Pushrod

What Is Pushrod?

A pushrod is a slender, rigid rod, typically fabricated from hollow steel or aluminum tubing, integral to the overhead valve (OHV), also known as cam-in-block, valvetrain architecture in internal combustion engines. It is positioned vertically or near-vertically within the engine block, extending upwards from the hydraulic or solid lifter (tappet), which rides directly on the camshaft lobe. The pushrod then passes through a dedicated bore or passage within the cylinder head to make contact with the rocker arm. Its fundamental purpose is to transmit the linear upward motion generated by the camshaft’s eccentric lobe, via the lifter, to the rocker arm. This mechanical transmission is a critical link in the precise orchestration of valve timing, converting the upward force into a downward force on the valve stem tip, thereby opening the intake or exhaust valve against its closing spring pressure to facilitate efficient gas exchange within the combustion chambers.

How It Works

The operation of a pushrod is a sequential mechanical process within the OHV valvetrain:

• Camshaft Rotation and Lobe Contact: As the engine’s crankshaft rotates, it drives the camshaft via a timing chain, belt, or gear set. Each eccentric cam lobe, designed with a specific profile, rotates and comes into contact with the base of a corresponding lifter (or tappet).
• Lifter Actuation: The rotating cam lobe’s rising profile (“ramp”) exerts an upward force on the lifter, causing it to move vertically upwards, away from the camshaft’s base circle.
• Pushrod Engagement and Transmission: The lower end of the pushrod is seated in a spherical cup or socket at the top of the lifter. As the lifter moves upward, it directly pushes the pushrod upwards.
• Rocker Arm Pivoting: The upper end of the pushrod engages a similar cup or socket on one end of the rocker arm. The rocker arm is a lever that pivots on a fixed fulcrum (either a rocker arm stud or a shaft). As the pushrod exerts upward pressure on its end of the rocker arm, the rocker arm pivots.
• Valve Stem Depression: This pivoting action causes the opposite end of the rocker arm to move downwards, applying force directly to the tip of the valve stem (either intake or exhaust).
• Valve Opening: The downward force from the rocker arm overcomes the opposing pressure of the valve spring, pushing the valve open. This action allows the precise ingress of the air-fuel mixture into the cylinder or the egress of spent exhaust gases.
• Valve Closing and Return Stroke: As the cam lobe rotates past its peak lift point and onto its closing ramp, the compressed valve spring’s energy forces the valve closed. This action simultaneously pushes the rocker arm and the pushrod back downwards, allowing the lifter to follow the receding profile of the cam lobe back to its base circle, completing one cycle.
• Lubrication (Optional): In many designs, pushrods are hollow and serve a secondary function as conduits for engine oil. Oil, supplied under pressure from the main oil gallery through the lifters, travels up the hollow pushrods to lubricate the rocker arm assembly and valve tips.

Common Failures & Troubleshooting

Pushrods, while robust, are subject to specific failure modes that can significantly impair engine operation.

• Symptoms of Failure:
  • Audible Valvetrain Noise: The most common symptom is a distinct ticking, tapping, or clattering noise emanating from the cylinder head area, often more pronounced at idle or during light engine load. This can indicate excessive valvetrain lash, a bent pushrod, or wear in other associated components.
  • Reduced Engine Performance: Misfires, rough idling, noticeable loss of power, poor acceleration, or decreased fuel efficiency can occur if a pushrod’s function is compromised, leading to improper valve timing or inadequate valve lift.
  • Check Engine Light (CEL) Illumination: The CEL may activate with diagnostic trouble codes (DTCs) related to misfires (e.g., P0301-P0308 for specific cylinders) or general valvetrain performance, although specific “pushrod failure” codes are rare.
  • Catastrophic Engine Damage: In severe cases, such as a completely broken pushrod or one that causes a valve to stick open, piston-to-valve contact can occur, leading to extensive damage to pistons, valves, and cylinder heads, potentially resulting in engine lock-up.
• Wear Mechanisms:
  • Bending: This is the predominant mode of pushrod failure. It commonly arises from excessive valvetrain stress, such as over-revving leading to valve float, hydraulic lifter pump-up, a sticking valve, or direct contact between the valve and piston. A bent pushrod effectively shortens its length, reducing valve lift and altering valve timing.
  • Tip Wear and Fretting: The spherical ends of the pushrod, where they interface with the lifter and rocker arm, can experience wear, pitting, or fretting due to insufficient lubrication, abrasive contaminants in the oil, material fatigue, or misalignment. This wear increases valvetrain lash.
  • Fracture or Breakage: While less frequent, complete pushrod fracture can occur from severe bending forces, long-term fatigue, or manufacturing defects, leading to a complete loss of valve actuation for that cylinder.
  • Corrosion or Pitting: In engines subjected to infrequent oil changes, moisture ingress, or prolonged storage, surface corrosion or pitting can weaken the pushrod material, predisposing it to failure.
• Diagnosis:
  • Visual Inspection: With the valve covers removed, manually rotate the engine or crank it (with fuel/ignition disabled) while observing the pushrods. Look for any visible bends, evidence of rubbing or interference (shiny spots), or signs of tip wear. Use a known straight edge or specialized pushrod checker tool to confirm straightness.
  • Valvetrain Noise Pinpointing: A mechanic’s stethoscope is invaluable for isolating the source of ticking or tapping noises to a specific cylinder or valvetrain component.
  • Compression and Leak-Down Tests: Low or zero compression in a cylinder, especially when combined with a leak-down test revealing air escaping through the intake or exhaust, can point to a valve not closing properly due to a compromised pushrod.
  • Lifter and Camshaft Lobe Inspection: Often, a pushrod failure is symptomatic of an underlying issue with a worn camshaft lobe, a collapsed hydraulic lifter, or a sticking lifter. These components should be inspected in conjunction with the pushrod.
• Repair and Replacement:
  • Pushrod Replacement: The direct repair involves replacing the affected pushrod(s). It is often good practice to replace all pushrods in a bank or cylinder if multiple show signs of distress or if the engine has high mileage.
  • Root Cause Identification and Rectification: Crucially, the underlying cause of the pushrod failure must be identified and addressed to prevent recurrence. This may include:
    • Replacing worn or collapsed hydraulic lifters.
    • Verifying correct valve spring pressure and replacing weak springs.
    • Ensuring adequate engine oil pressure and flow to the valvetrain components.
    • Checking for proper valve-to-piston clearance, especially after engine modifications.
    • Adjusting valvetrain geometry and hydraulic lifter preload to specifications.
  • Collateral Damage Assessment: Depending on the severity of the failure, further engine disassembly (e.g., cylinder head removal) may be necessary to inspect for collateral damage to valves, valve guides, valve seats, or pistons, particularly if piston-to-valve contact is suspected.
  • Upgraded Components: For high-performance, racing, or heavy-duty applications, upgrading to hardened, chrome-moly, or thicker-walled pushrods can significantly enhance durability and resistance to bending under extreme valvetrain loads.