Rockers, Pushrods, And Geometry
Transferring the motion of the camshaft to the valves is the job of the valvetrain. The key components here consist of the lifters, pushrods, and rockers, and can include studs and guideplates, depending upon the engine design. Considering a typical stud-mounted valvetrain as typified by a Chevrolet V-8 engine, there are geometry considerations that need to be kept in mind when setting up the valvetrain. With a stud-mounted rocker, the fulcrum position relative to the valve tip varies in relation to the pushrod length. Most engine combinations assembled with aftermarket rockers, cylinder heads, and/or lifters will typically require pushrods of a custom length. Determining the most appropriate length is part of the engine building process, with the objective being to provide the most advantageous geometry possible.
Ideally, the geometry of the valvetrain will provide for minimal scrub of the rocker at the valve tip, with the contact centered on the valve tip. By varying the pushrod length, the fulcrum height of the rocker is changed, providing the engine builder with a means to adjust the geometry. Typically, the valvetrain is mocked up with a light checking spring in place of the valvespring, and an adjustable pushrod is used to find the most advantaged length. The sweep of the rocker can be verified visually by coloring the valve tip with layout dye or a felt marker, and running the rocker through its motion.
As a general rule, the contact point of the rocker is moved inward to the valley side of the engine as the pushrod length is reduced, while the contact point is moved outward with longer pushrods. The shortest sweep is achieved if the length allows the rocker to achieve a 90-degree operating angle at mid lift. Depending upon the specific parts being used, and the associated rocker length, valve length, and stud position, an "ideal" geometry may not be possible, however, the builder must strive to achieve an acceptable "best compromise" through appropriate pushrod length selection.
In addition to the sweep motion of the rocker at the valve tip, an engine builder must consider the mechanical alignment of the rocker tip to the valve. This entails ensuring that the roller is centered on the valve. In an engine employing guideplates, the lateral alignment of the roller can be adjusted by shifting the guideplate laterally. Again, given variations in specific parts, it may not be possible to exactly center a pair of rockers without modifying the guideplates, however a "best compromise" solution of splitting the difference is usually sufficient unless there is a gross misalignment.
A spring tester is used to check the spring load at the installed height, giving the close
A set of light checking springs are installed in place of the valvesprings to assist in ch
A checking pushrod is adjustable in length, and allows the builder to mock up the valvetra
To aid in visualizing the rocker sweep, the tip of the valve should be marked with layout
The contact pattern of the rocker will be clearly seen in the witness marks once the is as
Here we have a virtually ideal pattern, with the contact patch well centered on the valve