noun. A vertical bolt that joins the body of a wagon or other vehicle to its front axle and usually acts as a pivot.

A large bolt connecting the fore part of a carriage with the fore axle. The axle rotates about it as a joint “when the carriage is turned.

A large bolt which passes through the truck and body-bolsters and center-plates of a carbody and the center of a truck.

Also kiny-pin.

noun. An iron rod in a roof- or bridge-truss, used in place of a king-post to prevent the inclined members from allowing an increase of the distance between the tie and the compression pieces.

noun. A vertical iron bolt, by which the forward axle and wheels of a vehicle or the trucks of a railroad car are connected with the other parts.

noun. The main bolt of a structure, especially the bolt in a motor vehicle that links the chassis to the axle, providing the steering pivot.

noun. bolt that provides a steering joint in a motor vehicle

"He repaired the kingbolt, but what the watch gained in one way it lost in another."

bolt  

kingpin