What is "crankpin"

n. The pin that attaches a connecting rod to a crank

In a reciprocating engine, the crankpins, also known as crank journals are the journals of the big end bearings, at the ends of the connecting rods opposite to the pistons.

If the engine has a crankshaft, then the crank pins are the journals of the off-centre bearings of the crankshaft. In a beam engine, the single crank pin is mounted on the flywheel; In a steam locomotive, the crank pins are often mounted directly on the driving wheels.