Facts – Manton Pushrods

A pushrod is an eccentrically loaded column due to angularity load and arc motion throughout pushrod travel. Pushrods want to deflect most toward the bottom of the column near the lifter side of the pushrod. This is because of the angularity load. In most cases it is best to use the largest diameter pushrod that will fit in the engine. The increased diameter will lessen deflection and allow better valve train control.

When checking and fitting for pushrod diameter it may be necessary to use a single taper or dual offset taper design, with the large end being toward the bottom. This places the larger diameter and increased mass properly to stiffen the pushrod where it wants to flex the most. The added clearance that the tapered design gives through the head and near the rockerarm can really be helpful. The taper on the tube can also help dampen harmonics in the valvetrain.

With a stiffer pushrod column, increased valvelift should be able to be measured statically in applications using a lot of spring pressure. The higher the engine speed the greater the increase will be at running speed. Keep in mind that by increasing wall thickness to a pushrod column does add strength, the percentage of increase is very small. The large gain in column strength comes from increasing the pushrod diameter.

Do not be overly concerned about pushrod weight. The pushrod is on the slow moving side of the valve train. The additional weight of a heavy wall pushrod usually provides a much needed increase in valve train stability.

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Note: In this simplified illustration, you can see that pushrod deflection and compression can cause reduced net valve lift, the result of a foreshortened pushrod. Valve timing (duration and timing) can also be affected by inadequate pushrod stiffness.

Note: Adding to the complexity of pushrod loading are compound angles resulting from offset pushrod cups (in lifters) and angularity relationships among the pushrod, valve lifter and rockerarm. Oblique angles contribute to side-loading and complex load patterns placed on the pushrod. Although some degree of pushrod “shock absorbing” is virtually unavoidable, minimizing such deflection and compression is critical for maintaining proper valve timing.