø15.9 | 1/2"-13 UNC | 13 | 0.142 | 0.875" (22 mm) | 0.875" (22 mm) |
| 1-1/4" | 88.9 | 4 × Ø15.9 | 1/2"-13 UNC | 13 | 0.142 | 0.875" (22 mm) | 0.875" (22 mm) |
| 1-1/2" | 98.4 | 4 × Ø15.9 | 1/2"-13 UNC | 13 | 0.142 | 0.875" (22 mm) | 0.875" (22 mm) |
| 2" | 120.7 | 4 × Ø19.1 | 5/8"-11 UNC | 11 | 0.226 | 1.0625" (27 mm) | 1.0625" (27 mm) |
| 2-1/2" | 139.7 | 4 × Ø19.1 | 5/8"-11 UNC | 11 | 0.226 | 1.0625" (27 mm) | 1.0625" (27 mm) |
| 3" | 152.4 | 4 × Ø19.1 | 5/8"-11 UNC | 11 | 0.226 | 1.0625" (27 mm) | 1.0625" (27 mm) |
| 3-1/2" | 177.8 | 8 × Ø19.1 | 5/8"-11 UNC | 11 | 0.226 | 1.0625" (27 mm) | 1.0625" (27 mm) |
| 4" | 190.5 | 8 × Ø19.1 | 5/8"-11 UNC | 11 | 0.226 | 1.0625" (27 mm) | 1.0625" (27 mm) |
| 5" | 215.9 | 8 × Ø22.2 | 3/4"-10 UNC | 10 | 0.334 | 1.25" (32 mm) | 1.25" (32 mm) |
| 6" | 241.3 | 8 × Ø22.2 | 3/4"-10 UNC | 10 | 0.334 | 1.25" (32 mm) | 1.25" (32 mm) |
| 8" | 298.5 | 8 × Ø22.2 | 3/4"-10 UNC | 10 | 0.334 | 1.25" (32 mm) | 1.25" (32 mm) |
| 10" | 362.0 | 12 × Ø25.4 | 7/8"-9 UNC | 9 | 0.462 | 1.4375" (37 mm) | 1.4375" (37 mm) |
| 12" | 431.8 | 12 × Ø25.4 | 7/8"-9 UNC | 9 | 0.462 | 1.4375" (37 mm) | 1.4375" (37 mm) |
| 14" | 476.3 | 12 × Ø28.6 | 1"-8 UNC | 8 | 0.606 | 1.625" (41 mm) | 1.625" (41 mm) |
| 16" | 539.8 | 16 × Ø28.6 | 1"-8 UNC | 8 | 0.606 | 1.625" (41 mm) | 1.625" (41 mm) |
| 18" | 577.9 | 16 × Ø31.8 | 1-1/8"-8 8UN | 8 | 0.790 | 1.8125" (46 mm) | 1.8125" (46 mm) |
| 20" | 635.0 | 20 × Ø31.8 | 1-1/8"-8 8UN | 8 | 0.790 | 1.8125" (46 mm) | 1.8125" (46 mm) |
| 22" | 692.2 | 20 × Ø34.9 | 1-1/4"-8 8UN | 8 | 1.000 | 2" (51 mm) | 2" (51 mm) |
| 24" | 749.3 | 20 × Ø34.9 | 1-1/4"-8 8UN | 8 | 1.000 | 2" (51 mm) | 2" (51 mm) |
Class 300 Flange Bolting
Covers NPS 1/2" - 24".Stud sizes derived from B16.5 bolt-hole diameters.
| NPS | Bolt Circle (mm) | Bolts | Stud Size | TPI | At (in²) | Nut (heavy hex) | Socket / Wrench |
|---|---|---|---|---|---|---|---|
| 1/2" | 66.7 | 4 × Ø15.9 | 1/2"-13 UNC | 13 | 0.142 | 0.875" (22 mm) | 0.875" (22 mm) |
| 3/4" | 82.6 | 4 × Ø15.9 | 1/2"-13 UNC | 13 | 0.142 | 0.875" (22 mm) | 0.875" (22 mm) |
| 1" | 88.9 | 4 × Ø19.1 | 5/8"-11 UNC | 11 | 0.226 | 1.0625" (27 mm) | 1.0625" (27 mm) |
| 1-1/4" | 98.4 | 4 × Ø19.1 | 5/8"-11 UNC | 11 | 0.226 | 1.0625" (27 mm) | 1.0625" (27 mm) |
| 1-1/2" | 114.3 | 4 × Ø19.1 | 5/8"-11 UNC | 11 | 0.226 | 1.0625" (27 mm) | 1.0625" (27 mm) |
| 2" | 127.0 | 8 × Ø19.1 | 5/8"-11 UNC | 11 | 0.226 | 1.0625" (27 mm) | 1.0625" (27 mm) |
| 2-1/2" | 149.2 | 8 × Ø22.2 | 3/4"-10 UNC | 10 | 0.334 | 1.25" (32 mm) | 1.25" (32 mm) |
| 3" | 168.3 | 8 × Ø22.2 | 3/4"-10 UNC | 10 | 0.334 | 1.25" (32 mm) | 1.25" (32 mm) |
| 3-1/2" | 184.2 | 8 × Ø22.2 | 3/4"-10 UNC | 10 | 0.334 | 1.25" (32 mm) | 1.25" (32 mm) |
| 4" | 200.0 | 8 × Ø22.2 | 3/4"-10 UNC | 10 | 0.334 | 1.25" (32 mm) | 1.25" (32 mm) |
| 5" | 235.0 | 8 × Ø22.2 | 3/4"-10 UNC | 10 | 0.334 | 1.25" (32 mm) | 1.25" (32 mm) |
| 6" | 269.9 | 12 × Ø22.2 | 3/4"-10 UNC | 10 | 0.334 | 1.25" (32 mm) | 1.25" (32 mm) |
| 8" | 330.2 | 12 × Ø25.4 | 7/8"-9 UNC | 9 | 0.462 | 1.4375" (37 mm) | 1.4375" (37 mm) |
| 10" | 387.4 | 16 × Ø28.6 | 1"-8 UNC | 8 | 0.606 | 1.625" (41 mm) | 1.625" (41 mm) |
| 12" | 450.8 | 16 × Ø31.8 | 1-1/8"-8 8UN | 8 | 0.790 | 1.8125" (46 mm) | 1.8125" (46 mm) |
| 14" | 514.4 | 20 × Ø31.8 | 1-1/8"-8 8UN | 8 | 0.790 | 1.8125" (46 mm) | 1.8125" (46 mm) |
| 16" | 571.5 | 20 × Ø34.9 | 1-1/4"-8 8UN | 8 | 1.000 | 2" (51 mm) | 2" (51 mm) |
| 18" | 628.6 | 24 × Ø34.9 | 1-1/4"-8 8UN | 8 | 1.000 | 2" (51 mm) | 2" (51 mm) |
| 20" | 685.8 | 24 × Ø41.3 | 1-1/2"-8 8UN | 8 | 1.492 | 2.375" (60 mm) | 2.375" (60 mm) |
| 22" | 749.3 | 24 × Ø41.3 | 1-1/2"-8 8UN | 8 | 1.492 | 2.375" (60 mm) | 2.375" (60 mm) |
| 24" | 812.8 | 24 × Ø41.3 | 1-1/2"-8 8UN | 8 | 1.492 | 2.375" (60 mm) | 2.375" (60 mm) |
Class 600 Flange Bolting
Covers NPS 1/2" - 24".Stud sizes derived from B16.5 bolt-hole diameters.
| NPS | Bolt Circle (mm) | Bolts | Stud Size | TPI | At (in²) | Nut (heavy hex) | Socket / Wrench |
|---|---|---|---|---|---|---|---|
| 1/2" | 66.7 | 4 × Ø15.9 | 1/2"-13 UNC | 13 | 0.142 | 0.875" (22 mm) | 0.875" (22 mm) |
| 3/4" | 82.6 | 4 × Ø15.9 | 1/2"-13 UNC | 13 | 0.142 | 0.875" (22 mm) | 0.875" (22 mm) |
| 1" | 88.9 | 4 × Ø19.1 | 5/8"-11 UNC | 11 | 0.226 | 1.0625" (27 mm) | 1.0625" (27 mm) |
| 1-1/4" | 98.4 | 4 × Ø19.1 | 5/8"-11 UNC | 11 | 0.226 | 1.0625" (27 mm) | 1.0625" (27 mm) |
| 1-1/2" | 114.3 | 4 × Ø22.2 | 3/4"-10 UNC | 10 | 0.334 | 1.25" (32 mm) | 1.25" (32 mm) |
| 2" | 127.0 | 8 × Ø19.1 | 5/8"-11 UNC | 11 | 0.226 | 1.0625" (27 mm) | 1.0625" (27 mm) |
| 2-1/2" | 149.2 | 8 × Ø22.2 | 3/4"-10 UNC | 10 | 0.334 | 1.25" (32 mm) | 1.25" (32 mm) |
| 3" | 168.3 | 8 × Ø25.4 | 7/8"-9 UNC | 9 | 0.462 | 1.4375" (37 mm) | 1.4375" (37 mm) |
| 3-1/2" | 184.2 | 8 × Ø28.6 | 1"-8 UNC | 8 | 0.606 | 1.625" (41 mm) | 1.625" (41 mm) |
| 4" | 215.9 | 8 × Ø28.6 | 1"-8 UNC | 8 | 0.606 | 1.625" (41 mm) | 1.625" (41 mm) |
| 5" | 266.7 | 8 × Ø31.8 | 1-1/8"-8 8UN | 8 | 0.790 | 1.8125" (46 mm) | 1.8125" (46 mm) |
| 6" | 292.1 | 12 × Ø31.8 | 1-1/8"-8 8UN | 8 | 0.790 | 1.8125" (46 mm) | 1.8125" (46 mm) |
| 8" | 349.2 | 12 × Ø34.9 | 1-1/4"-8 8UN | 8 | 1.000 | 2" (51 mm) | 2" (51 mm) |
| 10" | 431.8 | 16 × Ø38.1 | 1-3/8"-8 8UN | 8 | 1.234 | 2.1875" (56 mm) | 2.1875" (56 mm) |
| 12" | 489.0 | 20 × Ø41.3 | 1-1/2"-8 8UN | 8 | 1.492 | 2.375" (60 mm) | 2.375" (60 mm) |
| 14" | 527.0 | 20 × Ø44.5 | 1-5/8"-8 8UN | 8 | 1.775 | 2.5625" (65 mm) | 2.5625" (65 mm) |
| 16" | 603.2 | 20 × Ø50.8 | 1-7/8"-8 8UN | 8 | 2.414 | 2.9375" (75 mm) | 2.9375" (75 mm) |
| 18" | 654.0 | 20 × Ø54 | 2"-8 8UN | 8 | 2.771 | 3.125" (79 mm) | 3.125" (79 mm) |
| 20" | 723.9 | 24 × Ø60.3 | 2-1/4"-8 8UN | 8 | 3.557 | 3.375" (86 mm) | 3.375" (86 mm) |
| 22" | 784.2 | 24 × Ø63.5 | 2-1/4"-8 8UN | 8 | 3.557 | 3.375" (86 mm) | 3.375" (86 mm) |
| 24" | 838.2 | 24 × Ø63.5 | 2-1/4"-8 8UN | 8 | 3.557 | 3.375" (86 mm) | 3.375" (86 mm) |
Class 900 Flange Bolting
Covers NPS 1/2" - 24" (no 3-1/2" in Class 900).Stud sizes derived from B16.5 bolt-hole diameters.
| NPS | Bolt Circle (mm) | Bolts | Stud Size | TPI | At (in²) | Nut (heavy hex) | Socket / Wrench |
|---|---|---|---|---|---|---|---|
| 1/2" | 82.6 | 4 × Ø22.2 | 3/4"-10 UNC | 10 | 0.334 | 1.25" (32 mm) | 1.25" (32 mm) |
| 3/4" | 88.9 | 4 × Ø22.2 | 3/4"-10 UNC | 10 | 0.334 | 1.25" (32 mm) | 1.25" (32 mm) |
| 1" | 101.6 | 4 × Ø25.4 | 7/8"-9 UNC | 9 | 0.462 | 1.4375" (37 mm) | 1.4375" (37 mm) |
| 1-1/4" | 111.1 | 4 × Ø25.4 | 7/8"-9 UNC | 9 | 0.462 | 1.4375" (37 mm) | 1.4375" (37 mm) |
| 1-1/2" | 123.8 | 4 × Ø28.6 | 1"-8 UNC | 8 | 0.606 | 1.625" (41 mm) | 1.625" (41 mm) |
| 2" | 165.1 | 8 × Ø25.4 | 7/8"-9 UNC | 9 | 0.462 | 1.4375" (37 mm) | 1.4375" (37 mm) |
| 2-1/2" | 190.5 | 8 × Ø28.6 | 1"-8 UNC | 8 | 0.606 | 1.625" (41 mm) | 1.625" (41 mm) |
| 3" | 190.5 | 8 × Ø28.6 | 1"-8 UNC | 8 | 0.606 | 1.625" (41 mm) | 1.625" (41 mm) |
| 4" | 235.0 | 8 × Ø31.8 | 1-1/8"-8 8UN | 8 | 0.790 | 1.8125" (46 mm) | 1.8125" (46 mm) |
| 5" | 279.4 | 8 × Ø34.9 | 1-1/4"-8 8UN | 8 | 1.000 | 2" (51 mm) | 2" (51 mm) |
| 6" | 317.5 | 8 × Ø34.9 | 1-1/4"-8 8UN | 8 | 1.000 | 2" (51 mm) | 2" (51 mm) |
| 8" | 393.7 | 12 × Ø38.1 | 1-3/8"-8 8UN | 8 | 1.234 | 2.1875" (56 mm) | 2.1875" (56 mm) |
| 10" | 469.9 | 16 × Ø41.3 | 1-1/2"-8 8UN | 8 | 1.492 | 2.375" (60 mm) | 2.375" (60 mm) |
| 12" | 533.4 | 20 × Ø44.5 | 1-5/8"-8 8UN | 8 | 1.775 | 2.5625" (65 mm) | 2.5625" (65 mm) |
| 14" | 558.8 | 20 × Ø47.6 | 1-3/4"-8 8UN | 8 | 2.082 | 2.75" (70 mm) | 2.75" (70 mm) |
| 16" | 616.0 | 20 × Ø54 | 2"-8 8UN | 8 | 2.771 | 3.125" (79 mm) | 3.125" (79 mm) |
| 18" | 685.8 | 20 × Ø60.3 | 2-1/4"-8 8UN | 8 | 3.557 | 3.375" (86 mm) | 3.375" (86 mm) |
| 20" | 749.3 | 20 × Ø63.5 | 2-1/4"-8 8UN | 8 | 3.557 | 3.375" (86 mm) | 3.375" (86 mm) |
| 22" | 806.5 | 20 × Ø69.9 | 2-1/2"-8 8UN | 8 | 4.442 | 3.75" (95 mm) | 3.75" (95 mm) |
| 24" | 901.7 | 20 × Ø76.2 | 2-3/4"-8 8UN | 8 | 5.425 | 4.125" (105 mm) | 4.125" (105 mm) |
Class 1500 Flange Bolting
Covers NPS 1/2" - 24" (no 3-1/2" in Class 1500).Stud sizes derived from B16.5 bolt-hole diameters.
| NPS | Bolt Circle (mm) | Bolts | Stud Size | TPI | At (in²) | Nut (heavy hex) | Socket / Wrench |
|---|---|---|---|---|---|---|---|
| 1/2" | 82.6 | 4 × Ø22.2 | 3/4"-10 UNC | 10 | 0.334 | 1.25" (32 mm) | 1.25" (32 mm) |
| 3/4" | 88.9 | 4 × Ø22.2 | 3/4"-10 UNC | 10 | 0.334 | 1.25" (32 mm) | 1.25" (32 mm) |
| 1" | 101.6 | 4 × Ø25.4 | 7/8"-9 UNC | 9 | 0.462 | 1.4375" (37 mm) | 1.4375" (37 mm) |
| 1-1/4" | 111.1 | 4 × Ø25.4 | 7/8"-9 UNC | 9 | 0.462 | 1.4375" (37 mm) | 1.4375" (37 mm) |
| 1-1/2" | 123.8 | 4 × Ø28.6 | 1"-8 UNC | 8 | 0.606 | 1.625" (41 mm) | 1.625" (41 mm) |
| 2" | 165.1 | 8 × Ø25.4 | 7/8"-9 UNC | 9 | 0.462 | 1.4375" (37 mm) | 1.4375" (37 mm) |
| 2-1/2" | 190.5 | 8 × Ø28.6 | 1"-8 UNC | 8 | 0.606 | 1.625" (41 mm) | 1.625" (41 mm) |
| 3" | 203.2 | 8 × Ø31.8 | 1-1/8"-8 8UN | 8 | 0.790 | 1.8125" (46 mm) | 1.8125" (46 mm) |
| 4" | 241.3 | 8 × Ø34.9 | 1-1/4"-8 8UN | 8 | 1.000 | 2" (51 mm) | 2" (51 mm) |
| 5" | 292.1 | 8 × Ø41.3 | 1-1/2"-8 8UN | 8 | 1.492 | 2.375" (60 mm) | 2.375" (60 mm) |
| 6" | 317.5 | 12 × Ø38.1 | 1-3/8"-8 8UN | 8 | 1.234 | 2.1875" (56 mm) | 2.1875" (56 mm) |
| 8" | 393.7 | 12 × Ø47.6 | 1-3/4"-8 8UN | 8 | 2.082 | 2.75" (70 mm) | 2.75" (70 mm) |
| 10" | 482.6 | 12 × Ø54 | 2"-8 8UN | 8 | 2.771 | 3.125" (79 mm) | 3.125" (79 mm) |
| 12" | 571.5 | 16 × Ø60.3 | 2-1/4"-8 8UN | 8 | 3.557 | 3.375" (86 mm) | 3.375" (86 mm) |
| 14" | 635.0 | 16 × Ø63.5 | 2-1/4"-8 8UN | 8 | 3.557 | 3.375" (86 mm) | 3.375" (86 mm) |
| 16" | 704.8 | 16 × Ø69.9 | 2-1/2"-8 8UN | 8 | 4.442 | 3.75" (95 mm) | 3.75" (95 mm) |
| 18" | 774.7 | 16 × Ø76.2 | 2-3/4"-8 8UN | 8 | 5.425 | 4.125" (105 mm) | 4.125" (105 mm) |
| 20" | 831.8 | 16 × Ø82.6 | 3"-8 8UN | 8 | 6.506 | 4.5" (114 mm) | 4.5" (114 mm) |
| 22" | 898.5 | 16 × Ø88.9 | 3-1/4"-8 8UN | 8 | 7.686 | 4.875" (124 mm) | 4.875" (124 mm) |
| 24" | 990.6 | 16 × Ø101.6 | 3-3/4"-8 8UN | 8 | 10.339 | 5.625" (143 mm) | 5.625" (143 mm) |
Class 2500 Flange Bolting
Covers NPS 1/2" - 12" (B16.5 Class 2500 range).Stud sizes derived from B16.5 bolt-hole diameters.
| NPS | Bolt Circle (mm) | Bolts | Stud Size | TPI | At (in²) | Nut (heavy hex) | Socket / Wrench |
|---|---|---|---|---|---|---|---|
| 1/2" | 88.9 | 4 × Ø22.2 | 3/4"-10 UNC | 10 | 0.334 | 1.25" (32 mm) | 1.25" (32 mm) |
| 3/4" | 95.3 | 4 × Ø22.2 | 3/4"-10 UNC | 10 | 0.334 | 1.25" (32 mm) | 1.25" (32 mm) |
| 1" | 108.0 | 4 × Ø25.4 | 7/8"-9 UNC | 9 | 0.462 | 1.4375" (37 mm) | 1.4375" (37 mm) |
| 1-1/4" | 130.2 | 4 × Ø28.6 | 1"-8 UNC | 8 | 0.606 | 1.625" (41 mm) | 1.625" (41 mm) |
| 1-1/2" | 146.1 | 4 × Ø31.8 | 1-1/8"-8 8UN | 8 | 0.790 | 1.8125" (46 mm) | 1.8125" (46 mm) |
| 2" | 171.4 | 8 × Ø28.6 | 1"-8 UNC | 8 | 0.606 | 1.625" (41 mm) | 1.625" (41 mm) |
| 2-1/2" | 196.8 | 8 × Ø31.8 | 1-1/8"-8 8UN | 8 | 0.790 | 1.8125" (46 mm) | 1.8125" (46 mm) |
| 3" | 228.6 | 8 × Ø35.1 | 1-1/4"-8 8UN | 8 | 1.000 | 2" (51 mm) | 2" (51 mm) |
| 4" | 273.0 | 8 × Ø41.3 | 1-1/2"-8 8UN | 8 | 1.492 | 2.375" (60 mm) | 2.375" (60 mm) |
| 5" | 323.8 | 8 × Ø47.6 | 1-3/4"-8 8UN | 8 | 2.082 | 2.75" (70 mm) | 2.75" (70 mm) |
| 6" | 368.3 | 8 × Ø54 | 2"-8 8UN | 8 | 2.771 | 3.125" (79 mm) | 3.125" (79 mm) |
| 8" | 438.2 | 12 × Ø54 | 2"-8 8UN | 8 | 2.771 | 3.125" (79 mm) | 3.125" (79 mm) |
| 10" | 539.8 | 12 × Ø66.7 | 2-1/2"-8 8UN | 8 | 4.442 | 3.75" (95 mm) | 3.75" (95 mm) |
| 12" | 619.1 | 12 × Ø73 | 2-3/4"-8 8UN | 8 | 5.425 | 4.125" (105 mm) | 4.125" (105 mm) |
How to Read This Chart
From flange markings to a bolt list
- Identify the NPS (nominal pipe size) and class stamped on the flange face or callout.
- Open the table for that class and find the NPS row.
- Bolts column = count × bolt-hole diameter in mm (as tabulated in ASME B16.5).
- Stud Size = standard stud for those holes; Nut and Socket / Wrench columns give the heavy-hex across-flats you need for socket selection.
- Order studs with the correct thread series (UNC ≤ 1", 8UN above) and roughly 2 × flange thickness + nut height in length.
Bolt-hole to stud-size mapping
ASME B16.5 tabulates bolt holes, not stud sizes. The clearance is 1/8" (3.2 mm) for studs up to 2-1/4" and 1/4" (6.4 mm) above: a 22.2 mm hole takes a 3/4" (19.1 mm) stud, a 50.8 mm hole takes a 1-7/8" (47.6 mm) stud, and a 66.7 mm hole takes a 2-1/2" (63.5 mm) stud. The tensile stress area At column uses the ASME B1.1 formula A_t = 0.7854(d - 0.9743/n)^2, which feeds directly into torque calculation.
Calculate Flange Bolt Torque
Know the stud size? Now get the tightening torque — PCC-1 style target stress, lubrication-corrected K factor, three-pass tightening table and cross-pattern sequence.
Frequently Asked Questions
How do I determine the bolt size for a flange?
Three pieces of information come from the flange itself: the bolt count, the bolt-hole diameter, and the bolt circle, all tabulated in ASME B16.5 for each NPS and class. The stud diameter is the standard size below the hole diameter minus the clearance (1/8 in clearance for studs up to 2-1/4 in, 1/4 in above). This chart does that mapping for you: find your NPS row in the table for your flange class and read off the stud size, thread series, nut across-flats and wrench size.
What is the difference between UNC and 8UN threads on flange studs?
Studs of 1 inch diameter and smaller use Unified National Coarse (UNC) threads — 13 tpi for 1/2 in down to 8 tpi for 1 in. Studs larger than 1 inch use the 8UN constant-pitch series at 8 threads per inch. Both are covered by ASME B1.1, and the tensile stress area formula A_t = 0.7854(d - 0.9743/n)^2 applies to either series, so torque calculations work the same way.
Which nut and wrench size do flange studs use?
Flange studs are normally supplied with heavy-hex nuts per ASME B18.2.2 (A194 2H for B7 studs, 2HM for B7M, 8/8M for B8/B8M, 7 for L7). Heavy-hex across-flats is 1.5 times the nominal stud diameter for larger sizes — for example a 3/4 in stud takes a 1-1/4 in (32 mm) heavy-hex nut and therefore a 1-1/4 in socket. The chart lists the across-flats for every standard stud size.
Does Class rating change the bolt size for the same NPS?
Yes, significantly. A 6 in flange takes 8 bolts of 3/4 in in Class 150, 12 bolts of 1-1/4 in in Class 600, and 12 bolts of 1-7/8 in in Class 1500. Higher classes mean thicker flanges, larger bolt circles and more, larger bolts. Always read the table for the exact class — never reuse a bolt list across classes.
How long should flange stud bolts be?
Stud length depends on the combined flange thicknesses plus nut height, washer allowance and 2-3 threads of protrusion. For a standard pair of weld-neck flanges it is roughly 2 x (flange thickness t) plus the nut. Because flange thickness varies by class and size, use the flange dimension tables for your exact combination rather than a generic rule, and add extra length if hydraulic tensioners will be used.