Connecting Rod

The part in an internal combustion engine that connects the piston to the crankshaft. It changes reciprocating motion of the piston into rotary motion of the crankshaft or vice versa.

A connecting rod consists of a pin-end, a shank section, and a crank-end. Pin-end and crank-end pinholes at the upper and lower ends are machined to permit accurate fitting of bearings. These holes must be parallel.

Since the connecting rod is continually rotating and translating it experiences both high inertial loads and compressive forces from the combustion pressure applied through the piston.

The conrod mass is mc

The effective mass at the gudgeon pin end is

The effective mass at the crankpin or big end is

Big End

This is the end that connects to the crankpin and has the big end bearing.

This will see the maximum forces at engine overspeed. It is important to look at the blending of the big end into the main I-beam section of the conrod.

Connecting Rod Bearing

The bearing located in the large end of the connecting rod by which it is attached to the crankshaft.


This is the main cross-section through the beam that connects the little-end and the big-end.

The compression load at maximum engine torque can cause buckling of this beam.


As the length between the big and little ends is increased so the side forces on the piston decrease. Overall the effect is the engine efficiency increases as the conrod length is increased.

Little End

The gudgeon pin end, the part of the conrod that fits to the piston.

The critical design area is the top of the little end in tension when there is no load on the engine and it is running at or above maximum rpm in an overspeed condition. Therefore the width an thickness of the area around the little end should be analysed.


Piston Pin

A pin that rests in two bored holes in the piston and passes through the eye of the connecting rod, to join the two together flexibly.


Hiscox, Gardner Dexter, "Mechanical movements, powers and devices", 1911, New York, The Norman W. Henley publishing co.

See also: Big End, Connecting Rod End, Crank, Crankshaft, Internal Combustion Engine, Little End.

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Subjects: Engines