When S&S Cycle opened its doors back in 1958, the first product offered for sale was a solid lifter conversion kit that allowed performance-minded riders to get rid of the troublesome hydraulic units of the day. So if the first product S&S produced allowed performance enthusiasts to get rid of hydraulic tappets (a.k.a. lifters, cam followers, etc.), why are we even talking about them now? Well, that was 55 years ago, and the tappets we have now are a lot better than the ones George J. Smith was eliminating in 1958. Those old lifters were prone to collapse at high rpm, serving as a built-in mechanical rev limiter. If your valve train fails at a certain rpm, it doesn’t matter what other goodies you have in your engine. That’s as fast as your engine is going to turn. Period.
So what are tappets, and what do they do? A tappet is a device that slides or rolls on the cam lobe following the cam profile and translates the eccentric shape of the cam lobe to an up-and-down motion that opens and closes the valves. Harley-Davidson engines use roller tappets, which means they have a small wheel on the lower end that rolls on the cam as it turns. Tappets also provide a way to take up the lash between all the various components in the valve train. Hydraulic tappets contain a hydraulic piston that moves to automatically compensate for engine height growth from thermal expansion, keeping the valve train lash at zero.
Pushrod adjustment is a very frequent topic of phone calls to the S&S tech department. Like most things, it’s pretty simple when you understand it, but with the amount of myth, urban legend, advice from the end of the bar, and plain B.S. floating around, it’s kind of hard to sort out the facts.
In order to adjust a hydraulic tappet, you must use an adjustable pushrod. Stock engines don’t have adjustable pushrods since the factory relies on the self-adjusting feature of the hydraulic tappets to compensate for any machining tolerances in the construction of the engine as well as thermal expansion. It works just fine in that kind of narrowly defined application, but for a performance engine, adjustable pushrods are pretty handy. They can adjust for cams with non-stock base circles, decked heads, shortened cylinders, special valve jobs, roller rocker arms, and a host of other performance modifications that can affect the valve train.
You start with a cold engine, pull out the spark plugs, and roll the engine over slowly until both tappets for one cylinder are at the lowest point of their travel. The valves should be closed at that point. The pushrod-adjusting screw is turned, shortening the pushrod, until it no longer touches the cup in the top of the tappet. That allows the piston in the tappet to rise to its highest position. Then the pushrod is lengthened until the ball-end just touches the cup. You should just be able to turn the pushrod between the fingers with no shake or rattle—just a little roll. The pushrod is then lengthened in order to push the hydraulic piston in the tappet to the approximate center of its 0.2-inch available travel. The number of turns can vary according to the thread pitch of the adjuster screw. S&S pushrods have 32 threads per inch, so we recommend four turns (or 24 flats) of the adjuster nut. Once the locking nuts are tightened, you have to wait about 10 minutes for the oil to bleed out of the tappets. If the engine is turned over before the tappets have bled down, the valves will not be fully closed and could hit each other. You don’t want that to happen. If the pushrods are loose enough to be turned between your fingers (valves fully closed) after 10 minutes, you can turn the engine over until the tappets on the other cylinder are at their lowest point of travel and adjust them the same way. Pretty simple, really.
Sometimes during a tech call about tappet adjustment, “car guys” will ask, “But where do I stick my feeler gauge?” Being as polite as we are, we rarely tell them where to stick it. We just tell them we don’t use feeler gauges.
We’ve covered what hydraulic tappets are, what they do, and how they’re adjusted. Next, we’ll delve into what’s inside them that actually makes them work.