crankshaft  is the piston and crankshaft assemblies are called the engine's reciprocating assembly; the piston and rod reciprocate, or move up and down, in the cylinder, while the crankshaft changes the piston's linear or straight-line motion into usable rotary or spinning motion; piston move up and down together with 360 degree crankshaft, alternating with a 180 degree crankshaft