Piston and connecting rod from an automobile engine, showing the big end bearing at the bottom.
In a reciprocating engine, the crank pins are the bearing journals of the big end bearings, at the opposite ends of the connecting rods to the pistons. If the engine has a crankshaft, then the crank pins are the journals of the off-centre bearings of the crankshaft. In a beam engine the single crank pin is mounted on the flywheel; In a steam locomotive the crank pins are often mounted directly on the driving wheels.
Big end bearings are commonly plain bearings, but less commonly may be roller bearings, see crankshaft.
In a multi-cylinder engine, a crank pin can serve one or many cylinders, for example:
- In a straight engine each crank pin normally serves only one cylinder.
- In a V engine each crank pin usually serves two cylinders, one in each cylinder bank.
- In a radial engine each crank pin serves an entire row of cylinders.
Big end design
There are three common configurations of big end bearing:
- If a crank pin serves only one cylinder, then the big end is a relatively simple design, accommodating only one connecting rod. This design is the cheapest to produce, and is used in:
- All single cylinder engines.
- Most straight engines.
- All boxer engines.
- Some V-twin engines.
- If a crank pin serves more than one cylinder, then the corresponding cylinders may have an offset, to simplify the design of the big end bearing. This design is used in:
- Most V engines.
- Multiple row radial engines.
- If more than one cylinder is served by a single crank pin but there is no offset, then some or all of the connecting rods must be forked at the big end. This design in theory provides better engine balance than designs with an offset, but at the cost of considerable extra complexity and cost in both design and manufacture, and either more weight or closer manufacturing tolerances or both to achieve the same strength and reliability. Any extra weight added to the big end itself also carries a penalty of adding vibration and reducing balance. As the number of cylinders grows, the effect of the offset on balance becomes less important, and forked connecting rods become less common. They are mainly used in:
- Single-row radial engines.
- Some V-twin engines, notably including motorcycle engines.