A connecting rod is a part which connects a piston to a crankshaft in an internal combustion engine. For motorcycle engines, a high strength connecting rod is required to bear the combustion forces from piston and transfer to rotational torques to crankshaft. Forged connecting rods, made of chromoly steel, are manufactured by a series of processes including hot forging, rough machining of planes and bores, heat treatment, and precise grinding of planes and bores.
Fig. 1 Connecting rod assembled in the crankshaft
A motorcycle connecting rod is especially important in motorcycle engine because the bore race has to be hard (HRC60) to resist abrasion from crank pin and piston pin during high RPM operation. However, a hard material is brittle which causes connecting rod failure at rod area. To avoid from connecting rod breaking, the hardness at rod area is necessary to lower down to HRC35. The material at rod area will become ductile and then the motorcycle engine can operate safely with a good connecting rod.
Another important spec. of a connecting rod is the roundness of connecting rod bore. Roundness is the measure how closely the bore approaches to a perfect circle. The bore diameter is measured by different angles, and roundness is the difference of maximum and minimum of all the diameter measures. Usually speaking, a qualified motorcycle connecting rod requires roundness of 0.003mm for both big-end bore and small-end bore. Therefore, the perfect bore will ensure the engine operate smoothly and the durability of connecting rod will be greatly enhanced.