Valvetrain Geometry

AUTOMOTIVE ENGINE TECH TIPS

- Mike Felber - Felber

This issue I would like to address another common problem I see with automotive engines on airboats... Valvetrain Geometry, or in more common terms? Measuring Pushrod Length.

Correct valvetrain geometry is one of the most important parts of engine building that will dictate the life of the engine, especially at higher RPMs that we are more commonly seeing with higher redrive ratios.

For this example I will use the most common platform (Chevrolet) which uses a stud-mounted rocker arm. In this case many people tend to think that since the rocker is adjustable that pushrod length doesn't matter. In reality it can be more of a problem.

A pushrod length that is incorrect can cause premature valveguide wear, pushrod binding, interference of retainers and rockers etc. It will also sacrifice optimum lift of the valve.

There are many ways that people pitch to check valvetrain geometry and it usually involves some trick of the week tool. I'll show you guys how I do it which uses the only true tool to use in my opinion, or can be done without buying tools whatsoever (If you have access to several different length pushrods).

Many things affect the pushrod length.

Cam base circle. Each cam has a different base circle due to the lift. (Lift of a cam is increased by reducing the base circle, not making the lobe taller)