In mechanical engineering, reciprocating mass refers to any part that is forced to reverse direction while moving. Examples of this include pistons, connecting rods and lifters found in internal combustion engines.

Although useful, such parts are regarded as mechanical bottlenecks. Any time a part reverses direction, its inertia must be overcome. Consider that in automobile engine operating at 3,000 RPM the each piston must reverse directions 6,000 times per minute. That aborbs power produced by the engine,and creates a part that is potentially failure prone. Engineers seek to minimize reciprocating mass, where it cannot be avoided.

The avoidance of reciprocating mass are a primary advantage of turbines and one potential advantage of a rotary engine.