A push rod is a part of an internal combustion engine that rests in the top of a valve lifter and goes up into the rocker arm. As the lifter follows the cam lobe, the push rod actuates the rocker arm and moves the valve, opening and closing it to allow fuel and air in and exhaust out of the combustion chamber. This rod, being hollow, also channels oil up from the lifter and out of the rocker arm. This oil cools the valve spring as well as lubricates the rocker arm.
On some vehicles, the push rod operates within the constraints of a valve guide. On these applications, the rod must be hardened to withstand the contact of rubbing the guide plate. If it is not hardened, it would be ground thin and would bend. By hardening the rod, it can tolerate the friction with no ill effects or damage.
Another important function of the push rod is to center the rocker arm squarely over the valve tip. If the rod is too short or too long, the rocker arm will not sit on the center of the valve tip. This will cause damage to the valve tip and the rocker arm and will hinder performance from the engine. Many engine builders use an adjustable push rod called a rod length checker to measure the proper dimensions of the push rod needed for a particular application.
While many high-performance rods are made from a single piece of steel, some stock or low-performance rods are made with several parts. These rods use push-in rod ends that resemble small balls to ride in the lifter cups and the rocker arm cups. In a performance application, this type of rod can separate and cause severe damage to the engine. These rods are typically thin wall designs that would fail and bend under the load of racing valve springs, which typically have much greater resistance than a stock valve spring.
Push rods in early engines were often made of a solid piece of steel. The rod was often operating in the open above the cylinder head and was not used to transport oil to the top end of the engine. This allowed the rod to be created from a solid piece of steel, and a bearing was typically used in place of oil for the smooth operation of the rocker arm rod interface. It was soon noted that as engine revolutions and speed increased, the oil was needed to cool the valve springs and prevent them from breaking.