A common question at S&S Cycle and Fix My Hog: “Will automotive tappets perform like S&S tappets on my Harley Davidson?” This is not a crazy question, as both these lifters have identical outside body dimensions. In other words; they fit.
The answer, however, is no.
In this video lesson from S&S Cycle, Bruce Tessmer from S&S Cycle uses the S&S “End of the Line” engine testing machine to demonstrate why these tappets are not interchangeable. This engine is computer controlled with an electric motor that can spin the motor at any determined RPM. The machine uses heated oil to simulate actual running conditions.
This demonstration shows the most important difference between the Harley Davidson tappets and the Chevy V8 tappets. The most important difference is top end oil delivery. On a Harley Davidson V-Twin, oil must flow through the rocker arms to cool valves and valve springs.
In this demonstration, Bruce uses two cameras and ultra violet lighting to view top end oil delivery. The Chevy V8 tappets are installed in the front and the premium Harley Davidson tappets are installed in the rear. The test motor is run at 1k RPM with 13 PSI for oil pressure and 175° for oil temp. The result: the Harley Davidson tappets provide 80cc oil per minute to the rear valves while the Chevy tappets only provide 4cc oil per minute to the front.
The Chevy tappets are not inferior. They are simply manufactured for a different application. The Harley Davidson premium tappets are made to run in an air cooled engine with heavier oil (20W 50) and a wider spectrum for oil temp and pressure. The Chevy V8 tappets run in a liquid cooled engine with lighter weight oil (usually 5W 20). Both oil temp and pressure are more consistently regulated. Although Chevy V8 tappets may fit your Harley Davidson engine, they should not be used.
Thank you to S&S for sharing this video. Please visit their Tech sections.