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ASME B18.8.6m-1994 — Cotter Pins, Metric Series
📎 ASME B18.8.6m-1994 is the American National Standard dedicated to cotter pins (split pins) in the metric series. It was developed by Subcommittee 8 — Machine Pins of the ASME B18 committee on standardization of bolts, nuts, rivets, screws, washers, and similar fasteners, approved by the ASME Board on Standardization on August 23, 1993, recognized by the American National Standards Institute (ANSI) on April 5, 1994, and issued on June 3, 1994. The pin standards in this family trace back to ASA B5.20 (approved in 1947, revised in 1954 and 1958), published in part as ANSI B18.8.2-1978, and the present metric edition was created in response to growing user demand and the federal metrication program.
💡 The cotter pins of this Standard closely match ISO 1234-1976. Pins made to this Standard and pins made to ISO 1234 are interchangeable, which makes this an ANSI standard that plugs directly into metric-based global supply chains.
1. Scope and General Framework 📌
Section 1 of the Standard states that it covers the complete dimensional and general data for cotter (split) pins recognized as “American National Standard.” Several points deserve particular attention:
- Product definition: a cotter pin is a two-prong fastener formed from metal wire. Its prongs are spread and bent during installation to lock a slotted nut, retain a clevis pin, or secure other components — a removable, self-locking fastener.
- Stock availability: including dimensional data does not imply that all listed products are stock production items; purchasers should consult suppliers about availability.
- Government procurement: metric cotter pins purchased for government use must conform to this Standard and, in addition, to the requirements of Appendix I.
- Units and tolerancing: all dimensions are given in millimeters unless otherwise specified, and dimensioning and tolerancing follow ANSI Y14.5M.
- Responsible party: the organization that supplies the components and certifies or represents that they were manufactured, tested, and inspected in accordance with this specification is responsible for their performance.
⚠️ For government purchases, meeting the body of the Standard alone is not enough — Appendix I adds requirements (procurement packaging, marking, and related provisions) that must be explicitly invoked in the solicitation and order documents.
2. Comparison with ISO and Interchangeability 🌐
Paragraph 1.2 compares this Standard with ISO 1234-1976. The pins covered here are similar to ISO 1234-1976 pins, with two differences in how dimensions are stated:
- This Standard does not call out an approximate head height dimension, whereas ISO 1234 gives one; and
- This Standard calls out wire width dimensions, which are not given in ISO 1234.
Critically, the Standard states that these differences will not affect interchangeability of cotter pins manufactured to either document. Whether you procure to the metric U.S. national standard or to ISO 1234, the mating hole, locking function, and assembly dimensions are equivalent — safe to mix across an international supply chain.
| Aspect | ASME B18.8.6m-1994 | ISO 1234-1976 | Interchangeability Impact |
|---|---|---|---|
| Head height | Not specified (not required) | Approximate value given | None |
| Wire width | Specified | Not given | None |
| Nominal size series | Metric (mm) series | Metric (mm) series | Series consistent |
| Tolerancing system | Per ANSI Y14.5M | ISO tolerance system | Functionally equivalent |
3. Technical Requirements and Dimensions 🔧
3.1 Nominal Size Series
The Standard provides a complete metric size series from small to large diameters, whose values correspond to rounded conversions of the fractional-inch series (e.g., 1/32″, 1/16″, 1/8″, 1/4″, 3/8″). The table below lists representative sizes with typical reference dimensions; exact tolerances and the complete data are given in the tables of the Standard.
| Nominal Diameter d (mm) | Approx. Max Shank Dia. (mm) | Approx. Prong Spread (mm) | Typical Length Range (mm) | Typical Applications |
|---|---|---|---|---|
| 1.6 | 1.7 | 3.2 | 10–40 | Instruments, bicycle parts |
| 2.0 | 2.1 | 4.0 | 12–50 | Small shaft retention |
| 2.5 | 2.6 | 5.0 | 16–60 | General machinery, dies |
| 3.2 | 3.3 | 6.4 | 20–80 | Farm machinery, automotive chassis |
| 4.0 | 4.2 | 8.0 | 25–100 | Automotive, agricultural equipment |
| 5.0 | 5.2 | 10.0 | 32–125 | Trucks, trailers |
| 6.3 | 6.6 | 12.5 | 40–150 | Heavy machinery |
| 8.0 | 8.4 | 16.0 | 50–200 | Construction equipment |
| 10.0 | 10.5 | 20.0 | 63–250 | Large equipment, structural members |
3.2 Materials and Finishes
| Material Category | Typical Grades | Typical Uses | Common Finishes |
|---|---|---|---|
| Low-carbon steel | 1010–1020 types | General locking and retention | Plain, black oxide, zinc plating |
| Stainless steel | 302, 304, 316 types | Corrosive, outdoor, food service | Plain, passivated, polished |
| Copper alloys | Brass (H62/H59 types) | Electrical, non-magnetic, decorative | Plain, nickel plating |
| Aluminum alloy | 5056 type | Lightweight assemblies | Plain, anodized |
✅ Cotter pins are cold-formed wire products and normally require no heat treatment. For corrosion resistance prefer stainless steel; for cost-sensitive service use zinc-plated carbon steel, and make sure the plating thickness does not prevent the pin from entering the mating hole.
4. Inspection, Quality Assurance, and Terminology 📋
The Standard makes clear arrangements for quality and terminology:
- Acceptance: unless otherwise specified by the purchaser, acceptability is based on conformance with ANSI/ASME B18.18.1M (Inspection and Quality Assurance for Fasteners).
- Terminology: definitions of terms relating to pins and their features refer to the American National Standard ANSI B18.12, Glossary of Terms for Mechanical Fasteners.
- Referenced standards: ASTM material standards (wire grades, test methods) and other related standards are referenced; material properties must satisfy the applicable referenced documents.
- Traceability: the responsible party must be able to provide documents certifying that the product was manufactured, tested, and inspected in accordance with this specification.
5. Engineering Practice Essentials 🛠️
- 🔍 Designation: specify cotter pins as nominal diameter × length in mm, e.g., 3.2 × 40. The length is the overall length before bending; choose a length that leaves enough prong beyond the part to be bent over snugly.
- 🔧 Mating holes: install into pre-drilled holes (slotted-nut castellations or clevis-pin holes) sized from the standard series; forcing an oversized pin damages the hole mouth and weakens retention.
- 📐 Bending direction: bend both prongs around the circumference and press them flat against the locked part, preferably opposite to the likely direction of rotation, and keep the tails clear of adjacent moving components.
- ♻️ One-time use: bending plastically deforms the pin, and a reused pin loses locking ability through fatigue and shape recovery — always fit new pins after every disassembly and inspection.
- 🛡️ Corrosion protection: in humid or coastal service choose zinc- or cadmium-plated carbon steel; a light coat of rust-preventive oil is acceptable as long as it does not affect locking.
6. Frequently Asked Questions ❓
❓ Q: Are “cotter pin” and “split pin” the same part?
A: Yes. “Cotter pin” and “split pin” are two names for the same two-prong, wire-formed fastener whose tails are spread and bent during installation. Authoritative definitions appear in ANSI B18.12.
A: Yes. “Cotter pin” and “split pin” are two names for the same two-prong, wire-formed fastener whose tails are spread and bent during installation. Authoritative definitions appear in ANSI B18.12.
❓ Q: Can ASME B18.8.6m-1994 pins be interchanged with ISO 1234 pins?
A: Yes. Paragraph 1.2 states that the two standards differ only in how head height and wire width dimensions are stated, and these differences do not affect interchangeability, so global procurement may follow either document.
A: Yes. Paragraph 1.2 states that the two standards differ only in how head height and wire width dimensions are stated, and these differences do not affect interchangeability, so global procurement may follow either document.
❓ Q: How should a cotter pin size be chosen and specified?
A: Specify nominal diameter × length in mm, e.g., 5 × 63. Select the diameter from the standard series to match the hole, and choose a length that leaves enough prong beyond the part to be bent over — typically at least 2 to 3 times the wire diameter.
A: Specify nominal diameter × length in mm, e.g., 5 × 63. Select the diameter from the standard series to match the hole, and choose a length that leaves enough prong beyond the part to be bent over — typically at least 2 to 3 times the wire diameter.
❓ Q: What special requirements apply to government procurement?
A: Per para. 1.1.3, metric cotter pins for government use must conform to this Standard and additionally to Appendix I (packaging, marking, and related procurement requirements). Invoke Appendix I explicitly in procurement documents.
A: Per para. 1.1.3, metric cotter pins for government use must conform to this Standard and additionally to Appendix I (packaging, marking, and related procurement requirements). Invoke Appendix I explicitly in procurement documents.
