Every diesel engine relies on a precise sequence of valve operations to breathe, compress, ignite, and exhaust air and fuel. At the heart of this process are rocker arms — mechanical levers that translate the camshaft’s motion into valve movement. Whether intake or exhaust, rocker arms ensure that your engine performs efficiently and smoothly under heavy loads.
What Is a Rocker Arm?
A rocker arm is a pivoting lever located in the cylinder head that transfers motion from the camshaft (or pushrod) to the engine’s valves. It acts as a bridge between the camshaft and the intake/exhaust valves. When the cam lobe pushes on one side of the rocker arm, the opposite end presses down on the valve stem, opening it at precise timing intervals.
Types of Rocker Arm Systems in Diesel Engines
- Overhead Valve (OHV) Design: Common in older engines like the Cummins 5.9L or Detroit Series 60, where rocker arms are driven by pushrods.
- Overhead Cam (OHC) Design: Found in newer diesels such as the Volvo D13 and Cummins X15, where the camshaft directly actuates rocker arms above the valves.
Difference Between Intake and Exhaust Rocker Arms
Though they look similar, intake and exhaust rocker arms serve different purposes:
- Intake Rocker Arms: Control air/fuel entry into the combustion chamber. They open the intake valves during the suction stroke.
- Exhaust Rocker Arms: Control the release of exhaust gases. They open the exhaust valves during the power and exhaust strokes.
Some engines use a single rocker arm design for both valves, while high-performance diesels have separate arms for each.
Common Symptoms of Worn or Broken Rocker Arms
- Engine ticking or tapping noises.
- Loss of power or rough idle.
- Misfires caused by incomplete valve operation.
- Increased exhaust smoke due to poor combustion.
- Uneven wear patterns on valves or cam lobes.
Causes of Rocker Arm Failure
- Inadequate lubrication (oil starvation or clogged passages).
- Excessive valve lash or improper adjustment.
- Over-revving or high boost pressures causing excessive load.
- Fatigue from long operating hours or poor material quality.
How to Inspect and Replace Rocker Arms
During routine maintenance or overhaul:
- Remove valve covers and visually inspect for cracks or worn rollers.
- Measure valve lash and adjust according to OEM specs.
- Replace rocker arms showing pitting, looseness, or discoloration.
- Lubricate properly before reassembly to prevent dry starts.
Importance of Valve Lash Adjustment
Improper valve lash is one of the leading causes of rocker arm wear. Too tight, and valves can burn; too loose, and rocker impact causes premature wear. Always follow your engine’s service manual for valve adjustment intervals.
Heavy Duty Kits: Premium Diesel Rocker Arms
At Heavy Duty Kits, we carry rocker arms and valve train components for Cummins, Caterpillar, Detroit, Volvo, and Isuzu engines. All parts meet OEM tolerances to ensure long-lasting performance and precise valve timing.
FAQs
What do rocker arms do in a diesel engine?
They open and close the intake and exhaust valves by transferring camshaft motion.
How do I know if my rocker arms are bad?
Listen for ticking noises, power loss, or misfires — all signs of worn rocker arms.
Do rocker arms need regular maintenance?
Yes. Regular valve lash adjustments and oil changes help prevent rocker wear.