Pushrod Length and Valve Travel

Ready to finally measure that valve geometry? I am! I have some peace and quiet, a nice beer and everything set up:

Half of a Porsche 914 engine assembled with a dial indicator measuring valve travel


To achieve the valve travel I wanted, the orientation of the swivel foot and to minimize spacer thickness, I decided to machine 0.060" off of the bottom of each rocker arm. Here's the setup in the mill:

Machining the rocker arm of a Porsche 914 engine on a Bridgeport mill


And here the mill in action:


Now, with my dial indicator set up to measure the travel of each valve, I can adjust the height of the swivel foot, adjust the length of the pushrod and change the height of the rocker arm assembly by using spacers on the studs.

Dial indicator measuring valve travel on a Porsche 914 engine


I want to maximize valve travel (and keep the travel as closely matched between cylinders) and make sure that the swivel foot that presses down on the valve is as close to being straight when the valve is at the half way point of its stroke. For example, my exhaust valve travel is 0.486", so at 0.243" of travel, I want the orientation of the swivel foot to look like this:

Exhaust valve on a Porsche 914 engine at half lift


Here is a video showing a complete engine cycle with the dial indicator on the cylinder #2 intake valve. The exhaust valve opens letting the combusted gasses leave the cylinder, followed by the intake valve opening to bring in a fresh charge of fuel and air.

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All pictures and text copyright 2008