Cotter Pin Installation & Sizing Guide｜A Small Part That Safeguards Big Systems
- 翰君 陳
- Aug 19, 2025
- 2 min read
Updated: Sep 1, 2025
In many machines and structures, the cotter pin is a simple-looking part that provides crucial safety. Commonly used with nuts, shaft/hitch pins, and joints, it prevents components from loosening under vibration or long-term operation by inserting through a hole → then splitting the two legs to lock.
Don’t underestimate this thin metal pin—without it, an entire assembly can fail in an instant.
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Choose the Right Size for Reliable Retention
Selecting a cotter pin mainly depends on hole diameter and operating environment:
Hole vs. pin diameterThe pin diameter should be slightly smaller than the hole for smooth insertion.Example: For a Ø4 mm hole, use approx. Ø3.2 mm (1/8") cotter pin.
Pin lengthChoose a length slightly longer than the stacked thickness of the parts being secured so both legs can be fully bent and locked.Too short: inadequate locking.Too long: excessive protrusion that may interfere with nearby parts or appearance.
Material considerations
Carbon steel – Common and economical for general fastening
Stainless steel SUS304 / SUS316 – Excellent corrosion resistance for outdoor, marine, or food machinery environments
Zinc-plated options – Added corrosion protection where needed
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Pre‑Order Checklist
Before purchasing, confirm the following:
Hole size → Pin diameter slightly smaller than the hole for easy installation
Length → Long enough to fully bend both legs and clamp the part
Material → Choose carbon steel, stainless steel, or zinc‑plated based on the environment
Quantity → Cotter pins are typically single‑use; keep sufficient spares
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Install a Cotter Pin in Four Steps (≈3 minutes)
Insert through the holeAlign the pin’s shank with the hole and push it fully through.
Seat the headThe head should sit flush against the surface with no visible gap.
Split the legsUse needle‑nose pliers or by hand to bend the two legs in opposite directions:one leg pressed along the surface, the other bent the other way to lock.
Final checkEnsure both legs lie snug against the part without lifting or scratching surroundings.
Tips
Lightly tap the bent legs with a hammer to make them sit tighter.
Cotter pins are generally not reusable—replace after removal.
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Where You’ll Most Often See Cotter Pins
Securing nuts (with castellated or hex nuts)
Locking shaft pins or hinge pins
Anti‑loosening at heavy‑duty connection points
Automotive and motorcycle parts (wheel axles, brake linkages)
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Why Choose Sheng Fong Precision?
When choosing cotter pins, quality and supply reliability matter just as much.
Sheng Fong Precision offers:
Complete range – From common small sizes to large‑equipment specifications
Multiple materials – Carbon steel, stainless steel, zinc‑plated to match environments
Strict QC – Dimensional accuracy and surface finish inspected for secure installation
Fast delivery – In‑stock items; supports small lots and urgent orders
Expert support – Sizing and application advice to reduce installation errors
With Sheng Fong, your equipment stays safe and stable over the long term—reducing maintenance and replacement costs.
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📩 Contact us for cotter pin selection advice via LINE (ID: @s9000)
For the complete list of specifications and sizes, please refer to: https://lihi3.me/OWETp
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