Cotter / Split Pins — Engineering Reference | RR Hydraulic

Engineering Reference Document

Cotter / Split Pins

A world-class technical reference for EPC contractors, mechanical engineers, procurement heads, TPI inspection agencies, and global project buyers specifying cotter pins and split pins in castellated nut locking, clevis pin retention, shaft pin retention, valve linkage assemblies, and safety-critical anti-rotation applications across Oil & Gas, Power Generation, Petrochemical, Rotating Equipment, Offshore, and Heavy Machinery sectors worldwide.

ISO 1234 DIN 94 ASME B18.8.1 (inch) BS 1574 ISO 3506 A2 / A4 ASTM A153 (HDG) NACE MR0175 EN 10204 3.1 / 3.2 ISO 9001:2015

Part 01 / Technical Definition

Industry Context,
Type Classification
& Locking Mechanics

Cotter pins (split pins / split cotter pins) are semi-circular cross-section wire fasteners formed into a looped head and two diverging legs that are inserted through a transverse hole in a bolt, clevis pin, axle shaft, or castellated nut assembly, then bent to secure the retained component. They provide a positive mechanical lock that is entirely independent of friction, preload, or thread engagement — making them the definitive safety-critical anti-loosening device in mechanical and piping systems.

Cotter / Split Pins — RR Hydraulic Engineering Reference

1.1 — Technical Definition and Functional Role

A cotter pin (also termed split pin in British / ISO terminology, or cotter key in some US contexts) consists of a semi-circular wire folded back on itself at the head end to form a closed loop, with two parallel legs of equal or unequal length extending from the loop. The pin is sized by its nominal diameter — which is the diameter of the wire from which the semi-circular cross-section is formed — and its overall length (from head to toe of the shorter leg). The pin is inserted through a transverse drilled hole in a pin, shaft, bolt shank, or castellated nut slot, and the two legs are bent outward (or one leg up and one down) against the mating component to prevent withdrawal and provide positive mechanical retention.

In EPC and industrial mechanical systems, cotter pins appear in: castellated (slotted) hex nut locking on critical bolted joints (pressure vessel flanges, pipe supports, rotating equipment), clevis pin retention in hydraulic and mechanical linkages, valve handwheel shaft retention, axle pin retention on wheel and caster assemblies, pump impeller shaft pin locking, turbine and compressor control linkage retention, chain and conveyor pin retention, lifting equipment pin locking (shackles, hooks, swivels), and any application where the consequence of nut or pin backing-off would be safety-critical — including all pressure-retaining and structural applications where vibration loosening of conventional nuts must be absolutely prevented.

RR Hydraulic supplies cotter and split pins under all applicable international standards with full EN 10204 3.1 / 3.2 material traceability.

1.2 — Cotter / Split Pin Type Classification

Standard Cotter Pin (ISO 1234 / DIN 94)

The universal type: semi-circular cross-section wire, closed loop head, two equal-length legs. Per ISO 1234 and DIN 94. Installed through the transverse hole in the retained component; one leg bent over the top of the bolt/pin head, the other bent under the hex nut or back along the shank. The bent legs prevent the pin from backing out under vibration. Single-use only — the wire is work-hardened and embrittled by bending; never re-use a bent cotter pin. Available in carbon steel, SS, and brass.

Extended Prong (Safety Pin) Type

One leg significantly longer than the other — the long prong extends beyond the mating component surface, providing a visual indicator that the pin is correctly installed. The long leg is bent over while the short leg remains approximately straight. Per DIN 94 extended prong variant and ASME B18.8.1. Used in applications where visual confirmation of correct pin installation is required — lifting equipment, valve actuator linkages, and safety-critical pressure-retaining assemblies.

Hammer-Drive (Hammerlock) Split Pin

A variant with legs pre-spread slightly before installation — both legs are spread outward and locked by the insertion action rather than requiring post-installation bending. Used in high-volume automated assembly where individual manual leg-bending is impractical. Hammer-drive pins are inserted with a single tool stroke. Less common in precision EPC mechanical assembly than standard bend-leg types; primarily used in automotive and light machinery applications.

R-Clip (Hitch Pin Clip / Hair Pin Cotter)

Formed from a single wire into an R or hairpin shape — inserted through a transverse hole, then the spring tension of the wire retains it. Easily inserted and removed by hand without tools. Per DIN 11024 / ISO 11024. Used for quick-release, frequently-adjusted, or field-removable pin retention: equipment adjustment linkages, gate and hatch retention, valve stem position pins, and temporary assembly pins on modular equipment. Not for permanent or safety-critical retention where vibration loosening would be catastrophic.

Snap-On (Bridge / Bow) Cotter Pin

Formed with a bowed spring body that snaps over a shaft groove or pin groove — retained by spring tension in the groove rather than insertion through a hole. Per DIN 11023. Used for quick assembly and disassembly of equipment guards, panel retention, and non-structural pinned connections. Easily removed and replaced without tools or damage to the retained component. Not suitable for high-vibration or high-load structural pin retention.

Taper Cotter Pin (Taper Key)

A tapered flat or square cross-section pin driven through a transverse slot in a shaft-hub assembly to create a keyed connection. The taper creates a wedging self-locking action. Distinguished from the split (bend-leg) cotter pin by its solid cross-section and tapered form. Per DIN 258 (metric taper). Used in shaft-to-hub keying, connecting rod-to-piston connections in reciprocating machinery, and rod-to-yoke connections in valve actuators and mechanical linkages.

1.3 — Cotter Pin vs Alternative Locking Methods

Table 1.A — Cotter Pin vs Alternative Anti-Loosening Methods: Engineering Comparison

Method	Locking Mechanism	Vibration Resistance	Removability	Reusability	Visual Inspection	Primary EPC Application
Cotter / Split Pin	Positive mechanical — bent legs prevent axial pull-out	Excellent	Destructive (cut and discard)	Single use only	Excellent — visible	Castellated nuts; clevis pins; safety-critical
Castle / Slotted Nut	Paired with cotter pin — slots align with bolt hole	Excellent (with cotter)	Good — undo nut	Nut reusable; new cotter each time	Excellent	Pressure vessels; piping; rotating equipment
Nyloc Prevailing-Torque Nut	Nylon insert friction on thread	Very Good	Good (limited re-use)	5–10 uses	Not directly visible	General EPC; enclosures; moderate vibration
Thread Lock Adhesive	Chemical bonding of thread flanks	Excellent (270) / Good (243)	Moderate (heat to remove)	No (remove old adhesive first)	Not visible	Hydraulic; precision machinery; OEM
Nord-Lock Wedge Washer	Cam + wedge friction	Excellent (Junker tested)	Excellent	Multiple uses	Not directly visible	Critical rotating equipment; structural
Safety Wire	Wire twist locks bolt head to adjacent anchor	Excellent	Cut and discard	Single use	Excellent — visible	Aerospace; high-temp flanges; turbomachinery
Tab / Lock Washer	Tab bent against bolt flat	Excellent (mechanical)	Bend tab back (1 use only)	Single use	Good — visible	Shaft lock nuts; bearing retention
Double Nut (Jam Nut)	Nut-to-nut friction	Good	Excellent	Excellent	Visible	Rod ends; hanger rods; turnbuckles

1.4 — Functional Load Mechanics

Shear Load at Pin Cross-Section

The cotter pin functions primarily in shear — if the retained nut or pin attempts to move axially, the bent legs bear against the mating surface and the pin body is loaded in shear at the cross-section through the transverse hole. The shear capacity of a cotter pin is low relative to the bolt or clevis pin it retains — cotter pins are designed to prevent accidental backing-off under vibration, not to carry structural service loads. The pin size must match the transverse hole diameter in the bolt or castellated nut slot to function correctly.

Castellated Nut Slot Alignment

For castellated (slotted) nut applications, the bolt or stud must have a pre-drilled transverse hole positioned such that at least one nut slot aligns with the hole within the nut’s tightening torque range. If no slot aligns at the specified torque, the nut must be turned past the torque specification to achieve alignment — this is NOT acceptable in structural or pressure-retaining joints. The bolt/nut assembly must be designed with the hole position calculated from the torque-to-turn relationship to ensure alignment within the torque tolerance window.

Clevis Pin Retention

In clevis and yoke assemblies (hydraulic cylinder rod-end connections, valve actuator linkages, pump mechanical seals), a cotter pin through a transverse hole near the end of the clevis pin prevents the clevis pin from walking out of the clevis bore under cyclic transverse loading. The clevis pin OD and the cotter pin hole must be correctly matched — an oversized transverse hole relative to the cotter pin diameter allows lateral movement that will progressively wear through the cotter pin under dynamic loading.

Single-Use and Replacement Protocol

Cotter pins must be replaced every time the assembly is dismantled. The act of bending the legs during installation work-hardens the wire at the bend radius; straightening the legs for removal and re-bending for re-installation breaks the work-hardened material at the first bend radius. A cotter pin that has been removed and re-used may appear intact but will fracture under its first significant vibration load, providing no anti-loosening function. Cotter pin replacement is a mandatory step in every maintenance procedure — not optional to “save time”.

Bending Direction — Standard Practice

After insertion, the standard bending method per ISO 1234 and general mechanical engineering practice: one leg bent at 90° over the top of the bolt/pin head (or nut slot); the opposite leg bent at 90° downward along the bolt shank or under the nut. This configuration resists withdrawal in both directions of axial movement — essential for castellated nut applications where both tightening and loosening directions of nut rotation must be resisted. Bending both legs in the same direction (both up or both to the same side) provides single-direction resistance only.

Vibration Environment and Pin Sizing

Undersized cotter pins in high-vibration environments (compressor body bolting, reciprocating pump piping connections, rotating equipment covers) can fail by fatigue fracture at the transverse hole contact point — the leg is cyclically bent by the vibrating retained component. For high-vibration applications: use a cotter pin at the maximum diameter that will pass through the transverse hole; verify hole diameter is within the ISO 1234 tolerance for the nominal pin size; inspect cotter pins at every maintenance interval and replace regardless of visible condition.

1.5 — Shear Capacity Reference

Cotter Pin Shear Capacity — Single Leg at Transverse Hole

F_shear = τ_allow × A_pin = 0.6 × S_y × (π × d² / 8)

F_shear = Shear capacity per leg at the transverse hole (N)
τ_allow = Allowable shear stress = 0.6 × S_y (von Mises)
S_y = Yield strength of pin wire material (MPa)
d = Nominal pin diameter (mm) — semi-circular section area = π × d² / 8

Note: Cotter pins have semi-circular cross-sections (not full circular) — area = πd²/8
Two-leg cotter pin total: F_total = 2 × F_shear (both legs at the hole)

Design principle: Cotter pin shear capacity must exceed the axial retention load (vibration-induced nut back-off force or clevis pin withdrawal force) with a safety factor ≥ 2.0. The cotter pin is a retention device, not a structural load-carrying member.

Worked Example — Ø4 mm ISO 1234 CS cotter pin (S_y ≈ 200 MPa), castellated nut:
A_per_leg = π × 4² / 8 = 6.28 mm²
F_shear per leg = 0.6 × 200 × 6.28 = 754 N per leg
Total (2 legs) = 1,508 N = 1.51 kN axial retention capacity
This is sufficient to resist castellated nut back-off under vibration for a standard M20 bolt in normal process piping service; for compressor or high-vibration applications, use Ø6.3 mm pin minimum or supplement with additional anti-vibration measures.

Specifying cotter / split pins for an EPC, mechanical, or rotating equipment project?
Submit your pin diameter, length, material, quantity, and application for a documented RFQ within 24 hours.

Part 02 / Standards & Dimensional Design

Dimensional Reference,
Hole Sizes &
Standards Compliance

Cotter pin dimensions — nominal diameter, leg length, loop head geometry, and the transverse hole size range — are governed by ISO 1234 (metric), DIN 94 (metric), ASME B18.8.1 (inch), and BS 1574 (inch). Correct nominal pin diameter selection relative to the transverse hole diameter is the primary engineering decision. All applicable standards are supported with full certification at RR Hydraulic.

Cotter Pin Dimensional Reference — RR Hydraulic

Formal R.F.Q. — Cotter / Split Pins for EPC / Mechanical / Rotating Equipment
Submit pin diameter, length, material, standard, and quantity to sales@rrhydraulics.com for a fully certified offer.

2.1 — ISO 1234 / DIN 94 Split Pin Full Dimensional Table

Table 2.A — ISO 1234 / DIN 94 Cotter (Split) Pin: Full Dimensional Reference

Nominal Dia d (mm)	Wire Dia (mm)	Min Hole Dia (mm)	Max Hole Dia (mm)	Head Width b (mm)	Standard Lengths L (mm)	Leg Length After Bend (mm)	Cross-Section Area (mm²)
0.6	0.6	0.7	0.9	1.0	6, 8, 10	2.0	0.14
0.8	0.8	0.9	1.1	1.4	8, 10, 12	2.5	0.25
1.0	1.0	1.1	1.3	1.6	10, 12, 16, 20	3.0	0.39
1.2	1.2	1.3	1.5	2.0	12, 16, 20, 25	3.5	0.57
1.6	1.6	1.7	2.0	2.8	14, 16, 20, 25, 32	4.5	1.01
2.0	2.0	2.1	2.5	3.2	16, 20, 25, 32, 40	5.5	1.57
2.5	2.5	2.6	3.1	4.0	20, 25, 32, 40, 50	7.0	2.45
3.2	3.2	3.3	3.9	5.0	25, 32, 40, 50, 63	9.0	4.02
4.0	4.0	4.1	4.9	6.3	32, 40, 50, 63, 80	11.0	6.28
5.0	5.0	5.1	6.2	8.0	40, 50, 63, 80, 100	14.0	9.82
6.3	6.3	6.4	7.8	10.0	50, 63, 80, 100, 125	18.0	15.60
8.0	8.0	8.1	9.8	12.6	63, 80, 100, 125, 160	22.0	25.13
10.0	10.0	10.2	12.4	16.0	80, 100, 125, 160, 200	28.0	39.27
13.0	13.0	13.2	16.0	20.0	100, 125, 160, 200, 250	36.0	66.37

2.2 — ASME B18.8.1 Inch Series Cotter Pin Dimensional Table

Table 2.B — ASME B18.8.1 Inch Cotter Pin: Dimensional Reference

Size	Wire Dia (in)	Min Hole (in)	Max Hole (in)	Head Width (in)	Standard Lengths (in)	Approx Metric Dia
1/32″	0.031	0.032	0.048	0.06	½, ¾, 1	≈ 0.8 mm
3/64″	0.047	0.048	0.060	0.09	¾, 1, 1¼	≈ 1.2 mm
1/16″	0.062	0.064	0.078	0.12	¾, 1, 1¼, 1½	≈ 1.6 mm
5/64″	0.078	0.079	0.094	0.16	1, 1¼, 1½, 2	≈ 2.0 mm
3/32″	0.094	0.095	0.109	0.19	1, 1¼, 1½, 2, 2½	≈ 2.5 mm
1/8″	0.125	0.126	0.141	0.25	1½, 2, 2½, 3	≈ 3.2 mm
5/32″	0.156	0.158	0.172	0.31	2, 2½, 3, 3½	≈ 4.0 mm
3/16″	0.188	0.190	0.203	0.38	2, 2½, 3, 4, 5	≈ 4.8 mm
¼”	0.250	0.252	0.266	0.50	3, 4, 5, 6	≈ 6.3 mm
5/16″	0.312	0.314	0.330	0.62	4, 5, 6, 7	≈ 8.0 mm

2.3 — Castellated Nut Cotter Pin Hole Reference

Table 2.C — Castellated Nut + Cotter Pin Selection: Bolt Size vs Pin Diameter vs Drill Size

Bolt / Stud Size	Castellated Nut Slot Width (mm)	Transverse Hole in Bolt (mm)	ISO 1234 Pin Dia (mm)	ASME B18.8.1 Pin Size	Pin Length Required (mm)	Standard Reference
M6	2.0	1.6	1.6	1/16″	25–32	ISO 7035 / DIN 935
M8	2.5	2.0	2.0	5/64″	25–32	ISO 7035 / DIN 935
M10	3.0	2.5	2.5	3/32″	32–40	ISO 7035 / DIN 935
M12	3.5	3.2	3.2	1/8″	40–50	ISO 7035 / DIN 935
M16	4.5	4.0	4.0	5/32″	50–63	ISO 7035 / DIN 935
M20	5.5	5.0	5.0	3/16″	63–80	ISO 7035 / DIN 935
M24	6.5	6.3	6.3	¼”	80–100	ISO 7035 / DIN 935
M30	8.0	8.0	8.0	5/16″	100–125	ISO 7035 / DIN 935
M36	10.0	10.0	10.0	⅜”	125–160	ISO 7035 / DIN 935
M42	12.0	12.5	13.0	½”	160–200	ISO 7035 / DIN 935

2.4 — Applicable Standards and Compliance Framework

ISO 1234

Split pins (cotter pins) — metric series. The primary international standard for metric split pins defining nominal diameter, wire diameter, head width, leg length, transverse hole size range, dimensional tolerances, and material requirements. Available in steel, austenitic SS, and other materials per material designation on the order. The reference standard for all European and international EPC project cotter pin specification from Ø0.6 mm to Ø20 mm.

DIN 94

Split pins — metric. The legacy German standard aligned with ISO 1234. Still widely specified in German EPC equipment, machinery, and plant documentation. DIN 94 uses identical “d×L” designation format (e.g., 3.2×40 = Ø3.2 mm, 40 mm length). Dimensionally compatible with ISO 1234 for all standard sizes. DIN 94 stainless steel version remains specified on German process plant projects requiring corrosion-resistant pin retention in aggressive atmospheres.

ASME B18.8.1

Cotter pins — inch series. The governing US standard for inch-series cotter pins used in North American EPC projects, ASME-coded pressure equipment, and US-specification mechanical assemblies. Sizes from 1/32″ through ½” diameter; standard lengths from ½” to 6″. Material: low-carbon steel standard; SS per material designation. Used with inch castellated hex nuts per ASME B18.2.2 and clevis pins per ASME B18.8.1.

BS 1574

Split cotter pins — inch series, British standard. Used on legacy UK process plant and equipment documentation. BS 1574 sizes parallel ASME B18.8.1 but with slight dimensional differences in some sizes. Still encountered in maintenance and replacement procurement for pre-1990s UK-designed process plant throughout Asia, Africa, and the Middle East EPC markets. RR Hydraulic supplies BS 1574-compliant pins on request for legacy plant maintenance projects.

ISO 7035 / DIN 935

Prevailing torque type all-metal hexagon castle nuts (ISO 7035) and slotted hexagon nuts (DIN 935) — the companion nut standards for cotter pin applications. ISO 7035 / DIN 935 define the slot dimensions in the nut crown, the number of slots (6 or 8 per nut), and the slot width — which must match the cotter pin diameter range per Table 2.C. Always specify nut and cotter pin together to ensure slot-to-hole alignment compatibility.

DIN 11024 / ISO 11024

R-clips (hitch pin clips / hair pin cotters) — the quick-release spring-retention variant. Per DIN 11024 (metric) and ISO 11024. R-clips are inserted through a transverse hole and retained by spring tension rather than by bent legs — they are easily removed and replaced by hand without tools. Used for non-permanent, frequently-adjusted, or field-removable pin retention where a fully bent cotter pin would impede routine maintenance access.

ISO 3506

Mechanical properties of corrosion-resistant stainless steel fasteners. A2-70 (SS 304) and A4-70 (SS 316) wire material for SS cotter pins — the governing material standard for SS split pin supply in marine, offshore, chemical, and food-grade environments. A4-70 (SS 316) is the minimum grade for coastal and offshore cotter pin supply. Full passivation per ASTM A967 standard on all SS cotter pin supply. PMI on every SS lot to confirm SS 316 vs SS 304 material.

NACE MR0175 / ISO 15156

For cotter pins in H₂S sour service environments (O&G production, amine plants, sour crude handling): carbon steel cotter pins in these environments are susceptible to H₂S-induced stress corrosion cracking (HISC) and hydrogen embrittlement. Specify SS A4-70 (SS 316) cotter pins for all sour service applications. SS 316 is inherently NACE-compliant within standard ISO 3506 hardness limits and is resistant to H₂S-induced cracking in the ambient temperature range applicable to most EPC process plant applications.

Critical — Single-Use Rule: Cotter Pins Must Be Replaced Every Disassembly. A cotter pin that has been removed from service and straightened for re-use has been work-hardened at the bend radius. The wire at the original bend point is partially fractured and will break under the first significant vibration load with no warning. In pressure-retaining bolted joints (flanges, valve bonnets) and safety-critical clevis pin assemblies, a failed cotter pin provides zero retention — the consequence is a backed-off nut or ejected clevis pin under service conditions. Cotter pins are maintenance consumables — budget for replacement on every maintenance access and enforce replacement on every documented maintenance procedure. Never re-use, straighten, or re-bend a cotter pin removed from service.

Part 03 / Materials & Manufacturing

Material Grades,
Surface Finishes &
Manufacturing Process

Cotter pin material must be ductile enough to accept the bending operation on installation without fracture, while providing adequate strength for the retention function and corrosion resistance for the service environment. RR Hydraulic manufactures cotter pins in all standard grades — carbon steel, stainless steel, brass, phosphor bronze, and Monel — with full EN 10204 material traceability.

3.1 — Material Grade Overview and Properties

Table 3.A — Cotter Pin Material Comparison: Properties, Corrosion, Application

Material	Spec	Tensile Str. (MPa)	Bending Ductility	Corrosion Resistance	NACE MR0175	Temp Range (°C)	Primary Application
Low-Carbon Steel	ISO 1234 CS	320–450	Excellent	Low — coat required	Yes	−20 to +300	General EPC; indoor; coated outdoor
Spring Steel (harder)	ISO 1234 harder grade	500–700	Good	Low — coat required	Conditional (HRC check)	−20 to +300	Higher shear retention; vibrating systems
SS 304 (A2)	ISO 3506 A2	500–700	Very Good	Good	Yes	−196 to +300	Food, pharma, mild chemical, indoor SS assy
SS 316 (A4)	ISO 3506 A4	500–700	Very Good	Very Good	Yes	−196 to +300	Marine, offshore, chemical, NACE service
Brass (CuZn37)	ISO 8839 / DIN 17660	380–520	Excellent	Good	Yes	−200 to +150	Non-magnetic; electrical; non-sparking
Phosphor Bronze	CW453K	500–650	Very Good	Very Good	Yes	−200 to +150	Marine, non-sparking, non-magnetic
Monel 400	UNS N04400	480–550	Good	Excellent (seawater)	Yes	−200 to +400	Extreme marine; sour + seawater combined
Inconel 625	AMS 5666	827–1034	Good	Excellent	Yes	−196 to +650	High-temp; extreme sour; critical valve retention

3.2 — Corrosion Resistance Matrix

Table 3.B — Corrosion Resistance: Cotter Pin Material vs Service Environment

Material	Indoor Dry	Outdoor Inland	Coastal / Marine	H₂S / Sour	Chemical Process	High Temp (>200°C)	Non-Sparking
Carbon Steel (zinc)	Good	Fair	Poor	Conditional	Poor	Fair	No
SS 304 (A2)	Excellent	Excellent	Good	Fair	Good	Good to 300°C	No (slightly mag.)
SS 316 (A4)	Excellent	Excellent	Excellent	Good	Very Good	Good to 300°C	No (slightly mag.)
Brass CuZn37	Excellent	Very Good	Good	Fair	Fair	Limited >150°C	Yes
Phosphor Bronze	Excellent	Very Good	Very Good	Good	Good	Limited >150°C	Yes
Monel 400	Excellent	Excellent	Excellent	Very Good	Excellent	Good to 400°C	No
Inconel 625	Excellent	Excellent	Excellent	Excellent	Excellent	Excellent to 650°C	No

3.3 — Surface Finish Options

Table 3.C — Surface Finish Comparison for Carbon Steel Cotter Pins

Finish	Spec	Thickness (µm)	Salt Spray (h)	Bending Ductility Impact	Application
Plain / Bright	As-drawn	—	<24	None	Indoor dry assembly; immediate use
Black Oxide	MIL-DTL-13924	1–2	24–72	None	Indoor assembly; standard engineering supply
Zinc Electroplate	ASTM B633 / ISO 4042	5–12	96–300	Minimal — does not affect bending	General EPC outdoor; standard industrial
Yellow Zinc Chromate	ASTM B633 Type III	5–12 + Cr	200–500	Minimal	Standard outdoor EPC; process plant
Hot Dip Galvanise (HDG)	ASTM A153 / ISO 1461	45–85	1000+	Slight stiffening — verify bending OK	Outdoor structural; coastal marine-adjacent
Passivation (SS only)	ASTM A967	Passive layer	2000+	None	All SS cotter pins — standard finish
Nickel Plate	ASTM B689	5–20	200–500	Minimal	Non-magnetic applications; decorative

3.4 — Manufacturing Process

3.4.1 — Wire Drawing and Forming

Cotter pins are manufactured from wire stock (carbon steel, SS, brass, or exotic alloy) drawn to the precise nominal diameter per ISO 1234. The forming process: the wire is cut to the developed length, folded at the head loop using a forming die to create the closed-loop head and bring the two legs to parallel, then the legs are trimmed to the specified standard length. Critical: the wire must have adequate ductility to form the 180° head loop without cracking. Low-carbon steel wire (maximum 0.15% C) and fully annealed SS wire are specified for cotter pin manufacture — higher carbon or work-hardened wire cannot form the tight head loop radius without fracture.

3.4.2 — Quality Requirements

Head loop integrity: No cracks, splits, or partial fractures at the head bend radius — inspected visually and dimensionally on every lot; any head-loop crack renders the pin unusable
Leg parallelism: Both legs must be parallel within ±5° in the pre-installation condition; legs that diverge excessively will not pass through the transverse hole cleanly
Wire diameter tolerance: Per ISO 1234 wire diameter tolerance (h11 or equivalent) — oversize wire will not pass through the specified hole; undersize wire reduces retention force
Surface condition: No seams, laps, burrs, or sharp edges that could cut fingers during installation or score the bore of the transverse hole; deburring of cut wire ends mandatory
Bending test: Sampled lot units bent through 90° (one leg) and 180° (full wrap) in a mandrel bend test to confirm wire ductility; no cracking at the bend radius is the pass criterion

Part 04 / QC, Applications & Export

Inspection & QC,
Industry Applications
& Documentation

RR Hydraulic maintains full traceability from raw wire stock to final packed shipment on all cotter and split pin orders. Dimensional inspection, bending ductility test certificates, EN 10204 MTRs, and complete EPC export documentation packages are standard on all project-grade supply.

4.1 — Inspection & QC Protocol

100%

Dimensional Inspection

Wire diameter, head width, leg length, and overall pin length verified to ISO 1234 / DIN 94 / ASME B18.8.1 dimensional tables on every production lot. Wire diameter Go/No-Go gauging confirms the pin will pass through the minimum hole size specified for the nominal diameter. Leg parallelism checked on sampled units — divergence >5° causes difficult installation.

BEND

Bending Ductility Test

Mandatory on sampled lot. Single leg bent through 90° on a mandrel of 2× wire diameter; then completed to 180°. Pass criterion: no cracking, splitting, or fracture at the bend radius on either face of the wire. Confirms wire carbon content, temper, and forming process are correct for full installation bending without fracture risk. Results on material and mechanical properties certificate.

HEAD

Head Loop Integrity

100% visual inspection of the head loop bend radius under magnification (×5 or ×10) on SS, Monel, and exotic alloy lots. Carbon steel lots: sampled inspection. Any visible crack, seam, or partial fracture at the head loop automatically rejects the entire lot. The head loop is the highest-stressed point during installation bending — pre-existing cracks propagate immediately on first installation.

PMI

Positive Material ID

XRF verification on 100% of SS, Monel, phosphor bronze, and exotic alloy cotter pin lots. Differentiates SS 316 (A4) from SS 304 (A2) — critical for offshore, NACE, and marine service. Results on lot certificate cross-referenced to wire stock MTC heat number. Mandatory for all non-carbon-steel cotter pin supply.

COAT

Coating Inspection

Zinc plate thickness per ISO 4042; HDG per ASTM A90 / ISO 2178. Salt spray test per ISO 9227 on coated lot samples. Coating must not impair bending ductility — post-coating bend test on coated sample confirms coating has not embrittled the wire. HDG cotter pins: verify the galvanising temperature did not affect wire temper for spring-steel grade pins.

VISUAL

Surface & Burr Inspection

All pins inspected for burrs, sharp edges at cut wire ends, surface seams from wire drawing, and mechanical damage. Sharp leg-end burrs prevent clean insertion through the transverse hole and can score the hole bore — mandatory deburring of all cut ends. SS lots: no iron contamination on the SS surface (causes rust spots) — confirmed by wet tissue test on sample.

SHEAR

Shear Capacity Verification

Tensile test on sampled wire from each production lot confirms UTS and elongation per applicable material specification. Wire UTS cross-referenced to shear capacity formula (Section 1.5) to confirm design shear capacity for the nominal pin diameter and material. Results on lot MTC. Required for all high-vibration, safety-critical, and pressure-retaining bolted joint applications.

FAI

First Article Inspection

Complete dimensional, head loop integrity, bending ductility, PMI, coating, surface, and visual inspection on first production lot of each unique cotter pin configuration (diameter + length + material + coating) per project order. FAI report released before batch production proceeds — mandatory for all new project configurations.

4.2 — EN 10204 Material Test Certificate Requirements

Table 4.A — EN 10204 Certificate Types for Cotter / Split Pin Supply

Certificate	Content	Signatory	EPC Requirement	When Mandatory
2.1	Conformity declaration only	Manufacturer	General commercial supply	Non-critical general assembly
2.2	Non-specific test results	Manufacturer	Standard EPC mechanical assembly	General EPC cotter pin supply — most common
3.1	Wire lot traceable mech + chem	Manufacturer’s authorised QC	Safety-critical, NACE, SS offshore, pressure-retaining	All castellated nut bolting on pressure equipment, NACE, offshore SS
3.2	3.1 + TPI countersign	Manufacturer + SGS / BV / Lloyds	Critical offshore, nuclear, lifting equipment	Offshore structural pins, nuclear Class 1/2, lifting gear certification

4.3 — Applications by Industry

Castellated Nut Locking Pressure Vessel Flange Bolting Clevis Pin Retention Hydraulic Cylinder Linkages Valve Handwheel Shaft Retention Pump Impeller Shaft Locking Turbine Control Linkages Chain & Conveyor Pin Retention Lifting Equipment (Shackles) Offshore Topside Bolting Rotating Equipment Covers Gate Valve Bonnet Bolting Safety-Critical Axle Pins ATEX Zone Pin Retention LNG Cryogenic Assemblies Nuclear Instrumentation Locking

Castellated Nut Locking — Pressure-Retaining Bolting

The most safety-critical application of cotter pins in EPC. Castellated hex nuts (ISO 7035 / DIN 935) on pressure vessel flange bolts, valve bonnet studs, and pump casing bolts are locked with cotter pins through the stud transverse hole — providing absolute mechanical retention independent of bolt preload. Material: SS A4-70 for sour service and offshore; carbon steel zinc-plated for general service. Single-use rule applies absolutely — replace on every maintenance access. EN 10204 3.1 mandatory for all pressure-retaining bolting service.

Hydraulic Cylinder and Mechanical Linkages

Cotter pins through transverse holes in clevis pins retain the clevis pin in hydraulic cylinder end connections, valve actuator linkages, pump mechanical seal adjustment mechanisms, and mechanical override linkages. Brass or SS cotter pins for corrosive process atmosphere applications. Extended prong type preferred for linkages where visual inspection of pin installation is part of the maintenance verification procedure. Replace cotter pin on every hydraulic cylinder maintenance intervention.

Offshore Topside Castellated Bolting

All cotter pins on offshore topsides: SS A4-70 (SS 316) minimum — carbon steel with zinc coating corrodes rapidly in marine chloride atmosphere, causing pin seizure in the transverse hole and making removal by cutting extremely difficult in confined offshore maintenance access conditions. SS 316 cotter pins allow clean field removal with wire cutters. PMI on every SS lot. EN 10204 3.1 minimum; 3.2 with TPI for safety-critical pressure-retaining applications.

Lifting Equipment Pin Retention

Cotter pins retain shackle pins, hook pins, swivel pins, and block-and-tackle pivot pins on lifting equipment used in EPC construction, equipment erection, and maintenance. These are safety-critical applications — loss of a cotter pin during a lift can result in sudden release of the suspended load. Material: SS A4-70 for marine and outdoor service. Cotter pin size must be the maximum that fits the transverse hole — undersized pins in lifting equipment are a rejection criterion. Annual inspection per lifting gear regulations (LOLER / LEEA standards).

Rotating Equipment Covers and Shaft Retention

Cotter pins lock castellated nuts on rotating equipment: pump and compressor shaft end nuts, fan hub retention nuts, gearbox output shaft end nuts, and impeller retention nuts. High-vibration service — pin size must be the maximum the transverse hole allows; inspect at every planned maintenance shutdown. SS A4-70 for wet/corrosive service environments. Inconel 625 or Monel 400 for high-temperature process fluid pump shafts where standard SS exceeds service temperature.

ATEX Zone Assemblies — Non-Sparking Materials

In Zone 1 and Zone 2 classified explosion-hazard areas where non-sparking tools and hardware are required: specify brass (CuZn37) or phosphor bronze cotter pins — both are non-sparking per IEC 60079-0 / EN 60079 ATEX requirements. Brass cotter pins provide the full range of ISO 1234 sizes in a non-sparking material at minimal cost premium. Phosphor bronze for higher-load ATEX applications where the higher strength of PBr vs brass provides greater retention force in high-vibration environments.

4.4 — Export Packaging Specification

Cotter pins bulk-packed in polybags per nominal diameter, length, material, and coating — prevents mixing of sizes (similar-appearance different-diameter pins are easily confused at site)
VCI (Volatile Corrosion Inhibitor) poly bag packaging for all plain and zinc-plated carbon steel cotter pins — prevents surface rust and corrosion of the wire surface during ocean freight and site storage; rust on cotter pin legs prevents clean passage through transverse holes
SS cotter pins: packed separately from carbon steel hardware — iron contamination on SS wire surface causes rust spots; each SS polybag clearly labelled with grade (A2-70 / A4-70) and standard reference
Lot tagging: each polybag labelled with nominal diameter, length, material grade, coating, ISO 1234 / DIN 94 / ASME B18.8.1 standard reference, heat/lot number, quantity, and PO item reference
Maintenance kit packaging: assorted multi-size cotter pin kits in compartmented trays — one tray per bolt size range (M6–M16, M16–M30, M30+) to enable efficient field maintenance stocking on EPC construction and commissioning sites
ISPM-15 heat-treated timber crates for all international export; inner cartons per UN packaging requirements for metal hardware commodity classification
Documentation: packing list, EN 10204 MTC, bending ductility test certificate, coating certificate, PMI report (SS/exotic grades), and FAI report enclosed with each lot

4.5 — Complete EPC Project Documentation Package

Table 4.B — Full Documentation Package for EPC Cotter / Split Pin Supply

#	Document	Standard / Format	Mandatory / Conditional	Notes
01	Material Test Certificate (MTC)	EN 10204 2.2 / 3.1 / 3.2	Mandatory — grade per project spec	Wire stock lot heat-traceable
02	Chemical Composition Report	Wire lot certified lab analysis	Mandatory — 3.1 / 3.2 orders	Per ISO 1234 / ISO 3506 chemistry limits
03	Mechanical Properties Report	UTS + elongation per wire lot	Mandatory — 3.1 / 3.2 orders	Tensile + ductility confirmed
04	Bending Ductility Test Certificate	ISO 1234 bend test / sampled lot	Mandatory — all safety-critical supply	90° + 180° bend — no cracking
05	Dimensional Inspection Report	Per ISO 1234 / DIN 94 / ASME B18.8.1	Mandatory	Wire dia, head width, leg length confirmed
06	Head Loop Integrity Report	Visual + magnified inspection	Mandatory — SS / exotic alloy lots	No cracks at head bend radius
07	Surface Finish / Coating Certificate	ISO 4042 / ASTM B633 / ASTM A153	Mandatory — all coated supply	Coating thickness + salt spray per lot
08	PMI Report (XRF)	Per lot — SS / exotic grades	Mandatory — non-CS grades	A4-70 vs A2-70 differentiation; Monel confirmed
09	NACE Compliance Statement	Material + hardness declaration	Conditional — sour service supply	SS A4-70 confirmed; carbon steel excluded
10	Non-Sparking Certificate (ATEX)	Material composition + test record	Conditional — ATEX zone supply	Brass / phosphor bronze confirmed non-sparking
11	First Article Inspection (FAI) Report	Project-specific format	Mandatory — new configs / first lots	Released before batch production
12	TPI Witness Certificate	SGS / BV / DNV / Lloyds	Conditional — EN 10204 3.2 / lifting gear	Co-witness dimensional and bend test
13	ISO 9001:2015 Certificate	Third-party QMS certification	Mandatory — EPC projects	Scope covers cotter pin manufacture
14	Country of Origin Certificate	Chamber of Commerce	Mandatory — all export	HS tariff code for customs classification
15	Packing List	Item-level per shipment	Mandatory	Cross-references lot numbers and MTC
16	Commercial Invoice	Per INCOTERMS 2020	Mandatory	Includes HS tariff code
17	Bill of Lading / Air Waybill	Per freight mode	Mandatory	Issued by freight forwarder

4.6 — ISO and Quality System Compliance

ISO 9001:2015

Quality Management System covering wire stock procurement and certification, forming process control (loop head geometry), bending ductility testing, dimensional inspection, surface finish QC, coating process control, and full lot traceability from wire stock to finished pin. Mandatory for all EPC, safety-critical, and O&G project procurement qualification. RR Hydraulic holds current ISO 9001:2015 certification covering cotter pin manufacture.

ISO 10474

Steel and steel products — inspection documents. Source framework for EN 10204 certificate types. Some legacy EPC project specifications (particularly for pressure vessel and piping bolting) reference ISO 10474 Type 3.1.B for cotter pin material certification — this maps to EN 10204 3.1. RR Hydraulic provides documentation in either format and cross-maps for legacy specification compliance on request.

ISO 4413

Safety requirements for hydraulic fluid power systems. Cotter pins are used in hydraulic cylinder clevis pin retention and valve actuator linkage assemblies in ISO 4413 hydraulic systems. ISO 4413 mandates that all pin retention devices in hydraulic actuator connections be correctly installed and documented — cotter pin replacement is a mandatory step in the hydraulic cylinder maintenance procedure under ISO 4413 safety management requirements.

ASME B31.3 / B31.1

Process piping and power piping codes. Castellated nut assemblies with cotter pin retention are used on pressure-retaining flanged joint bolting in ASME B31.3 process piping systems. The cotter pin material must be compatible with the process fluid and service temperature — NACE MR0175 compliance required for sour service piping; SS A4-70 standard for offshore and marine atmosphere. Cotter pin condition is a required inspection item on ASME pressure system inspection records.

Ready to source cotter / split pins for your EPC, mechanical, or rotating equipment project?
Submit your pin diameter, length, material, coating, quantity, and application to RR Hydraulic for a complete, certified commercial offer.