How the Harley-Davidson Sportster Came to Have Four Cams

A unique feature of Harley-Davidson's 'Sportster' engines since their introduction in 1957, is their use of four separate camshafts, all geared together. This is quite a contrast to a classic American car V8, which has a single camshaft. Usually, American engineering is pretty attentive to basic economies like reduced parts counts. Consider that Harley's first OHV big twin, the E and EL of 1936 to 37, began a tradition of having but a single camshaft with four lobes, which continued until the recent Twin Cam broke from the tradition.

I became curious about this. I knew that the Sportster had inherited its unit construction (crank and gearbox in the same case) and its four separate camshafts from the last of the flatheads—the K-model. The KR racing engine, receiving a final comprehensive soup-up over the winter of 1967/’68, emerged to make the 500 Triumph OHV twins obsolete just before the AMA’s racing formula switched to 750 four-strokes in 1970.

The K inherited its four cam design from the previous ‘W’ (1937-1952), which in turn inherited it from model R (1932-1936), begat by model D (of 1929-31).

And then things get murky. Where did this four-cam idea originate?

In the very early days, reduced parts count was such a priority that a single cam lobe was often used to operate both intake and exhaust valves by locating two rockers at points around it to give the desired timing.

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In 1923, Indian's engineer, Irish former racer C.B. Franklin made a trip to the British Isles to see what was up. The following year, H-D's founding fathers made similar trips—to England, Europe, and Down Under.

Why all this globe-trotting? U.S.-made motorcycles found ready sale abroad because of their ruggedness, so it was important to learn what was selling. What they all discovered was that the most popular machine type, worldwide, was the 350 single. Adventuresome young men might dream of a 500, but four-to-one, the purse dictated the purchase of a 350.

As a direct result of these voyages of discovery:

What caused both U.S. makers to adopt this idea of one camshaft per valve? Clearly, having a look at other makers’ ideas stirred up their thinking. Up to this time, it had been normal to use lever followers, often with rollers to bear on the cam lobes. Now it appears Harley may have decided to adopt roller-tipped sliding square or cylindrical tappets of the kind being used in large numbers by makers of radial aircraft engines—notably the Wright-Lawrance J-series engines, one of which would in 1927 power Charles Lindbergh’s solo flight across the Atlantic. Because these had to be located directly above the cam lobe, the simplest way to have the distinctive parallel pushrods that the A, B, and C-model Harley singles employ was to have a separate camshaft under each. Even though Franklin’s Indian racing singles had two cams, he employed the old type of pivoted roller-tipped operating lever—not the aircraft-style sliding tappets.

When Harley decided to get into the new “45 market” with the D-model of 1929, the cam and tappet arrangement of the A, B, and C “export singles” was evidently adopted. It was then carried forward through R, W, and K/KH models, and then in 1957 was continued when the OHV cylinders and heads of the new Sportster were put atop the K-series lower end. Its distinctive parallel-pushrod appearance comes down to us in the present day. Much of this continuity was surely determined by the need to economize during the Great Depression (beginning Oct 1929) by sticking with what could be produced on existing tooling. Tooling changes are expensive.

Yet a decade after the design of the singles, when Harley engineering was developing the OHV E and EL, they chose to cut costs and reduce parts count by adopting a single four-lobe cam, cylindrical roller tappets carried in two tappet blocks, and the quite different appearance of angled rather than parallel pushrods. This was continued through the Evo Big Twin. The Big Twin's present Twin-Cam embodiment retains the two tappet blocks, but positions them over two cams rather than a single one.

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