Rocker Arm Geometry And Valvetrain Alignment

What has been alluded to in the past as well as the present is a view of the sweep of the rocker tip across the end of the valve stem that conforms closely to maintaining the correct VTG. This only takes into consideration half of the rocker arm geometry to the valve side, and we'll cover the other side later. Here is what has been offered.

It has been said that with a roller tip rocker, the contact point with the valve stem should, at the beginning of the valve opening cycle, move toward the exhaust side of the head, reach an apex at approximately mid-cycle, and then move back toward the intake side of the head to about the same position as when the cycle started as the valve becomes fully opened. If this occurs, then the geometry is necessarily a certain configuration and cannot be any other way.

In geometry, when this occurs, the angle formed by a line through the rocker shaft axis and the roller axis is at or very near 90 degrees to the valve stem when the valve is at the mid point in its travel toward full open position. Miller stated as the patented goal of his Mid-Lift(r) system that the angle at mid-lift would be exactly 90 degrees. Opinions vary on that point, but few would disagree that deviating too far from this concept is not advantageous.

By positioning the rocker arm height in a certain way, the contact sweep path may or may not be centered on the valve stem. In a perfect world, this would be the case. But there are variations in the machining of the blocks, placement of the rocker studs, rotational length of different rocker arms, and lengths of the valves that will alter slightly the position of the rocker tip sweep path on the end of the valve stem. The critical element is not the positioning of the sweep path on the valve tip; it is the geometric angles of the rocker pivots to the valve stem.

Of course, we don't want to see the roller rocker contact point move any closer than 0.020 inch to the edge of the valve stem, as Smokey has stated, but being off-centered has little negative effect on components such as valve guides and valvetrain harmonics and no effect on performance. It is not a performance enhancement to have the sweep path centered on the valve stem, nor does it necessarily mean that we have the correct VTG if it is centered.

The ideal geometric configuration must be repeated on the pushrod side of the rocker, too. With a little effort on the part of the builder and a correctly designed rocker arm, we can choose the right length of pushrod so that our geometry is correct. The key is to find a position of the pushrod cup that will mimic the geometry we have achieved on the other side of the rocker shaft and then choose a pushrod length that will match that location.

If we treat the center of rotation of the tip of the pushrod the same as we did the center of the roller tip shaft when we were finding angular measurements, then as the valve is being lifted at mid-cycle, the angle from a line drawn through the rocker shaft axis and the pushrod tip axis to the pushrod should be very close to 90 degrees. The fact that we have an adjustment for valve lash means that we can use pushrods of different lengths to achieve better VTG.

Again, every company has different design goals, and being exactly 90 degrees at mid-cycle is not the precise goal of all designers. Wrong is not necessarily defined as being a few degrees deviant from 90.

In the quest for proper VTG on the pushrod side, we shouldn't back out the adjuster so far as to weaken the structure by not having sufficient threads in place to resist the forces the rocker will experience. In some cases a compromise may be necessary, but only to a point. Being way off on the VTG may indicate that you have the wrong parts for the types of heads, or the rockers you have aren't meant to be used with the length of valves you have installed.

For example, if I need to turn out my adjuster for proper VTG to the point at which there are not sufficient threads to contain the valvetrain forces, I can use a longer valve, move my rocker up while maintaining proper VTG on the valve side, and then screw in the adjuster further to use more threads when I am adjusting valve lash on solid lifters or preload on hydraulic lifters. Be sure to watch where the roller rocker is sweeping the valve end and make sure it is not coming too close to the edge.