Although the pushrod appears nothing more than a mere pipe, this component is crucial to engine performance and reliability. Resting on top of a valve filter and running all the way up into the rocker arm, the rod is used for activating the rocker arm and operating the valve. As the valve opens and closes, fuel and air get in while the exhaust comes out of the combustion chamber. With this pipe, the rocker arm is positioned at the center over the valve tip. The rod also transfers oil from the filter and out of the rocker arm all the way to the cylinder head. If not for a properly working rod, the valve spring won't be cooled and the rocker won't be lubricated. In some vehicles, the pushrod may be designed to operate along with the valve guide.
Why do pushrods break?
When the valve opens and closes under extreme pressure from the coil spring, the rod is exposed to vibration and shock. The closed valve has to be opened quickly, and in order to do so, this requires a certain amount of energy. The rod has to withstand excessive force that ranges in magnitude while maintaining accuracy in motion based on the camshaft's profile. As the engine speed increases, it will be exposed to greater strain based on the aggressiveness of the camshaft's profile as well as the pressure from the valve spring. The pushrod may deflect and bend if it is not manufactured and designed for heavy-duty use or extreme applications.
How to find the right pushrod
- The rod should be designed for a specific application. It should have the right length, prescribed wall thickness, as well as the end configuration to meet the engine requirements and match the valvetrain design
- If a valve guide or guide plate is used, the rod has to be hardened or at least made sturdy enough to withstand friction as it moves or rubs against the plate. If the rod isn't strong enough or is made from a weak material, it may get thin and eventually bend.
- The rod should be in precise length. It can't be too short or too long. Otherwise, it won't be able to set the rocker arm at the center of the valve tip. This will result in a damaged tip and rocker arm and may ruin the engine's performance. Usually, pushrods come in fixed lengths. Depending on the valvetrain design, pushrods of different lengths may be used in the same engine. The adjustable rod is used to figure out the required length or measurements of the pushrod according to a specific application or for accurate rocker arm geometry.
- Not all pushrods are built with the same diameter. Check the engine's requirements and valvetrain design as the diameter of the rod is crucial for load use.
- Pushrods are typically made of high-tempered steel. For engines geared toward higher performance, rods made of chrome-moly material may be used for full protection.
- They may come as a single piece or may be made into several parts or pieces, which include the stem that has roller balls at both ends. The single piece is commonly used for high-performance applications to prevent separation that can result in serious engine damage as the thin wall design may cause the rod to bend easily because of the load of the racing valve springs. Stock pushrods or those used for low performance are typically designed into several parts or pieces.