To clamp two flanges together, nuts and bolts are required. The number of bolts is determined by the number of bolt holes in the flange, while the bolt diameter and length depend on the flange type and its pressure class. Stud bolts are widely used in the petrochemical industry and are specified under ASTM standards: ASTM A193 for high-temperature or high-pressure service, and ASTM A320 for low-temperature service. Note, "Heavy" in Heavy Hex Nuts or Heavy Hex Bolts indicates the nut or bolt heads are larger than normal. The larger thicker body spreads the load over a bigger area, have a higher torque capacity, and increased resistance to vibration.
For general information about flanges, gaskets and bolts we have a separate article which you may want to read first. If you just want to know the number of bolts for a flange you need to use our flange tables. This article is specifically about stud bolt grade and hex bolt grade pairings, chemical compositions and dimensions.
As in the table below, the most common pairings include: B7 x 2H (default), B16 x 7 (higher temp), L7 x 7 (low temp), and B8M x 8M. The non-stainless grades may rust if used outdoors hence you may need to buy them coated i.e. Zinc or Galvanised.
Typical Bolt and Nut Combinations
| Bolt grade | Nut grade (ASTM A194) | Notes |
|---|
| B7 | 2H | Default combo for pressure bolting (flanges). |
| B7M | 2HM | “M” = controlled hardness for sour service / cracking risk. |
| B16 | 7 | Default Higher-temp alloy stud; nut grade 7 is the usual match. |
| B16M | 7M | Sour service variant of the high-temp combo (hardness controlled). |
| B8 (304 SS) | 8 | Stainless corrosion service; moderate strength. |
| B8M (316 SS) | 8M | Stainless corrosion service; better chlorides than B8/8. |
| B8T (321 SS) | 8T | Stainless for elevated temperature (321). |
| B8C (347 SS) | 8C | Stainless for elevated temperature (347). |
| L7 | 7 (often) | Default Low-temp alloy bolting; commonly paired with grade 7 nuts (low-temp service intent). |
| B8 (304 SS) | 8 | Low-temp + corrosion resistant stainless. |
| B8M (316 SS) | 8M | Default Low-temp + better corrosion resistance stainless. |
| Grade 660 (A/B/C/D) | 8 / 8M (often) | High-temp, high-strength precipitation-hardening stud; stainless nuts used—pairing is application-specific (don’t assume strength match). |
Note: Match “M” with “M”: B7M→2HM, B16M→7M (hardness controlled) / High temp: use B16 bolts + A194 Grade 7 nuts / Stainless pairs with stainless: B8→8, B8M→8M, etc. / A453 660: nut choice depends on temperature, galling risk, coatings, applied lubrication, and project specification.
Further information on Bolting Grades
ASTM
Standard | Grade | Material | Relative Strength | Notes |
|---|
| A193 | B5 | Alloy steel - Type 501 Stainless Steel, 4-6% Chromium | | |
| A193 | B6 | Alloy steel - Type 410 Stainless Steel, 12% Chromium | | |
| A193 | B7 | Alloy steel - Chromium-Molybdenum, AISI 4140 | High | Most common - Flanges, pressure vessels |
| A193 | B7M | Alloy steel - Chromium-Molybdenum, AISI 4140 | High | Most common - Flanges, pressure vessels |
| A193 | B8 | 304 Stainless Steel | Medium | Most common - Corrosion resistant bolting |
| A193 | B8 Class 2 | 304 Stainless Steel, Strain Hardened | | |
| A193 | B8M | 316 Stainless Steel | Medium | Corrosion resistant bolting |
| A193 | B8M Class 2 | 316 Stainless Steel, Strain Hardened | | |
| A193 | B8C | 347 Stainless Steel | | |
| A193 | B8T | 321 Stainless Steel | | |
| A193 | B16 | Alloy steel - Chromium-Molybdenum-Vanadium | High | High-temperature - Better elevated-temp strength than B7 |
| A320 | L7 / L7M / L43 | Alloy steel - Chromium-Molybdenum, AISI 4140 | High | Low-temperature - Charpy impact tested |
| A453 | 660 A | Precipitation-hardening stainless | Very high | High-temperature - Common turbine grade |
| A453 | 660 B | Precipitation-hardening stainless | Very high | High-temperature - Higher strength than 660A |
| A453 | 660 C | Precipitation-hardening stainless | Very high | High-temperature - Improved creep resistance |
| A453 | 660 D | Precipitation-hardening stainless | Very high | High-temperature - Highest temp capability |
Further information on Nut Grades
ASTM A194
Nut Grade | Material | Heat Treated | Notes |
|---|
| 2H | Carbon steel | Yes | General pressure service - common heavy hex nut |
| 2HM | Carbon steel | Yes | Sour service - Controlled hardness |
| 3 | 501 Stainless steel | | |
| 4 | Carbon-Molybdenum | Yes | Now obsolete specification for high-tensile nuts. |
| 6 | 410 Stainless steel | | |
| 7 | Alloy steel - 4140 / 4142 / 4145 | Yes | High-temperature service - Better creep resistance |
| 7M | Alloy steel - 4140H / 4142H / 4145H | Yes | Sour service - Hardness limited |
| 8 | 304 Stainless steel | Solution treated | Corrosion resistant - Not high strength |
| 8A | 304 Stainless steel | | |
| 8C | 347 Stainless steel | Solution treated | High-temperature stainless - Stabilized SS |
| 8T | 321 Stainless steel | Solution treated | High-temperature stainless - Titanium stabilized |
| 8M | 316 Stainless steel | Solution treated | Corrosion resistant - Better chloride resistance |
| 8MA | 316 Stainless steel | | |
Chemical Composition of Bolt Grades
The chemical composition of bolts is a critical factor in determining their strength, durability, and suitability for specific service conditions. Bolts are manufactured from a range of carbon steels, alloy steels, and stainless steels, with precise amounts of elements such as carbon, manganese, chromium, molybdenum, and nickel added to achieve required mechanical and corrosion-resistance properties. These compositions are tightly controlled by international standards (such as ASTM and ISO), ensuring consistent performance in demanding applications including structural, petrochemical, and high-temperature or low-temperature service. Understanding bolt chemistry is essential for correct material selection, compliance, and long-term reliability.
| | ASTM A193 B7/B7M | ASTM A320 L7/L7M | ASTM A193 B16 |
| Element | Range | Variation | Range | Variation | Range | Variation |
| Carbon | 0.37–0.49 | 0.02 | 0.38–0.48 | 0.02 | 0.36–0.47 | 0.02 |
| Manganese | 0.65–1.10 | 0.04 | 0.75–1.00 | 0.04 | 0.45–0.70 | 0.03 |
| Phosphorus | 0.035 max | 0.005 over | 0.035 max | 0.005 over | 0.035 max | 0.005 over |
| Sulfur | 0.040 max | 0.005 over | 0.040 max | 0.005 over | 0.040 max | 0.005 over |
| Silicon | 0.15–0.35 | 0.02 | 0.15–0.35 | 0.02 | 0.15–0.35 | 0.02 |
| Chromium | 0.75–1.20 | 0.05 | 0.80–1.10 | 0.05 | 0.80–1.15 | 0.05 |
| Molybdenum | 0.15–0.25 | 0.02 | 0.15–0.25 | 0.02 | 0.50–0.65 | 0.03 |
| Vanadium | - | - | - | - | 0.25–0.35 | 0.03 |
| Aluminium | - | - | - | - | 0.015 max | - |
| Nickel | - | - | - | - | - | - |
| | ASTM A193 B7/B7M | ASTM A320 L7/L7M | ASTM A193 B16 |
| Element | Range | Variation | Range | Variation | Range | Variation |
| Carbon | 0.15 max | 0.01 over | 0.08 max | 0.01 over | 0.08 max | 0.01 over |
| Manganese | 1.00 max | 0.03 over | 2.00 max | 0.04 over | 2.00 max | 0.04 over |
| Phosphorus | 0.040 max | 0.005 over | 0.045 max | 0.010 over | 0.045 max | 0.010 over |
| Sulfur | 0.030 max | 0.005 over | 0.030 max | 0.005 over | 0.030 max | 0.005 over |
| Silicon | 1.00 max | 0.05 over | 1.00 max | 0.05 over | 1.00 max | 0.05 over |
| Chromium | 11.5–13.5 | 0.15 | 18.0–20.0 | 0.20 | 16.0–18.0 | 0.20 |
| Molybdenum | - | - | - | - | 2.00–3.00 | 0.10 |
| Vanadium | - | - | - | - | - | - |
| Aluminium | - | - | - | - | - | - |
| Nickel | - | - | 8.0–11.0 | 0.15 | 10.0–14.0 | 0.15 |
Stud bolts (Imperial) Dimensions - ASME B16.5
The data in the tables below show the size and number of studs required based on the flange size and class. The lengths are based on raised face weld neck flanges and are for guidance only. RTJ flanges and blind flanges are likely to need longer studs.
| | CLASS 150 | | | | CLASS 300 | |
| Flange size | No. of bolts | Stud dia | Stud length | TPI (UNC) | TPI (UNS) | | Flange size | No. of bolts | Stud dia | Stud length | TPI (UNC) | TPI (UNS) |
| 1/2" | 4 | 1/2" | 2.1/2" | 13 | - | | 1/2" | 4 | 1/2" | 2.3/4" | 13 | - |
| 3/4" | 4 | 1/2" | 2.3/4" | 13 | - | | 3/4" | 4 | 5/8" | 3.1/4" | 11 | - |
| 1" | 4 | 1/2" | 2.3/4" | 13 | - | | 1" | 4 | 5/8" | 3.1/4" | 11 | - |
| 1 1/4" | 4 | 1/2" | 3" | 13 | - | | 1 1/4" | 4 | 5/8" | 3.1/2" | 11 | - |
| 1 1/2" | 4 | 1/2" | 3" | 13 | - | | 1 1/2" | 4 | 5/8" | 3.3/4" | 11 | - |
| 2" | 4 | 5/8" | 3.1/2" | 11 | - | | 2" | 8 | 5/8" | 3.3/4" | 11 | - |
| 2 1/2" | 4 | 5/8" | 3.3/4" | 11 | - | | 2 1/2" | 8 | 3/4" | 4.1/4" | 10 | - |
| 3" | 4 | 5/8" | 3.3/4" | 11 | - | | 3" | 8 | 3/4" | 4.1/4" | 10 | - |
| 3 1/2" | 8 | 5/8" | 3.3/4" | 11 | - | | 3 1/2" | 8 | 3/4" | 4.1/2" | 10 | - |
| 4" | 8 | 5/8" | 3.3/4" | 11 | - | | 4" | 8 | 3/4" | 4.3/4" | 10 | - |
| 5" | 8 | 3/4" | 4" | 10 | - | | 5" | 8 | 3/4" | 5" | 10 | - |
| 6" | 8 | 3/4" | 4.1/4" | 10 | - | | 6" | 12 | 3/4" | 5" | 10 | - |
| 8" | 8 | 3/4" | 4.1/2" | 10 | - | | 8" | 12 | 7/8" | 5.3/4" | 9 | - |
| 10" | 12 | 7/8" | 5" | 9 | - | | 10" | 16 | 1" | 6.3/4" | 8 | 8 |
| 12" | 12 | 7/8" | 5" | 9 | - | | 12" | 16 | 1.1/8" | 7.1/2" | 7 | 8 |
| 14" | 12 | 1" | 5.3/4" | 8 | 8 | | 14" | 20 | 1.1/8" | 7.1/2" | 7 | 8 |
| 16" | 16 | 1" | 5.3/4" | 8 | 8 | | 16" | 20 | 1.1/4" | 8 | 7 | 8 |
| 18" | 16 | 1.1/8" | 6.1/4" | 7 | 8 | | 18" | 24 | 1.1/4" | 8.1/4" | 7 | 8 |
| 20" | 20 | 1.1/8" | 6.3/4" | 7 | 8 | | 20" | 24 | 1.1/4" | 8.3/4" | 7 | 8 |
| 22" | 20 | 1.1/4" | 7.1/4" | 7 | 8 | | 22" | 24 | 1.1/4" | 9.1/2" | 7 | 8 |
| 24" | 20 | 1.1/4" | 7.1/2" | 7 | 8 | | 24" | 24 | 1.1/2" | 9.3/4" | 6 | 8 |
| | | | | | | | | | | | | |
| | CLASS 600 | | | | CLASS 900 | |
| Flange size | No. of bolts | Stud dia | Stud length | TPI (UNC) | TPI (UNS) | | Flange size | No. of bolts | Stud dia | Stud length | TPI (UNC) | TPI (UNS) |
| 1/2" | 4 | 1/2" | 3.1/4" | 13 | - | | 1/2" | 4 | 3/4" | 4.1/4" | 10 | - |
| 3/4" | 4 | 5/8" | 3.3/4" | 11 | - | | 3/4" | 4 | 3/4" | 4.3/4" | 10 | - |
| 1" | 4 | 5/8" | 3.3/4" | 11 | - | | 1" | 4 | 7/8" | 5.1/4" | 9 | - |
| 1 1/4" | 4 | 5/8" | 4 | 11 | - | | 1 1/4" | 4 | 7/8" | 5.1/4" | 9 | - |
| 1 1/2" | 4 | 3/4" | 4.1/2" | 10 | - | | 1 1/2" | 4 | 1" | 6" | 8 | - |
| 2" | 8 | 5/8" | 4.1/2" | 11 | - | | 2" | 8 | 7/8" | 6" | 9 | - |
| 2 1/2" | 8 | 3/4" | 5 | 10 | - | | 2 1/2" | 8 | 1" | 6.3/4" | 8 | 8 |
| 3" | 8 | 3/4" | 5.1/4" | 10 | - | | 3" | 8 | 7/8" | 6" | 9 | - |
| 3 1/2" | 8 | 7/8" | 5.3/4" | 9 | - | | 3 1/2" | n/a | - | - | - | - |
| 4" | 8 | 7/8" | 6" | 9 | - | | 4" | 8 | 1.1/8" | 7.1/4" | 7 | 8 |
| 5" | 8 | 1" | 7" | 8 | 8 | | 5" | 8 | 1.1/4" | 8" | 7 | 8 |
| 6" | 12 | 1" | 7.1/4" | 8 | 8 | | 6" | 12 | 1.1/8" | 8" | 7 | 8 |
| 8" | 12 | 1.1/8" | 8" | 7 | 8 | | 8" | 12 | 1.3/8" | 9.1/4" | 6 | 8 |
| 10" | 16 | 1.1/4" | 9" | 7 | 8 | | 10" | 16 | 1.3/8" | 9.3/4" | 6 | 8 |
| 12" | 20 | 1.1/4" | 9.1/4" | 7 | 8 | | 12" | 20 | 1.3/8" | 10.1/2" | 6 | 8 |
| 14" | 20 | 1.3/8" | 9.3/4" | 6 | 8 | | 14" | 20 | 1.1/2" | 11.1/4" | 6 | 8 |
| 16" | 20 | 1.1/2" | 10.1/2" | 6 | 8 | | 16" | 20 | 1.5/8" | 11.3/4" | - | 8 |
| 18" | 20 | 1.5/8" | 11.1/4" | - | 8 | | 18" | 20 | 1.7/8" | 13.1/2" | - | 8 |
| 20" | 24 | 1.5/8" | 12 | - | 8 | | 20" | 20 | 2" | 14.1/2" | 4.5 | 8 |
| 22" | 24 | 1.3/4" | 12.3/4" | 5 | 8 | | 22" | n/a | - | - | - | - |
| 24" | 24 | 1.7/8" | 13.1/2" | - | 8 | | 24" | 20 | 2.1/2" | 18" | 4 | 8 |
Stud bolts (Metric) Dimensions - DIN976
The threads on these stud blots are in accordance with ISO 261.
| | Diameter | Pitch Thread | |
| | M6 | 1 | |
| | M8 | 1.25 | |
| | M10 | 1.5 | |
| | M12 | 1.75 | |
| | M14 | 2 | |
| | M16 | 2 | |
| | M18 | 2.5 | |
| | M20 | 2.5 | |
| | M22 | 2.5 | |
| | M24 | 3 | |
Heavy Hex Nuts - Imperial
The heavy hex nut dimensions are in accordance with ANSI B18.2.2. As per the image, these have a washer face on one side. Heavy hex nuts are larger than standard hex nuts.
| | Diameter | TPI (UNC) | TPI (UNS) | A/F | Thickness (T) | |
| | 1/4" | 20 | - | 1/2" | 1/4" | |
| | 5/16" | 18 | - | 9/16" | 5/16" | |
| | 3/8" | 16 | - | 11/16" | 3/8" | |
| | 7/16" | 14 | - | 3/4" | 7/16" | |
| | 1/2" | 13 | - | 7/8" | 1/2" | |
| | 9/16" | 12 | - | 15/16" | 9/16" | |
| | 5/8" | 11 | - | 1.1/6" | 5/8" | |
| | 3/4" | 10 | - | 1.1/4" | 3/4" | |
| | 7/8" | 9 | - | 1.7/16" | 7/8" | |
| | 1" | 8 | 8 | 1.5/8" | 1" | |
| | 1.1/8" | 7 | 8 | 1.13/16" | 1.1/8" | |
| | 1.1/4" | 7 | 8 | 2" | 1.1/4" | |
| | 1.3/8" | 6 | 8 | 2.3/16" | 1.3/8" | |
| | 1.1/2" | 6 | 8 | 2.3/8" | 1.1/2" | |
| | 1.5/8" | - | 8 | 2.9/16" | 1.5/8" | |
| | 1.3/4" | 5 | 8 | 2.3/4" | 1.3/4" | |
| | 1.7/8" | - | 8 | 2.15/16" | 1.7/8" | |
| | 2" | 4.5 | 8 | 3.1/8" | 2" | |
| | 2.1/4" | 4.5 | 8 | 3.1/2" | 2.1/4" | |
| | 2.1/2" | 4 | 8 | 3.7/8" | 2.1/2" | |
Heavy Hex Nuts - Metric
The heavy hex nut dimensions are in accordance with DIN934. Heavy hex nuts are larger than standard hex nuts.
| | Diameter (D) | Thread Pitch | A/F | Thickness (T) | |
| | M8 | 1.25 | 13 | 8 | |
| | M10 | 1.50 | 17 | 10 | |
| | M12 | 1.75 | 19 | 12 | |
| | M14 | 2.00 | 22 | 14 | |
| | M16 | 2.00 | 24 | 16 | |
| | M18 | 2.50 | 27 | 18 | |
| | M20 | 2.50 | 30 | 20 | |
| | M22 | 2.50 | 32 | 22 | |
| | M24 | 3.00 | 36 | 24 | |
Heavy Hex Bolts - ASME B18.2.1
Heavy hex bolts have a larger head than standard hex head bolts.
| | A/F (F) | A/C (C) | Height (H) | Thread (T) |
| Dia (D) | Basic | Min | Max | Min | Max | Basic | Min | Max | Up to 6" | Over 6" |
| 1/2" | 7/8" | 0.850 | 0.875 | 0.969 | 1.010 | 11/32" | 0.302 | 0.364 | 1.25 | 1.50 |
| 5/8" | 1.1/6" | 1.031 | 1.062 | 1.175 | 1.227 | 27/64" | 0.378 | 0.444 | 1.50 | 1.75 |
| 3/4" | 1.1/4" | 1.212 | 1.250 | 1.383 | 1.443 | 1/2" | 0.455 | 0.524 | 1.75 | 2.00 |
| 7/8" | 1.7/16" | 1.394 | 1.438 | 1.589 | 1.660 | 37/64" | 0.531 | 0.604 | 2.00 | 2.25 |
| 1" | 1.5/8" | 1.575 | 1.625 | 1.796 | 1.876 | 43/64" | 0.591 | 0.700 | 2.25 | 2.50 |
| 1.1/8" | 1.13/16" | 1.756 | 1.812 | 2.002 | 2.093 | 3/4" | 0.658 | 0.780 | 2.50 | 2.75 |
| 1.1/4" | 2" | 1.938 | 2.000 | 2.209 | 2.309 | 27/32" | 0.749 | 0.876 | 2.75 | 3.00 |
| 1.3/8" | 2.3/16" | 2.119 | 2.188 | 2.416 | 2.526 | 29/32" | 0.810 | 0.940 | 3.00 | 3.25 |
| 1.1/2" | 2.3/8" | 2.300 | 2.375 | 2.622 | 2.742 | 1" | 0.902 | 1.036 | 3.25 | 3.50 |
| 1.5/8" | 2.9/16" | 2.481 | 2.562 | 2.829 | 2.959 | 1.3/32" | 0.978 | 1.116 | 3.50 | 3.75 |
| 1.3/4" | 2.3/4" | 2.662 | 2.750 | 3.035 | 3.175 | 1.5/32" | 1.054 | 1.196 | 3.75 | 4.00 |
| 1.7/8" | 2.15/16" | 2.844 | 2.938 | 3.242 | 3.392 | 1.1/4" | 1.130 | 1.276 | 4.00 | 4.25 |
| 2" | 3.1/8" | 3.025 | 3.125 | 3.449 | 3.608 | 1.11/32" | 1.175 | 1.388 | 4.25 | 4.50 |
| 2.1/4" | 3.1/2" | 3.388 | 3.500 | 3.862 | 4.041 | 1.1/2" | 1.327 | 1.548 | 4.75 | 5.00 |
| 2.1/2" | 3.7/8" | 3.750 | 3.875 | 4.275 | 4.474 | 1.21/32" | 1.479 | 1.708 | 5.25 | 5.50 |