Definition
In a radial engine, the single connecting rod that attaches directly to the crankshaft throw. The pistons of all the other cylinders in that row connect to the master rod through smaller link rods (also called articulated rods) attached to knuckle pins around the master rod's flange.
Plain English
In a radial engine, the cylinders are arranged in a circle around the crankshaft. Only one of those cylinders has a connecting rod that bolts straight onto the crankshaft. That special rod is the master rod. Every other cylinder's rod is hinged off the side of it.
Context Anchor
Seen in aircraft engine maintenance, especially when studying or inspecting radial piston engines.
Derivation
Called 'master' because it is the principal rod -- the one that controls the motion of all the others. The link rods attached to it follow whatever path the master rod sets.
Why Pilots Care
Proper function of the master rod is essential for transmitting power from every piston to the crankshaft; wear or failure can ground the aircraft and require major engine work.
Analogy
Think of the master rod as the main handle that several other handles pull on. The other rods do not each connect separately to the crankshaft; they work through the master rod.
Intuition Check
Do not read “master rod” as just any important rod in the engine. In this context, it is a specific main connecting rod used in a radial piston engine.
Example Sentence 1
On a radial engine, the master rod is installed in one specific cylinder, and that cylinder is identified in the engine manual.
Example Sentence 2
All cylinders on that bank transmit their force through link rods that pivot on the master rod.