What does a clevis pin do?
It joins a clevis fork to a tang and lets the connection pivot. It has a cylindrical shank, a flat head at one end and a cross-drilled hole near the other. A second fastener goes through the cross hole and stops the pin backing out (Q1-07). The pin carries radial shear, and in a clevis joint it is in double shear while the retaining clip carries no working load. Clevis pins are made to ASME B18.8.1 and ISO 2341 (which replaced DIN 1444).
One habit is worth avoiding: specifying a pin by diameter and length alone. That skips the retention method, the shear capacity and the installation clearance, which is where joints actually come apart.
How do you choose the diameter?
By the load, and the check is done elsewhere: shear and bearing pressure at the greatest force across the joint (Q1-05), and the sizing from cylinder force with a safety factor (K03-Q1-14). Then match it to the holes. The pin diameter must match the hole diameter. Too small and it wobbles, too large and it will not go in.
What does clearance cost you?
Lost motion. Clearance holes let the joint absorb thermal growth and tolerance stack-up without galling, so some clearance is needed. But the clearances add up along the joint. Suppose the pin is 0.1 mm smaller than the bore in the fork arms and 0.1 mm smaller than the bore in the tang. The tang can then move 0.1 + 0.1 = 0.2 mm relative to the fork before the pin takes the load. Every time the load reverses, the joint closes that gap with an impact, which is what hammers a loose clevis and wears its holes (Q1-12). A pin that is a tight fit can bind, so choose the smallest clearance the thermal growth, the finish and the tolerances allow, and take the tolerance class from the clevis maker's drawing (Q1-02).
How long should the pin be?
Long enough to reach the retainer, with the stack measured, not assumed. The length must be slightly longer than the total thickness of the parts, so a cotter pin or retaining ring can be fitted at the end.
Worked on an example: a fork with an inside width of 20 mm and two arms 10 mm thick has an overall width across the arms of 40 mm. If a washer of 3 mm goes under the retainer on the open side, the cross-hole centre must sit at least 43 mm plus half the hole diameter from the underside of the head, plus any float the retainer maker asks for. Measure the stack of both fork arms, the part between them and any spacer or washer. A pin that is too long sticks out and catches; too short and the retainer will not seat.
What material and hardness?
Use a steel pin and size it for the load. Low-carbon steel serves general use, medium-carbon steel gives higher strength for moderate loads, and stainless steel suits outdoor, food or humid places. Pins can be forged, with larger tolerances and rougher ends, or CNC-machined, which is precise. A soft pin in a generic clevis tends to fail in a sequence: the pin and bore wear, the clearance grows, the load turns into impact, and the thread or the body of the clevis loosens or cracks. A hardened pin keeps the wear out of the first step.
Hardness also decides which part wears. When a hard pin works in a softer bore, the bore wears. When the bore is a replaceable bushing that is cheap to change, that is the right place for the wear; when it is the clevis or the machine bracket, it is the expensive one that is lost. So decide which part should take the wear and make it the softer one: usually a cheap, replaceable bushing, not the clevis or the bracket. No single hardness figure applies to every pin, so ask the maker for the hardness that goes with the clevis and the load.
Why does pin clearance matter more at a pedal pad?
A pedal or vehicle linkage pin makes many small movements in a place with mud, dust and water, and its wear shows up as free play at the pad. Two points from this article change weight.
First, the lost motion at the pin is magnified. The earlier example gave 0.2 mm of lost motion from two 0.1 mm clearances. At a pushrod pin on a pedal with an assumed 4:1 ratio, the pad moves four times as far as the pushrod, so 0.2 mm at the pin is about 0.8 mm at the pad, which adds directly to pedal free play and grows as the pin and bushing wear.
Second, the pin usually turns in a bushing, so the rule about which part should take the wear applies directly. Make the bushing the part that is cheap to replace, and keep the pin harder than the bore it runs in. Choose a fit that is free enough to keep turning when dirt or corrosion gets in, since a tight fit is what seizes first in the dirt, and not so loose that play becomes noticeable at the pad. For bushing options, see SYZ's bushing buyers guide.
For a brake or clutch pedal, the vehicle maker specifies the pin size, the fit and the hardness.