horizontal pin by means of which a piston is attached freely to a connecting rod in a diesel engine

A small pin fitted within the piston assembly of an internal combustion engine which is linked by the connecting [con] rod to the drive shaft of the engine.

British term for a piston pin or wrist pin.

In internal combustion engines piston engines, the gudgeon pin is that which connects the piston to the connecting rod and provides a bearing for the connecting rod to pivot as it moves. In very early engine designs (including those driven by steam and also many very large stationary or marine engines, the gudgeon pin is located in a sliding crosshead that connects to the piston via a rod.