These are the parts that "connect" the crankshaft and the piston assemblies. The crankshaft side is large and has an end cap which is held on with 2 nuts. The pistons are held on the small end of the rod with a hollow wrist pin.
Rod which is a link connecting reciprocating parts and rotating parts, of a machine, for example the crankshaft and piston of an automobile, with the rod having a pinned connection at one end and the free end rotating around the pin. The reciprocating motion of the piston within the cylinder is sent to the crankshaft, which allows it to rotate.