Automotive Tappets: Too Good To Be True?

It was probably a result of guys who work on cars and bikes noticing that Chevy tappets look just like what they took out of their Harley-Davidson engine. There’s nothing like a little solid research to put the kibosh on something that sounds like a brilliant plan.

VT_Screen-Shot-2016-03-07-at-1.54.35-PM[dropcap]I[/dropcap]magine my joy, as a young lad, when I discovered that I could replace my bike’s wheel bearings with inexpensive trailer bearings from the local auto parts store. Not only was I saving some cash, but I had discovered something that not everybody knew, or so I thought. In any case, I was pretty pleased with myself. Then I figured out that I could go back to the auto parts store and buy ignition points for Chevrolet 6-cylinder cars and they would fit in my Harley cone motor. Gettin’ smarter every day!

Well, let’s leave these fond memories of a simpler time and fast forward to present day. Guys are still trying to save a buck by substituting automotive parts in their V-twins. It’s always tempting to find ways to get things done for less money. Thrift is good, right? Sometimes.

The example that comes to mind is the idea of using tappets for a Chevrolet V8 engine in 1999-later big twins, which includes the 88-in., 96-in. 103-in., 110-in. and 120R engines. It was probably a result of guys who work on cars and bikes noticing that Chevy tappets look just like what they took out of their Harley-Davidson engine. Yes, both tappets look identical! They even have identical dimensions. GM tappets will, in fact, fit in a Harley big twin engine. These also fit in 2000-later Sportster models, but is it a good idea to use them?

There’s nothing like a little solid research to put the kibosh on something that sounds like a brilliant plan. At S&S, we checked out a lot of different tappets before we started selling our own. We tested a lot of brands, and yes, we tested tappets for GM V8 engines. Here’s what we found.

Although the automotive tappets look identical to tappets for a Harley-Davidson engine, they are not the same. The difference is in top end oiling. As you are no doubt aware, modern (since 1984) Harley-Davidson engines oil the top end through the tappets and pushrods. The tappets are an important component in metering oil to the rocker arms, and from there, to the valves and springs.
The two most important functions of motor oil in an air-cooled engine are, in order of importance, lubrication and cooling. Moving parts tend to wear out with no lubrication, and any way you can take some combustion heat away is a bonus. In fact, 1999 and later big twins even have little nozzles called piston oiling jets that squirt oil up on the bottoms of the piston to provide positive lubrication and to provide some cooling. In addition to combustion heat in the heads, you’d be surprised how much heat is generated just by the flexing of the valve springs, so it’s really important to have adequate oil flow to the cylinder heads.

Did you know that motor oil glows under ultra violet light?

It may not seem important now, but remember it. There may be a quiz later. We devised a testing fixture that allowed us to measure oil flow through a tappet under controlled conditions. The values we got weren’t a direct measurement of the amount of oil the top end would get in a running engine, but the tests were very repeatable and the results are valid for comparison. The results showed quite a bit of variation between the different tappets on the market, but with one exception, nothing raised a red flag. When we saw the volume of oil delivered by stock Delco tappets for modern GM V8 engines we were all like OMG! Seriously? For real?!

Comparing an S&S Premium tappet with the Delco tappet, the S&S tappet delivered 80cc per minute in the test while the Delco delivered 4cc. Quite a difference! This prompted further investigation.

We put together a special engine with S&S tappets running the valve train in the rear cylinder and Delco tappets for the front cylinder. This engine was placed on the S&S End Of Line Testing (EOLT) machine that we use to test every engine we build before it gets shipped. The EOLT machine consists of a powerful computer controlled electric motor that spins the engine under test to various rpm, while electronic data acquisition sensors measure what’s going on in the engine. The oil going into the engine is heated so the oil pressure and viscosity is representative of what a running engine would see. We took the top rocker covers off and trained video cameras on the intake valves, turned out the lights, turned on some powerful ultraviolet lamps, and started turning the engine at about 1,000 rpm…idle speed. The computer showed that the oil temperature was about 175° F, and the oil pressure was about 13 psi.

After what seemed like a long time, (42 seconds) oil started to glow at the end of the rear intake rocker arm that was fed by the S&S tappets. After what actually was a long time, (6 minutes) no oil had come out of the end of the rocker arms fed by the GM tappets, so we shut the engine down. This showed us a couple of important things. First, it’s not a good idea to rev your engine right after you start it, no matter what tappets are in it. Secondly, it’s probably not a good idea to run your engine at all if you are using automotive tappets.

Does this mean that the automotive tappets are inferior? Absolutely not! It means that they were designed for a different application. In a liquid cooled automotive engine the oil doesn’t generally get as hot, and oil pressure is generally about 50 psi on the highway and seldom less than 20 psi even at idle. In an air-cooled V-twin engine the oil can get really hot, and oil pressure may only be 20 psi at highway speeds, and often is less than 5 psi at idle. But wait! There’s more! The engine oil specified for a GM V8 is 5W20 compared with 20W50 for the V-twin. So feeding thicker oil through a tappet designed for thinner oil, at lower temperatures, and higher oil pressure seems like it’ll work for what reason? The fact is that is doesn’t work very well at all.

Even though we sell replacement rocker arms, we don’t recommend using automotive tappets in your Harley-Davidson engine. So it’s a matter of pay me now or pay me later. Save a few bucks by using automotive tappets, and pay for some expensive top end repairs down the road (and not too far down the road either).

Motorcycles are only fun when they’re running, so let’s try to keep them running for as long as we can!

Bruce Tessmer has worked in both technical and marketing positions during his 27 years with S&S Cycle Inc. His technical background and the love of teaching are the driving force behind the tech articles and S&S instructional videos he produces.

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