Lynch Pins and Hairpin Clips

About Lynch Pins and Hairpin Clips

The lynch pin and the hairpin clip are the smallest parts securing a connection on a farm machine — and the ones most often lost. They are used everywhere from the PTO shaft to the drawbar, from the opener shoe to the rake group, which makes them the first item a farmer keeps in stock.

What a lynch pin does is simple but critical: it keeps a shaft or a connecting pin in place. A plain pin walks out under vibration; the spring leg of a lynch pin, locking back onto the body, prevents that. Because the leg opens and closes by hand, no tool is needed to fit or remove it — which is exactly what is wanted when a machine has to come apart in the field.

Two dimensions decide the choice: the pin body diameter and the effective length. Body diameter follows the hole; effective length must exceed the thickness of the connection. On R clip type hairpin clips the spring force of the leg matters as well — a weak leg works itself out under vibration.

The international equivalent of an agricultural lynch pin is DIN 11023, a 1979 standard titled simply “Agricultural machinery — Linch pins”. The same part is known in the field as a lynch pin or a locking pin. You do not need the standard number to order — the body diameter and the effective length are enough, because production follows the dimensions given.

Our catalogue holds 4, 5, 6, 8 and 10 mm lynch pins, 6/8/10 mm D pins, an 8 mm square pin, a drawbar pin and a 3–5 mm hairpin clip series. D pins are produced with a spring ring. If a size is not on the list it is produced from a sample or from dimensions — there is no tooling dependency on the pin side.

How to take the dimensions of a lynch pin

A mix-up over dimensions is the commonest reason the wrong part arrives. Four measurements are enough to order; all of them can be taken from the old part with a caliper or a ruler.

• Body diameter — the diameter of the straight body of the pin. This is the figure catalogues use as the name of the product: “8 mm lynch pin” means the body diameter. It is chosen to suit the hole it goes into.
• Effective length — the length of the working part of the body that passes through the connection. It must be longer than the combined thickness of the parts being joined, otherwise the spring leg has nowhere to close.
• Overall length — the length measured end to end. In tight positions this is what decides whether the pin will fit at all.
• Spring leg / ring size — the opening of the spring that closes over the body, and the thickness of its wire. These set the closing force of the leg; a weak leg works itself out under vibration.

DIN 11023 nominal sizes

The international reference for an agricultural machinery lynch pin is DIN 11023, titled “Agricultural machinery — Linch pins”. The standard identifies a pin not by a single diameter but by a nominal size pair — diameter and length are always quoted together. The 6, 8 and 10 mm lynch pins in our catalogue are produced to these nominal sizes:

• 5 × 32 — the smallest nominal size.
• 6 × 42 — the 6 mm lynch pin in our catalogue is produced to this nominal size.
• 8 × 42 — the 8 mm lynch pin in our catalogue is produced to this nominal size.
• 10 × 45 — the 10 mm lynch pin in our catalogue is produced to this nominal size.
• 12 × 45 and 12 × 55 — two lengths of the same diameter; the length is chosen to suit the thickness of the connection.
• 17 × 60 — the largest nominal size, for heavy connections.

What “8 mm pin” actually means in the field

A lynch pin is named for the hole it goes into, not for its own body diameter. When an “8 mm lynch pin” is asked for, what is meant is a pin that fits an 8 mm hole; the measured diameter of the body sits a little under that figure, because the pin has to go in and come out without binding. It is therefore normal for an old part not to measure exactly 8.00 mm under a caliper.

For the same reason one maker’s “8 mm” and another’s can differ by tenths. As long as the nominal size is the same, both go into the same hole; the difference lies in the tolerance the maker has chosen.

When ordering, the soundest route is to state the diameter of the hole, or to send the old part — then the question of tolerance never arises.

Why another maker’s dimension table cannot be copied

Lynch pin tables given with letters such as d1, d2, l1, l2, b1 and b2 circulate widely online. Those letters are not a standard: the same letter can denote the body diameter in one maker’s table and the ring opening in another’s. In comparing records that use the same letter set, we have seen l1 point to different quantities.

Copying a table and ordering from it is therefore risky. The safe route is always the same: measure the old part in your hand, or lay it next to a ruler and photograph it. If you have a sample there is no need to measure at all — an exact copy is produced.

The international reference for an agricultural machinery lynch pin is DIN 11023, but you do not need to know the standard number; the body diameter and the effective length are enough.

Where it is used on tractors and agricultural machines

A lynch pin does its work wherever a shaft or connecting pin must be stopped from working its way out. On the tractor and implement side it turns up most often at these connections:

• Drawbar and hitch — the point where a trailer or field implement connects to the tractor. A pin that walks out here means the implement comes away.
• PTO shaft connections — holding the shaft on axis in shaft and guard assemblies.
• Seed drill opener assembly — on the shoe and press arm connections.
• Rake and comb bar assemblies — on the shaft connections of the tine carrier bars.
• Three-point linkage and top link — securing the link pins; here it usually works together with a hairpin clip.

What it is also called in the field

The same part is asked for under different names depending on the region and the seller. Locking pin, safety pin, retaining pin and quick-release pin usually all mean the same lynch pin. It is spelled both lynch pin and linch pin; because the ring is R-shaped it is also called an R-pin.

The same is true of the hairpin clip: R-clip, R-pin hairpin and hairpin cotter are the same part. One distinction does matter — a split pin (cotter pin) is NOT a lynch pin: it has no spring, it is secured by bending its legs after fitting, and it is not reused once removed.

Which name you use when ordering makes no difference; the body diameter and a photograph of the part in your hand are enough.

What sets the price

There is no single list price for a lynch pin, because the same pin costs many times more or less depending on quantity. Four things set the price: size, quantity, coating and packing. The most decisive of these is quantity — setting up the coiling line is a fixed job that is divided by the quantity, so one piece and a thousand pieces come off the same line but not at the same price.

Stating the quantity when you ask for a quotation is therefore the fastest route to an accurate price. Naming the coating you want — galvanized or stainless — settles it further.

We are the manufacturer, not a dealer: the pin is coiled in our own plant, and because there is no tooling dependency a size that is not on the list carries no tooling cost.