Connecting Rods

It’s the connecting rods’ (conrods) job to join the pistons to the crankshaft.

Connecting Rods
Connnecting Rods

The rod has three main areas: the big end (which bolts to the crank), the little end, (which fixes to the piston, usually with a pin that passes through the rod and into the piston) and the beam (which is the main body of the rod).

The rod will be under constantly changing compressive and tensile loads; compressive as the explosion takes place, as well as the piston reaches the bottom of its travel and has to start moving up again, and tensile as the piston reaches the top of its travel and the crank starts to pull it down.

The material and the design must therefore ensure that the rod can not only sustain these permanent stretching and compressing forces, but that it can do so at high engine speeds.

As with the crank, there are three main manufacturing methods:

  • Cast (where molten metal flows into a mould)
  • Forged (where a semi-solid lump of metal is forced into a mould under pressure and heat)
  • Billet (where the rod is machined from a solid lump of metal)

However, a variation on the theme is the sintered forged rod, where powdered material is forced into a mould under pressure and temperature.