In a reciprocal piston engine, the connecting rod connects the piston to the crankshaft. It can rotate at both ends so that its angle can change as the piston moves up and down and the crankshaft rotates.

The top end, with a small opening for the piston pin is called the small end. The lower end, that connects to the crankshaft with a larger opening, is called the big end. The big end holds the crank's bearing shells and also has a pinhead size hole bored to the crank bearing journal that squirts lubricating engine oil on the cylinder wall.