Valve Train Geometry

[Jim Harding]

In an earlier post of mine about rocker arms and pushrods, (Erson FE Rocker Arms), Brent attached a diagram of a rocker arm and valve tip to demonstrate proper valve train geometry. I have also seen this diagram in Comp Cams catalogs. They state, “What is the correct length pushrod for your application? The one that produces correct valve train geometry. What is correct valve train geometry? When the rocker arm roller tip rolls from the intake side of the valve tip, across the center of the tip (at approximately mid lift), to the exhaust side of the valve tip (at full lift) and back.” They then refer to the diagram to visually explain this.

When it came time for me to measure for new pushrods for my new set of rocker arms, I tried to apply this method. Problem is I could see no direct correlation between push rod length and roller to valve tip geometry. As can be seen in the picture below, there is no pushrod. The rocker roller is positioned over the valve tip. The only thing the pushrod would do is impart movement of the roller to travel over the valve tip as it pushes down on the valve and spring. How far it travels would be in direct relation to how much lift the cam imparts on the lifter and pushrod.

So what mechanism will adjust the position of the rocker arm roller to the preferred position? At first glance, it would seem that the distance between the center line of the rocker shaft and the center line of the valve stem would have to be adjusted. Or the length of the rocker arm (valve end) would have to be lengthened or shortened. In my case, the Erson assembly drops in over the studs and sits on the head. There is very minute slop or play when positioning the assembly, and the installation instructions do not address this. And of course there is no adjustment to change the position of the roller ball in the rocker arm. You get what you get, and hope the designers did their homework when they produced their product.

So what do you do to determine the correct pushrod length? Erson recommends that when selecting the length, have between one and three threads of the lash adjustment screw visible on the underside of the rocker arm. I chose two. Then with the rocker arm assembly installed, using an adjustable pushrod length checker, lengthen or shorten it while positioning a feeler gauge between the roller and valve. What you should end up with is a pushrod length that will allow you to set the valve lash as recommended by the cam maker and have from one to three threads of the adjustment screw visible. There was no mention of “valve train geometry”. When I talked to the tech guy at Smith Brothers Pushrods, he basically said the same thing.

I admit I’m no expert mechanic, and I’m throwing this out here for discussion. Maybe I’m missing something here that someone with more knowledge and understanding can educate me and others that may also be scratching their heads over this subject.