LO 9-5 Identify and describe the purpose, construction, and operation of components used in the valve train

LO 9-05: Identify and describe the purpose, construction, and operation of components used in the valve train.

Rocker Arms

Rocker arms convert the linear change of the cam lobe profile to the reciprocating motion used to open and close the valves. The arms may be constructed of stamped, cast, or forged steel. A roller is incorporated into the levers of overhead cam engines to reduce friction. A much wider roller is used on the exhaust valve rocker lever because cylinder pressures are higher when the exhaust valve opens, and the loading of the roller is much greater than on the intake valve. The roller for an injector lobe is the widest because injection pressures can reach as high as 33,000 psi (2275 bar) on engines such as the Cummins ISX. Hardened wear pads may be welded to the arm, or the arm may be induction hardened for increased durability. If rocker arms are of different lengths on the engine, the exhaust rocker lever is often longer than the intake lever Figure 46. This is due to the cam profile of the exhaust cam lobe. A less aggressive profile with a smaller lift is used on the camshaft’s exhaust lobe to minimize wear. A low rocker arm ratio means a greater mechanical advantage is used on the exhaust lever to multiply the cam lobe lift and open the valve more rapidly. This translates into less friction on the exhaust cam lobe. A larger exhaust rocker arm lever ratio is not a rule, though—there are many exceptions.

FIGURE 09-46: This Detroit Series 60 diesel uses a rocker arm with two levers to actuate the valves. Note that the injector and exhaust valve rollers are wider due to the greater loading forces.

©2026 - CDX