Kingpin
The main pivot pin in the steering mechanism of heavy-duty truck axles and vintage vehicle steering knuckles.
A kingpin is the main steel pivot pin that connects the steering knuckle to the solid axle beam in heavy-duty trucks, buses, and older classic passenger cars. It acts as the steering pivot axis around which the wheel assembly rotates when the steering wheel is turned. Kingpins predate modern ball joint suspension designs and remain the standard for high-load commercial vehicles due to their extreme strength.
Kingpins ride inside metal bushings pressed into the axle ends, and require regular lubrication with grease via zerk fittings to prevent friction wear. If a truck goes too long without grease, the kingpin bushings will wear out, creating play in the steering wheel and wheel wobble. This leads to unstable steering (shimmy) and rapid front tire wear.
Symptoms of worn kingpin bushings include steering play, front-end vibration at highway speeds, uneven tire wear, and hard steering. Mechanics test for kingpin wear by raising the vehicle and checking for vertical and horizontal play in the wheel knuckle. Replacing kingpins requires pressing the old pins out with a high-tonnage hydraulic press, pressing in new bushings, and reaming the bushings to exact specifications to fit the new pin.