ASME B18 fastener standards are often searched together, but they do not describe the same product. A buyer looking for an ASME B18.2.1 hex bolt, an ASME B18.2.2 hex nut, or an ASME B18.6.3 tapping screw must begin with the product function—not with an assumed interchangeability. This guide explains the scopes, the links between inch and metric systems, and the checks that make an RFQ easier to quote correctly.
Quick answer: which ASME B18 standard do I need?
| Standard | Primary product scope | Typical use | System |
|---|---|---|---|
| ASME B18.2.1 also searched as ANSI B18.2.1 |
Square, hex and heavy-hex bolts and screws | Externally threaded fasteners assembled with a matching nut | Inch series |
| ASME B18.2.2 also searched as ANSI B18.2.2 |
Machine-screw, hex, square, hex-flange and coupling nuts | Internally threaded mating components for general applications | Inch series |
| ASME B18.6.3 | Machine screws, tapping screws and metallic-drive screws | Thread-forming or sheet-metal fastening; selected machine-screw forms | Inch series |
| ISO 4032 also searched as ISO4032 |
Hexagon regular nuts, style 1 | Metric coarse-thread hex nuts in steel or stainless steel | Metric |
Key point: B18.2.1 and B18.2.2 are commonly specified as a mating bolt-and-nut family, but the correct thread series, diameter, material and property requirements still have to be stated. ASME B18.6.3 is not a substitute for either standard; it addresses a different group of screws.
ASME B18.2.1: bolts and screws with external threads
ASME B18.2.1 covers dimensional requirements for common inch-series square-head, hex-head and heavy-hex head bolts and screws. In purchasing language, it is the starting point when the drawing calls for an external-thread fastener with a defined head style, nominal diameter, thread and length.
It is useful to separate geometry from performance. A B18.2.1 callout helps establish the product form and dimensions; it does not by itself choose the material grade, coating, corrosion resistance, mechanical property class or application-specific certification. Those items belong in the complete specification and purchase order.
- Confirm nominal size and thread designation, such as UNC or UNF.
- State head style and required length.
- Specify material, grade and finish separately where required.
- For critical joints, define inspection, traceability and document requirements.
ASME B18.2.2: the matching nut standard
ASME B18.2.2 covers general and dimensional data for inch-series square and hex nuts, including machine-screw nuts, hex-flange nuts and coupling nuts. It is therefore the appropriate reference when the RFQ concerns the internally threaded partner—not the bolt.
A common RFQ error is to write only “ANSI B18.2.2 nut” without defining the mating thread or material. That leaves several practical questions unanswered: hex or heavy-hex form, thread size and pitch, finish, strength level, and whether the application needs plain, flange or coupling nuts. The best RFQs show the mating bolt callout and the environment together.
ASME B18.6.3: machine, tapping and metallic-drive screws
ASME B18.6.3 is the relevant inch-series standard for machine screws, tapping screws and metallic-drive screws. It should not be treated as a “hex nut” standard simply because it sits within the wider B18 fastener family. For enclosure panels, light-gauge metal assemblies, appliance components and other thread-forming applications, its scope is often the more relevant starting point.
Selection depends on what the screw will engage. A machine screw normally works with a preformed internal thread or a nut; a tapping screw forms or cuts its mating thread in an appropriate substrate. Before requesting a quote, define the substrate, pilot-hole condition, head and drive style, diameter, point style, plating and test needs. This prevents a visually similar screw being supplied with the wrong functional point or thread.
ISO 4032 vs ASME B18.2.2: similar purpose, not automatic equivalents
ISO 4032 specifies metric hexagon regular nuts (style 1). The current ISO 4032:2023 edition describes steel and stainless-steel coarse-pitch nuts from M5 to M39 in product grades A and B. ASME B18.2.2 is an inch-series standard. Both may appear in general-purpose fastener sourcing, but their thread systems, dimensions and designation methods are different.
| Question | ISO 4032 | ASME B18.2.2 |
|---|---|---|
| Thread family | Metric coarse thread | Unified inch thread designations as specified |
| Product form | Regular style-1 hex nut | Several inch-series nut forms, including hex and coupling nuts |
| Best RFQ approach | State M size, pitch when needed, class/material and finish | State inch thread, nut form, material/grade and finish |
| Can one replace the other? | No—verify engineered interchangeability; do not assume it. | |
Related searches: ASME B18.6.9 and ASME B18.24
Two related terms are regularly mixed into broad “ASME B18” searches. ASME B18.6.9 covers inch-series wing nuts, not standard hex nuts. ASME B18.24 is a part-identifying-number (PIN) code system for B18 fastener products; it supports identification and traceability but does not replace the underlying product standard. Include these standards only when the required product or identification method actually matches their scope.
How to write a quote-ready fastener specification
- Identify the component: bolt, hex nut, machine screw, tapping screw or wing nut.
- Name the governing standard: for example, ASME B18.2.1, ASME B18.2.2, ASME B18.6.3 or ISO 4032.
- Define the thread: nominal diameter, series/pitch and right- or left-hand direction where applicable.
- Define material and protection: carbon steel, stainless steel or another alloy; plus coating, passivation or plating.
- Define functional requirements: strength/property class, head and drive, bearing surface, locking feature, tolerances and application environment.
- Define quality requirements: quantity, inspection level, marking, material certificates and packaging.
Practical selection example
For an inch-series assembly using an external hex bolt and a separately supplied hex nut, start with B18.2.1 for the bolt form and B18.2.2 for the nut form, then add the unified thread designation, material grade and coating. For a sheet-metal cover that needs to form its own thread, start by reviewing B18.6.3 and define the material thickness and required point/drive configuration. For a metric machine using M-series hardware, ISO 4032 may be the appropriate nut reference—but only alongside the specified mating metric bolt and property requirements.
Frequently asked questions
Is ANSI B18.2.1 the same as ASME B18.2.1?
Many drawings and purchase documents retain the legacy “ANSI” wording. Confirm the exact revision and customer requirement, then reference the current applicable ASME edition where appropriate.
Can an ASME B18.2.2 nut be used with an ISO 4032 bolt?
Do not assume compatibility. Inch and metric thread systems must be matched by a qualified engineering specification.
Does ASME B18.6.3 cover hex bolts?
No. It covers machine screws, tapping screws and metallic-drive screws; standard hex bolts are addressed under the B18.2.1 family.
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