What is a kingpin?

A kingpin is on a semi trailer. It is what locks into the fifth wheel on a semi tractor, to connect the tractor and trailer together.

Answer

There is another kingpin that had been common on older vans, that took the place of balljoints, if I remember right. I do remember the technicians dreaded replacing them.

I have a '57 Chevy truck that has kingpins in the front axle. they hold the brake spindles onto the axle. i have had to replace them before, and they have to be replaced with new bushings that may have to be machined to fit correctly. they are very common in straight axle vehicles especially "classic" vehicles.