ANSI Flange Bolt Size Chart: Class 150 to 2500

Contents

1 1. What Is an ANSI Flange Bolt Size Chart?
- 1.1 Bolts Versus Stud Bolts
2 2. ANSI Flange Standards and Pressure Classes
3 3. Flange Bolt Terminology and Dimensions
4 4. ANSI Class 150 Flange Bolt Size Chart
5 5. ANSI Class 300 Flange Bolt Size Chart
6 6. ANSI Class 400 and Class 600 Flange Bolt Size Chart
- 6.1 ANSI Class 400 Flange Bolt Size Chart
- 6.2 ANSI Class 600 Flange Bolt Size Chart
7 7. ANSI Class 900, 1500, and 2500 Flange Bolt Size Chart
8 9. How to Select the Correct Flange Bolts, Nuts, and Materials
9 Conclusion

Bolts are essential components of a flanged joint because they provide the clamping force required to compress the gasket and maintain a leak-tight connection. Selecting the wrong bolt diameter, number of bolts, or bolt length can prevent proper flange alignment, produce uneven gasket compression, and increase the risk of leakage during operation.

An ANSI flange bolt size chart provides the bolting requirements for standard pipe flanges according to nominal pipe size and pressure class. The chart commonly lists the number of bolts, bolt or stud diameter, bolt-circle diameter, and recommended stud-bolt length. These dimensions vary because larger flanges and higher pressure classes generally require more or larger fasteners to withstand the internal pressure and external piping loads.

Although these products are often called “ANSI flanges,” their dimensions are currently governed primarily by ASME standards. ASME B16.5 covers pipe flanges and flanged fittings from NPS ½ through NPS 24, while ASME B16.47 covers large-diameter steel flanges from NPS 26 through NPS 60.

This article provides ANSI flange bolt size charts for common pressure classes and explains how to select the correct bolts, stud bolts, nuts, materials, and lengths for a flanged piping connection.

1. What Is an ANSI Flange Bolt Size Chart?

ANSI Flange Bolt Size Chart

An ANSI flange bolt size chart is a reference table showing the standard fastener requirements for joining two ASME flanges. Engineers, fabricators, pipefitters, and maintenance technicians use these charts to determine the correct bolting arrangement for a particular nominal pipe size and pressure class.

A typical flange bolt chart contains the following information:

Nominal pipe size, expressed as NPS
Flange pressure class
Number of bolt holes
Bolt-hole diameter
Required bolt or stud diameter
Bolt-circle diameter
Recommended stud-bolt length
Number and size of nuts

For example, two NPS 4 flanges may require different bolting arrangements if one is Class 150 and the other is Class 600. The higher-pressure flange is normally thicker and designed with larger or more robust bolting to provide the required joint strength.

The number of bolt holes in a standard flange is normally a multiple of four. The holes are equally spaced around the bolt circle, and flanges are generally installed so that the vertical and horizontal centerlines pass between adjacent bolt holes rather than through them. This convention helps maintain consistent flange orientation and simplifies the installation of connected equipment and piping.

Bolts Versus Stud Bolts

The term “flange bolt” is frequently used in a general sense, but industrial flanged joints commonly use stud bolts rather than conventional headed bolts. A stud bolt is threaded along its length and secured with a heavy hex nut at each end. This arrangement offers several advantages:

Clamping force can be applied more evenly.
Installation and removal are easier in confined piping systems.
Studs can accommodate different flange and gasket thicknesses.
Damaged nuts can be replaced without necessarily replacing the entire stud.

The dimensions in a bolt size chart must therefore be interpreted carefully. “Bolt diameter” usually refers to the nominal thread diameter of the stud, while “bolt length” may refer to either a stud-bolt length or a machine-bolt length. These values are not always interchangeable.

2. ANSI Flange Standards and Pressure Classes

The term “ANSI flange” remains widely used in industry, but ANSI does not directly publish the principal dimensional requirements for modern pipe flanges. ANSI accredits the standards-development process, while the detailed flange requirements are published by ASME. Consequently, “ANSI flange dimensions” and “ASME flange dimensions” often refer to the same standardized flange system.

ASME B16.5

ASME B16.5 covers pipe flanges and flanged fittings for nominal pipe sizes from NPS ½ through NPS 24. It includes requirements for:

Flange dimensions
Pressure–temperature ratings
Materials
Tolerances
Markings
Testing
Bolt holes and bolting dimensions

The standard includes pressure classes 150, 300, 400, 600, 900, 1500, and 2500. However, not every flange type, material, or pipe size is available in every pressure class.

ASME B16.47

ASME B16.47 covers large-diameter steel flanges from NPS 26 through NPS 60. These flanges are divided into two dimensional series:

Series A: Generally heavier, thicker, and designed with larger bolt-circle diameters and fewer but larger bolts.
Series B: Generally lighter and more compact, with smaller bolt-circle diameters and a greater number of smaller bolts.

Series A and Series B flanges of the same nominal size and pressure class are not directly interchangeable. Their outside diameters, bolt circles, bolt-hole quantities, and bolt sizes may differ.

ANSI/ASME Pressure Classes

An ANSI or ASME flange class is a pressure–temperature rating designation rather than a direct pressure value. For example, a Class 300 flange is not automatically limited to 300 psi under every operating condition. Its allowable working pressure depends on several factors, including:

Flange material
Operating temperature
Pressure class
Applicable material group
Requirements of the governing design code

The common flange pressure classes are:

Pressure class	Typical description
Class 150	Commonly used for relatively low-pressure services
Class 300	Used for moderate-pressure piping systems
Class 400	Less common intermediate pressure class
Class 600	Used for higher-pressure industrial service
Class 900	Used in high-pressure process piping
Class 1500	Used in very high-pressure applications
Class 2500	Used for extremely high-pressure service

As pressure class increases, the flange generally becomes thicker and may require larger bolts, more bolts, or both. However, bolt requirements should always be taken directly from the applicable dimensional table rather than estimated solely from the pressure class.

The flange standard determines the basic number and diameter of the fasteners, but separate standards and specifications govern bolting materials and installation. Common examples include ASTM A193 for alloy-steel and stainless-steel bolting, ASTM A320 for low-temperature bolting, ASTM A194 for nuts, and ASME PCC-1 for bolted-flange joint assembly practices.

3. Flange Bolt Terminology and Dimensions

Flange Bolt Terminology and Dimensions

Correctly reading a flange bolt chart requires an understanding of the dimensions used to describe the flange, bolt holes, and fasteners. Although the general term “flange bolts” is commonly used, most industrial ASME flange joints are assembled with fully threaded or continuous-thread stud bolts and two heavy hex nuts.

The principal flange bolting terms are described below.

Nominal Pipe Size

Nominal Pipe Size, abbreviated as NPS, identifies the nominal size of the pipe and matching flange. It is a designation rather than the exact inside or outside diameter of the pipe.

For example, an NPS 4 flange connects to an NPS 4 pipe, but neither the flange opening nor the pipe outside diameter necessarily measures exactly 4 inches.

Pressure Class

The pressure class identifies the flange’s pressure–temperature rating category. Common ASME B16.5 classes include 150, 300, 400, 600, 900, 1500, and 2500.

The pressure class affects several flange and bolting dimensions, including:

Flange thickness
Flange outside diameter
Number of bolts
Stud-bolt diameter
Bolt-hole diameter
Bolt-circle diameter
Required stud-bolt length

Flanges of the same NPS but different pressure classes generally cannot be bolted together because their bolt-hole patterns and other dimensions may differ.

Number of Bolts

The number of bolts indicates how many stud bolts or machine bolts are required to assemble the joint. Standard ASME flange bolt patterns normally use bolt quantities in multiples of four.

The quantity listed in a chart is the number required for one flange joint—not the combined number of holes in both flanges. For example, two mating flanges with eight aligned holes require eight stud bolts and sixteen nuts.

Stud-Bolt Diameter

The stud-bolt diameter is the nominal outside diameter of the threaded fastener. It is always smaller than the flange bolt-hole diameter to provide installation clearance.

For example, a flange with ¾-inch bolt holes normally uses ⅝-inch stud bolts. The bolt diameter should be selected from the applicable flange standard and must not be increased or reduced without engineering verification.

Bolt-Hole Diameter

The bolt-hole diameter is the diameter of each drilled flange hole. Bolt holes are manufactured larger than the specified stud diameter so the studs can pass through both flanges even when minor manufacturing and assembly tolerances are present.

The clearance should not be interpreted as permission to use a larger fastener. A ⅞-inch hole, for example, is commonly intended for a ¾-inch stud rather than a ⅞-inch stud.

Bolt-Circle Diameter

The bolt-circle diameter, commonly abbreviated as BCD, is the diameter of an imaginary circle passing through the centers of all bolt holes.

Bolt holes are equally spaced around this circle. For two flanges to mate correctly, they must have the same:

Number of bolt holes
Bolt-hole diameter
Bolt-circle diameter
Flange-facing configuration

Stud-Bolt Length

Stud-bolt length is generally measured from the first complete thread at one end to the first complete thread at the other end, excluding the end chamfers. The required length must accommodate:

Thickness of both flanges
Compressed gasket thickness
Raised-face or RTJ configuration
Two heavy hex nuts
Any specified washers
Sufficient thread projection beyond each nut

Stud length can differ between raised-face and ring-type joint flanges. It may also change when connecting a flange to a valve, blind flange, spectacle blind, insulating gasket kit, or other nonstandard component.

RF and RTJ Lengths

RF means raised face. An RF flange joint typically uses a spiral-wound, compressed-fiber, or other gasket positioned within the bolt circle.

RTJ means ring-type joint. An RTJ connection uses a metallic ring gasket installed in matching machined grooves. Its geometry may require a different stud length from the equivalent raised-face joint.

The following charts provide common ASME B16.5 stud-bolt dimensions. Final bolting must be verified against the applicable project specification, flange type, gasket arrangement, and current standard edition.

4. ANSI Class 150 Flange Bolt Size Chart

ANSI Class 150 Flange Bolt Size Chart

Class 150 is widely used in water, utility, chemical-processing, oil and gas, and general industrial piping. The required stud diameter increases gradually with the flange size, while the number of studs rises from four on small flanges to twenty on the largest ASME B16.5 Class 150 sizes.

The following chart applies to standard ASME B16.5 Class 150 flanges from NPS ½ through NPS 24. All dimensions are in inches.

NPS	Number of studs	Stud diameter	RF stud length	RTJ stud length
½	4	½	2¼	—
¾	4	½	2½	—
1	4	½	2½	3
1¼	4	½	2¾	3¼
1½	4	½	2¾	3¼
2	4	⅝	3¼	3¾
2½	4	⅝	3½	4
3	4	⅝	3½	4
3½	8	⅝	3½	4
4	8	⅝	3½	4
5	8	¾	3¾	4¼
6	8	¾	4	4½
8	8	¾	4¼	4¾
10	12	⅞	4½	5
12	12	⅞	4¾	5¼
14	12	1	5¼	5¾
16	16	1	5¼	5¾
18	16	1⅛	5¾	6¼
20	20	1⅛	6¼	6¾
24	20	1¼	6¾	7¼

The chart shows that the same stud diameter may be used across several adjacent pipe sizes. However, the required quantity and length can still change. For example, NPS 3 and NPS 4 Class 150 flanges both use ⅝-inch studs, but NPS 3 requires four studs while NPS 4 requires eight.

Class 150 RTJ dimensions are not available for every small flange size. A dash in the table means that no corresponding standard RTJ stud length is listed in the referenced chart—not that an arbitrary length should be selected. The dimensions above align with commonly published ASME B16.5 bolting references, but actual joint configuration must be checked before procurement.

5. ANSI Class 300 Flange Bolt Size Chart

Class 300 flanges are generally thicker and designed for higher pressure–temperature ratings than Class 150 flanges of the same material and nominal size. They usually require longer, larger, or more numerous stud bolts.

Class 150 and Class 300 flanges should not be considered interchangeable. Even when two sizes happen to use the same stud diameter, their flange thicknesses, bolt circles, hole quantities, and stud lengths may differ.

The following chart gives common ASME B16.5 Class 300 stud-bolt dimensions. All dimensions are in inches.

NPS	Number of studs	Stud diameter	RF stud length	RTJ stud length
½	4	½	2½	3
¾	4	⅝	3	3½
1	4	⅝	3	3½
1¼	4	⅝	3¼	3¾
1½	4	¾	3½	4
2	8	⅝	3½	4
2½	8	¾	4	4½
3	8	¾	4¼	4¾
3½	8	¾	4¼	5
4	8	¾	4½	5
5	8	¾	4¾	5¼
6	12	¾	4¾	5½
8	12	⅞	5½	6
10	16	1	6¼	6¾
12	16	1⅛	6¾	7¼
14	20	1⅛	7	7½
16	20	1¼	7½	8
18	24	1¼	7¾	8¼
20	24	1¼	8	8¾
24	24	1½	9	10

An important example is the NPS 2 Class 300 flange. It uses eight ⅝-inch studs, whereas the corresponding NPS 2 Class 150 flange uses only four studs of the same diameter. This demonstrates why bolt diameter alone is insufficient for identifying a flange.

The listed lengths are useful for standard flange-to-flange joints. Different lengths may be required when the joint includes:

A blind flange
A valve or equipment nozzle
An orifice flange
A spectacle blind or spacer
An insulating gasket kit
Additional washers
A nonstandard gasket thickness
Flanges with unequal thicknesses

For purchasing or fabrication, confirm the stud length from the actual joint stack-up and allow sufficient thread engagement in both nuts. The ASME B16.5 bolting chart provides the standard dimensional basis, while controlled joint assembly practices are addressed separately. ASME B16.5 flange bolt chart

6. ANSI Class 400 and Class 600 Flange Bolt Size Chart

Class 400 and Class 600 flanges are used in higher-pressure piping systems where Class 150 or Class 300 flanges cannot provide an adequate pressure–temperature rating. Class 400 is less commonly specified, while Class 600 is widely used in oil and gas, petrochemical, power-generation, steam, and high-pressure process applications.

For smaller sizes, Class 400 and Class 600 flanges share several bolting dimensions. Differences begin to appear as flange size increases because Class 600 flanges generally require larger studs to produce greater clamping capacity.

ANSI Class 400 Flange Bolt Size Chart

The following table gives the required stud-bolt quantity and nominal diameter for ASME B16.5 Class 400 flanges.

NPS	Number of studs	Stud diameter
½	4	½ in.
¾	4	⅝ in.
1	4	⅝ in.
1¼	4	⅝ in.
1½	4	¾ in.
2	8	⅝ in.
2½	8	¾ in.
3	8	¾ in.
3½	8	⅞ in.
4	8	⅞ in.
5	8	⅞ in.
6	12	⅞ in.
8	12	1 in.
10	16	1⅛ in.
12	16	1¼ in.
14	20	1¼ in.
16	20	1⅜ in.
18	24	1⅜ in.
20	24	1½ in.
24	24	1¾ in.

ANSI Class 600 Flange Bolt Size Chart

The following table applies to ASME B16.5 Class 600 flanges.

NPS	Number of studs	Stud diameter
½	4	½ in.
¾	4	⅝ in.
1	4	⅝ in.
1¼	4	⅝ in.
1½	4	¾ in.
2	8	⅝ in.
2½	8	¾ in.
3	8	¾ in.
3½	8	⅞ in.
4	8	⅞ in.
5	8	1 in.
6	12	1 in.
8	12	1⅛ in.
10	16	1¼ in.
12	20	1¼ in.
14	20	1⅜ in.
16	20	1½ in.
18	20	1⅝ in.
20	24	1⅝ in.
24	24	1⅞ in.

Up to NPS 4, the listed Class 400 and Class 600 bolting arrangements are substantially the same. From NPS 5 upward, Class 600 typically uses larger stud diameters or a different number of studs. For example, an NPS 12 Class 400 flange uses sixteen 1¼-inch studs, while an NPS 12 Class 600 flange uses twenty studs of the same diameter.

These similarities should not be interpreted as proof that Class 400 and Class 600 flanges are interchangeable. Their thicknesses and other dimensions can differ even where their bolt patterns are similar. The flange class must always be confirmed from its markings, documentation, or dimensional inspection.

7. ANSI Class 900, 1500, and 2500 Flange Bolt Size Chart

Classes 900, 1500, and 2500 are intended for high- and very-high-pressure applications. They are frequently used in petroleum production, refineries, chemical plants, power stations, and high-pressure gas systems.

Higher pressure classes do not always use more studs. In some cases, the design uses fewer studs with considerably larger diameters. Therefore, both the quantity and diameter must be obtained from the appropriate chart.

ANSI Class 900 Flange Bolt Size Chart

NPS	Number of studs	Stud diameter
½	4	¾ in.
¾	4	¾ in.
1	4	⅞ in.
1¼	4	⅞ in.
1½	4	1 in.
2	8	⅞ in.
2½	8	1 in.
3	8	⅞ in.
4	8	1⅛ in.
5	8	1¼ in.
6	12	1⅛ in.
8	12	1⅜ in.
10	16	1⅜ in.
12	20	1⅜ in.
14	20	1½ in.
16	20	1⅝ in.
18	20	1⅞ in.
20	20	2 in.
24	20	2½ in.

The required stud diameter does not increase uniformly with NPS. For example, NPS 2 uses eight ⅞-inch studs, NPS 2½ uses eight 1-inch studs, and NPS 3 returns to eight ⅞-inch studs. This is a standard dimensional arrangement and should not be “corrected” based on size progression alone.

ANSI Class 1500 Flange Bolt Size Chart

NPS	Number of studs	Stud diameter
½	4	¾ in.
¾	4	¾ in.
1	4	⅞ in.
1¼	4	⅞ in.
1½	4	1 in.
2	8	⅞ in.
2½	8	1 in.
3	8	1⅛ in.
4	8	1¼ in.
5	8	1½ in.
6	12	1⅜ in.
8	12	1⅝ in.
10	12	1⅞ in.
12	16	2 in.
14	16	2¼ in.
16	16	2½ in.
18	16	2¾ in.
20	16	3 in.
24	16	3½ in.

Large Class 1500 flanges require very large fasteners. An NPS 24 Class 1500 flange, for example, uses sixteen 3½-inch-diameter studs. Such joints require carefully controlled tightening procedures, suitable hydraulic tensioning or torquing equipment, and verified lubricant and torque values.

ANSI Class 2500 Flange Bolt Size Chart

Under ASME B16.5, the standard Class 2500 size range extends only through NPS 12.

NPS	Number of studs	Stud diameter
½	4	¾ in.
¾	4	¾ in.
1	4	⅞ in.
1¼	4	1 in.
1½	4	1⅛ in.
2	8	1 in.
2½	8	1⅛ in.
3	8	1¼ in.
4	8	1½ in.
5	8	1¾ in.
6	8	2 in.
8	12	2 in.
10	12	2½ in.
12	12	2¾ in.

A Class 2500 flange should not be selected solely from its nominal pressure class. The allowable working pressure still depends on flange material and operating temperature. The complete pressure–temperature rating must be checked in the applicable material table.

The tables in this section show only bolt quantity and diameter. Stud length must be determined separately because it depends on flange thickness, facing, gasket type, nut dimensions, and the actual components included in the joint.

Stud-bolt diameter and quantity are fixed by the flange size and pressure class, but the required length depends on the complete joint arrangement. A stud that is too short may not fully engage both nuts, while an excessively long stud adds unnecessary weight, cost, and installation difficulty.

For a basic flange-to-flange connection, the required stud length can be estimated as:

$L = T_{1} + T_{2} + G + 2 H + 2 P$

Where:

$L$ = required stud-bolt length
$T_{1}$ = thickness of the first flange
$T_{2}$ = thickness of the second flange
$G$ = compressed gasket thickness or flange-face allowance
$H$ = height of one nut
$P$ = desired thread projection beyond one nut

This expression is a practical stack-up method rather than a substitute for the standard bolting table. The exact measurement convention and allowances must follow the project specification and applicable flange standard.

Step 1: Identify Both Connected Components

First determine whether the joint connects:

Two identical flanges
A flange and a valve
A flange and an equipment nozzle
A standard flange and a blind flange
Two flanges with different pressure classes
A flange and a spectacle blind or spacer

Do not assume that both sides have the same thickness. A valve body, blind flange, or equipment nozzle may require a significantly longer stud than a standard flange-to-flange joint.

Step 2: Determine the Flange Thicknesses

Obtain the thickness of each flange from the applicable dimensional drawing or ASME table. Flange thickness varies according to:

Nominal pipe size
Pressure class
Flange type
Flange facing
Applicable standard

A blind flange is normally thicker than a comparable slip-on or weld-neck flange and therefore may require longer studs.

Step 3: Account for the Gasket and Facing

For a raised-face joint, account for the compressed gasket thickness and the relationship between the raised faces. The gasket’s uncompressed thickness should not automatically be added to the stack-up because some gasket types compress substantially during tightening.

For an RTJ connection, the ring gasket sits in matching grooves and the final separation between the flanges is controlled by the flange and ring geometry. Standard RF and RTJ stud lengths can therefore differ.

Step 4: Add Nut Heights

A stud-bolted flange joint normally uses two heavy hex nuts per stud. The thickness of both nuts must be included in the calculated length.

Nut dimensions depend on the stud diameter and applicable nut standard. ASTM A194 heavy hex nuts are frequently used with ASTM A193 and ASTM A320 stud bolts.

Step 5: Add Thread Projection

After tightening, the stud should generally extend beyond the outer face of each nut sufficiently to show complete thread engagement. A common project requirement is at least one complete thread beyond each nut, although the permitted projection may vary.

Excessive projection should also be avoided, particularly where:

Installation clearance is limited
Bolt tensioning equipment will be used
Personnel could contact exposed studs
Protective bolt caps are installed
Corrosion protection must cover the exposed threads

Step 6: Include Additional Joint Components

The basic chart length must be increased if the joint includes additional thickness from components such as:

Washers
Insulating sleeves and washers
Flange isolation kits
Spectacle blinds
Paddle blinds
Spacers
Orifice plates
Corrosion protectors
Bolt-tensioning allowances

For hydraulic bolt tensioning, additional exposed thread length may be required so the tensioning tool can grip the stud correctly.

Example of Stud-Length Calculation

Consider a joint with the following assumed dimensions:

First flange thickness: 1.25 in.
Second flange thickness: 1.25 in.
Compressed gasket allowance: 0.125 in.
Height of each nut: 0.875 in.
Thread projection at each end: 0.125 in.

The estimated stud length is:

$L = 1.25 + 1.25 + 0.125 + 2 (0.875) + 2 (0.125)$ $L = 4.625 in.$

The calculated value must then be rounded up to an available standard stud length, such as 4¾ inches, subject to verification of thread engagement and project requirements.

For standard flange-to-flange arrangements, using a verified ASME bolting chart is faster and safer than calculating every stud individually. A stack-up calculation becomes particularly important when nonstandard components or unequal flange thicknesses are present.

9. How to Select the Correct Flange Bolts, Nuts, and Materials

Selecting flange bolting involves more than matching the stud diameter and quantity shown in a dimensional chart. The fasteners must also have suitable mechanical strength, corrosion resistance, temperature capability, thread form, and compatibility with the flange and service conditions.

An appropriate bolting specification should identify:

Stud-bolt material and grade
Nut material and grade
Stud diameter and length
Thread series and fit
Coating or surface treatment
Required lubricant
Tightening method
Target bolt load or torque
Applicable material and assembly standards

Select the Required Diameter and Quantity

Begin by confirming the following flange information:

Nominal pipe size
Pressure class
Flange standard
Flange facing
Flange type
Connected component

Use the appropriate ASME B16.5 or ASME B16.47 table to determine the number and nominal diameter of the studs. Do not estimate bolt size based only on pipe diameter or operating pressure.

A flange marked NPS 4 Class 300, for example, requires a different bolting arrangement from an NPS 4 Class 150 flange. Large-diameter ASME B16.47 Series A and Series B flanges can also have different bolt patterns despite having the same NPS and pressure class.

Select the Stud-Bolt Material

The bolting material must be suitable for the design temperature, pressure, environment, and flange material. Common specifications include ASTM A193, ASTM A320, and ASTM F593.

ASTM A193 Grade B7

ASTM A193 Grade B7 is one of the most widely used bolting materials for carbon-steel and low-alloy steel flanges. It is manufactured from chromium-molybdenum alloy steel and is commonly used in petroleum, chemical-processing, and power-generation applications.

Typical applications include:

General process piping
Carbon-steel flange connections
Elevated-temperature service
Oil and gas facilities
Pressure vessels and valves

B7 studs are commonly paired with ASTM A194 Grade 2H heavy hex nuts.

ASTM A193 Grade B8

Grade B8 is based on Type 304 stainless steel. It provides better corrosion resistance than alloy-steel B7 bolting and is frequently used with stainless-steel piping and equipment.

Depending on the specified class and heat treatment, its mechanical properties can differ significantly. The complete material designation must therefore be stated rather than specifying only “B8.”

ASTM A193 Grade B8M

Grade B8M is based on Type 316 stainless steel and offers improved resistance to chloride-bearing and corrosive environments compared with Type 304-based B8.

B8M bolting is commonly paired with compatible ASTM A194 Grade 8M nuts. However, stainless-steel stud-and-nut combinations are susceptible to galling, so appropriate lubrication and installation controls are important.

ASTM A320 Grade L7

ASTM A320 Grade L7 is commonly selected for low-temperature service. It has mechanical properties similar to A193 B7 but includes low-temperature impact-testing requirements.

L7 studs are often paired with ASTM A194 Grade 4 or Grade 7 nuts, depending on the project specification and operating conditions.

Common Stud-and-Nut Combinations

Service or stud grade	Common nut grade	Typical application
ASTM A193 B7	ASTM A194 2H	General carbon-steel process piping
ASTM A193 B7M	ASTM A194 2HM	Controlled-hardness or sour service
ASTM A193 B8	ASTM A194 8	Type 304 stainless-steel service
ASTM A193 B8M	ASTM A194 8M	Type 316 stainless-steel service
ASTM A320 L7	ASTM A194 4 or 7	Low-temperature service
ASTM A320 L7M	ASTM A194 7M	Low-temperature controlled-hardness service
ASTM A320 B8M	ASTM A194 8M	Low-temperature stainless-steel service

These are common combinations rather than universal requirements. Material selection must comply with the piping material specification, applicable code, design temperature, and corrosive-service requirements.

Consider Sour-Service Requirements

Bolting used in wet hydrogen sulfide service may require controlled hardness and compliance with the project’s adopted sour-service standard.

Grades such as ASTM A193 B7M and ASTM A320 L7M may be specified instead of conventional B7 or L7. The corresponding nuts must also meet the required hardness and material conditions.

Using standard B7 bolting without reviewing the sour-service requirements can create a risk of sulfide stress cracking.

Select the Correct Thread Series

Inch-series flange studs normally use Unified National threads. The thread series generally depends on the stud diameter:

UNC threads are commonly used for smaller diameters.
8UN threads are widely used for stud diameters of 1 inch and larger.

The procurement description should state the required diameter, thread series, thread fit, and length. For example:

ASTM A193 B7 stud bolt, 1¼-8UN × 8 in. long, with two ASTM A194 2H heavy hex nuts.

Thread requirements should be taken from the applicable flange, bolting, and project specifications rather than assumed from diameter alone.

Consider Coatings and Corrosion Protection

Uncoated alloy-steel bolting can corrode rapidly in outdoor, marine, offshore, chemical, or buried service. Common protective options include:

Hot-dip galvanizing
Mechanical galvanizing
Zinc electroplating
Zinc-flake coating
PTFE-based coating
Fluoropolymer coating
Nickel-based coating
Corrosion-inhibiting lubricant

The coating must be compatible with the service temperature, nuts, installation lubricant, and required tightening method. A coating changes the friction between the stud and nut, which directly affects the relationship between applied torque and achieved bolt load.

Torque values developed for uncoated bolting must not automatically be applied to PTFE-coated, galvanized, or differently lubricated fasteners.

Prevent Stainless-Steel Galling

Stainless-steel studs and nuts can seize during tightening because of adhesive wear between the mating threads. Galling is more likely when installation involves:

High tightening speed
Dry threads
Similar stainless-steel grades
Damaged or dirty threads
Excessive torque
Repeated assembly

The risk can be reduced by using an approved anti-seize compound, keeping the threads clean, tightening at a controlled speed, and selecting an appropriate stud-and-nut material combination.

Verify Nut and Washer Requirements

Most standard flange joints use one heavy hex nut at each end of every stud. Washers are not automatically required for all ASME flange joints, but they may be specified when:

Hydraulic tensioning is used
An insulating gasket kit is installed
Coated flange surfaces require protection
The project specification requires hardened washers
Oversized or slotted holes are present
Load distribution beneath the nut must be improved

Washers add to the joint stack-up and must be included when determining stud length.

Confirm the Tightening Procedure

Correct bolt dimensions and materials cannot ensure a leak-tight joint unless the bolts are installed properly. The tightening procedure should control:

Flange alignment
Gasket position
Thread condition
Lubrication
Tightening sequence
Number of tightening passes
Target torque or bolt load
Final circumferential verification

Bolts are normally tightened progressively in a cross-pattern so gasket compression is distributed evenly. Large or critical joints may require hydraulic torque wrenches, ultrasonic bolt measurement, or hydraulic bolt tensioners.

ASME PCC-1 provides guidance for assembling bolted flange joints, including tightening methods, joint preparation, bolt-load control, and assembler qualification.

Final Selection Checklist

Before releasing flange bolting for purchase or installation, verify:

Correct flange standard
Correct NPS and pressure class
Correct number of studs
Correct stud diameter
Correct stud length
Correct thread series
Correct stud material and grade
Compatible nut material and grade
Appropriate coating and lubricant
Gasket and facing compatibility
Adequate thread engagement
Approved tightening procedure
Compliance with the piping material specification

Conclusion

An ANSI flange bolt size chart provides the number, diameter, and standard length of the studs required for a flanged piping connection. These dimensions vary with nominal pipe size, pressure class, flange facing, and joint configuration.

ASME B16.5 governs standard flanges from NPS ½ through NPS 24, while ASME B16.47 covers large-diameter flanges from NPS 26 through NPS 60. Although the term “ANSI flange” remains common, modern flange dimensions are generally specified using these ASME standards.

The bolt chart should be used to establish the basic fastener dimensions, but it does not complete the entire bolting selection process. Engineers and installers must also verify stud length, material grade, nut compatibility, coating, lubricant, service temperature, corrosion conditions, and tightening procedure.

For critical or high-pressure joints, always confirm the bolting arrangement against the latest applicable standard, approved piping material specification, equipment drawings, and project bolted-joint procedure before purchasing or installing the fasteners.

How Does a Hydraulic Accumulator Work?

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