Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ASME B18.8.1-2014 — Clevis Pins and Cotter Pins (Inch Series)
📌 ASME B18.8.1-2014 is the inch-series standard for pins within the ASME B18 fastener system, maintained by Subcommittee 8 (Machine Pins) and a revision of the 1994 edition (reaffirmed 2010). The Standard covers clevis pins in sizes 3/16 in. through 2 in. and cotter (split) pins in sizes 1/32 in. through 3/4 in., of various materials. Clevis pins provide the pivot in clevis-type joints; cotter pins pass through a hole in the pin end and are bent over to lock it in place. The two are normally used together to form a quick-dismantling hinge connection.
💡 The clevis pin + cotter pin combination is the classic “quick-lock” pivot used in agricultural machinery, towing hitches, hydraulic cylinder trunnions, winches, and countless linkage mechanisms: assembly and disassembly need only a pair of pliers — no special tools, low cost, high reliability.
1. Scope and General Framework 📌
Key provisions of the Standard include:
- Size range: clevis pins 3/16 through 2 in.; cotter pins 1/32 through 3/4 in.
- Units: all dimensions in inches, applying before plating or coating; tolerancing per ASME Y14.5.
- Responsibility: the responsible party for product performance is the organization that supplies the components and certifies they were manufactured, tested, and inspected per this specification.
- Inspection and quality assurance: unless otherwise specified by the purchaser, acceptability is based on conformance with ASME B18.18 (Quality Assurance for Fasteners).
- Referenced standards: terminology per ASME B18.12; coating and material references include ASTM A153/A153M (hot-dip zinc), A380 (stainless cleaning/passivation), and A493 (stainless wire and rod for cold heading and cold forging).
The Standard also defines the cotter pin length gage (Figure 1) and its use, plus tables of preferred sizes and lengths for cotter pins.
⚠️ A cotter pin carries only a “locking” load — it is not a load-bearing pin. The shear capacity of a clevis pin is calculated from pin diameter and material. Common engineering mistakes include small pins in oversized holes and re-bending the same cotter pin, both of which lead to locking failure.
2. Main Changes in the 2014 Revision 🔧
In 2013, a major U.S. cotter pin manufacturer brought several issues to the B18.8 Subcommittee for modification. None of the changes affect the pins’ function or quality:
| Subject | Change | Purpose |
|---|---|---|
| Clevis pins | Added sizes 9/16, 1-1/8, 1-1/4, 1-3/8, 1-1/2, 1-5/8, 1-3/4, 1-7/8, and 2 in.; added Table 2 defining effective length G from shank length M | Larger sizes and clearer length definition |
| Cotter pin total shank diameter | Opened the tolerance on total shank diameter A in Table 1 | Easier production; consistent with other feature tolerances |
| Gage hole diameter tolerance | Increased tolerance for holes of 3/16-in. nominal and up | Consistent ratio of gage hole to nominal diameter for smaller sizes |
| Gap requirements | Para. 3.4.2 gap wording revised to 3 ranges; gap increased for 3/8-in. and larger diameters | Better match to actual assembly |
| Ductility bend | Para. 3.5.3 clarifies radius: bend back 180 deg around a gage pin whose diameter is specified relative to nominal pin size | Removes ambiguity of “bent back upon itself once” |
Cotter pin types include the extended prong type and the hammer lock type; preferred sizes and lengths for each are given in the tables.
3. Engineering Practice Essentials 🛠️
- 🔍 Selection workflow: determine clevis pin diameter and material from pivot loading → choose effective length G from clamping width → select cotter pin size from the pin end hole → choose type by installation access (extended prong for general use, hammer lock for severe vibration).
- 🛡️ Anti-loosening design: after insertion, both prongs must be correctly spread and bent around the pin (180 deg) — never bend only one side or trim without bending; inspect periodically in high-temperature or severe-vibration service.
- 📋 Material and coating: carbon steel cotter pins are usually zinc or hot-dip galvanized (ASTM A153) for corrosion resistance; stainless pins are made from A493 cold-heading wire and cleaned/passivated per A380.
- 📊 Inspection: sample acceptance per B18.18, checking pin diameter tolerance, prong spread, and ductility (bend-back without fracture).
4. Frequently Asked Questions ❓
❓ Q: How is the effective length G of a clevis pin determined?
A: Effective length G is derived from shank length M and end structure per Table 2, ensuring the installed pin protrudes sufficiently on both sides of the clevis for cotter pin installation without excessive overhang beyond the clevis.
A: Effective length G is derived from shank length M and end structure per Table 2, ensuring the installed pin protrudes sufficiently on both sides of the clevis for cotter pin installation without excessive overhang beyond the clevis.
❓ Q: Can a cotter pin be reused?
A: Cotter pins are designed for single use. Re-bending fatigues the prongs or loses elastic locking force. The ductility requirement (bending back 180 deg around a specified gage pin without fracture) is a manufacturing acceptance criterion, not a license for reuse.
A: Cotter pins are designed for single use. Re-bending fatigues the prongs or loses elastic locking force. The ductility requirement (bending back 180 deg around a specified gage pin without fracture) is a manufacturing acceptance criterion, not a license for reuse.
❓ Q: Why did the 2014 revision open the cotter pin shank diameter tolerance?
A: The former tolerance was unnecessarily tight, making production difficult and inconsistent with other feature tolerances. The opened tolerance eases manufacture while meeting functional requirements; the gage-hole and gap changes align with assembly practice and gage ratios — none affect function or quality.
A: The former tolerance was unnecessarily tight, making production difficult and inconsistent with other feature tolerances. The opened tolerance eases manufacture while meeting functional requirements; the gage-hole and gap changes align with assembly practice and gage ratios — none affect function or quality.
🔍 Q: How does B18.8.1 relate to the metric cotter pin standard B18.8.200M?
A: B18.8.1 is the inch series while B18.8.200M is the metric series (cotter pins, headless and headed clevis pins). Both belong to the B18.8 machine pin system; choose the standard by the units used in the project and never mix metric and inch sizes.
A: B18.8.1 is the inch series while B18.8.200M is the metric series (cotter pins, headless and headed clevis pins). Both belong to the B18.8 machine pin system; choose the standard by the units used in the project and never mix metric and inch sizes.
