This display illustrates the mechanism that is used in a radial aircraft engine to obtain rotary propeller motion using multiple cylinders arranged in a circular (radial) fashion.
The splines at the end of the crankshaft are used to engage the hub of the propeller that has the propeller blades attached to it. There are a series of connecting rods, one for each cylinder, that connect the crankshaft to the pistons that are in the cylinders.
Each of these connecting rods is attached to the crankshaft by use of a knuckle pin. One of the connecting rods is quite different from all of the others. This rod is called the “master rod.” The remaining connecting rods are attached at their lower ends by knuckle pins to the holes that are part of a large flange formed by part of the master rod.
The master rod is similar to any other connecting rod except that it is constructed to provide for the attachment of the articulated rods. Each articulated connecting rod has a bushing of nonferrous metal, usually bronze, pressed or shrunk into place to serve as a knuckle pin bearing. The knuckle pins may be held tightly in the master rod holes by a press fit or they may be of the full floating type depending upon engine design.
Source: Excerpts from aviation-history.com