What Is a Lynch Pin? Types and Choosing the Size

What a lynch pin does

A lynch pin carries no load; the shaft or bolt carries it. The pin's job is to stop that connection from working itself loose. In other words it is a safety element.

What does the work is not the body but the spring ring attached to it. The ring closes onto the pin and grips the shaft once it is through. When vibration pushes the pin out, the opening force of the ring holds it back. Once that force drops, the pin works its way out in the field — most lost pins do not fall out, they are pushed out.

It is used widely: PTO guards, drawbars and hitch pins, trailer couplings, seeding units, rake arms, seed drill units. Anywhere a fastening needs to open and close by hand without tools.

Types of lynch pin — and how they really differ

Several names are used interchangeably in the field, yet these are four different parts. Most of the confusion starts here.

• Lynch pin (R ring) — a straight body with a spring ring riveted or welded to it. The ring closes onto the body. Our catalogue covers 6, 8 and 10 mm.
• R pin / hairpin clip — it has no body; the part itself is bent spring wire, named for its resemblance to the letter R. It goes straight through the hole. Our catalogue covers 3, 4 and 5 mm.
• D pin — a pin whose spring ring is D shaped. The wider ring opens in a single movement even with gloves on, so it is preferred where the pin is removed often. Our catalogue covers 6, 8 and 10 mm.
• Square pin — a body with a square section. Used where the connection must not rotate; a round pin turns in its seat, a square one does not. Our catalogue covers 8 mm.
• Split pin (cotter pin) — this is NOT a lynch pin. It is a two-legged, single-use part whose legs are bent open after fitting. A lynch pin is used again and again; a split pin is not. Using a split pin where the joint is opened often is the wrong choice.

Which dimension to ask for

Two dimensions decide a lynch pin; a third is rarely needed.

The first is body diameter. It is chosen to suit the hole, and it is the number in front of the pin name — "10 mm lynch pin" refers to the body thickness, not the length. Measure the hole with callipers; failing that, measure the body of the old pin.

The second is effective length: the part of the body from where the ring starts to the tip. This must be longer than the total thickness of the connection the pin passes through. Too short and the ring will not seat on the shaft, so the pin comes out at the first jolt. Too long does not stop it working, the tip simply protrudes.

If you cannot measure, the quickest route is a photo of the old pin next to a ruler; the quotation form accepts file attachments. Sizes outside the catalogue can also be produced — the coiling line works with no tooling dependency.

Fitting, removing, and why pins go missing

A lynch pin needs no tools; that is the whole point of it. The body goes through the hole and the ring is pressed closed onto the shaft. Make sure the ring has fully caught the shaft — a half-seated ring looks fitted but does not hold.

To remove it, the ring is opened with a thumb and the pin pulled back. If the ring has stiffened or the coating has stuck, lifting it with a flat screwdriver is enough; no separate removal tool is needed. Forcing the ring with pliers permanently reduces its force — that pin will never hold again.

If a pin keeps going missing from the same place, the pin is usually not the culprit. The hole may have gone oval, the shaft may be worn, or the ring may be fatigued. A fatigued ring shows itself like this: when closed it does not touch the shaft, leaving a gap. Such a pin is replaced; trying to correct it does not work.

Galvanised coating is not a cosmetic choice on this part. Corrosion thins the section of the ring, and lost section directly reduces the opening force — so a rusted pin stops doing its job well before it breaks.