More on the Harley-Davidson Sportster’s Four Cams

After speculating that Harley leadership, on their 1924 overseas trips, might have seen something to suggest they give their brand new 'A' and 'B' 350 singles two camshafts each (see "How the Sportster Came to Have Four Cams"), I looked at what had been hot on the British scene at the time.

That turns out to have been AJS's TT-winning OHV 350 singles, one of which had rather sensationally won the 1921 Senior race (beating all the 500s), so remarkable was its performance. Since World War I, AJS had won the Jr. TT in 1920, 1921, and 1922, coming second in 1923. Outstanding.

Why was the AJS so fast? The company was an early adopter of overhead valves in a racing scene still populated mainly by side-valve bikes.

And further, it turns out that those AJS machines had two separate camshafts, each located directly under its “mushroom-style” flat tappet and pushrod. Each camshaft’s drive gear engaged the crankshaft pinion at the necessary two-to-one ratio.

If another example were needed, it was provided by Humber, who provided a useful variation. The “shoe,” on each of its two tappets was rectangular so that it was prevented from rotating by the nearby presence of a flat surface. Thus, whilst adjusting the tappets of your Humber you needed only two wrenches—one for the threaded clearance adjuster and one for its locknut. You did not need a third one to prevent a round tappet from rotating.

AJS/Matchless OHV singles (like the battered 1952 AJ desert bike I owned briefly in 1963) continued to employ the two camshaft valve train with round-shanked flat tappets and parallel pushrods to the end of their production in 1966.

This doesn't tell us whether or not those peripatetic Harley execs even looked at the AJS/Humber valve train. Harleys would also in time adopt roller tappets, which in 1924 were being used in the Wright/Lawrance J4 Whirlwind radial aircraft engine. Because radial engines employ large diameter cam rings rather than cam shafts, it was not possible for them to use flat tappets (because each lobe on the cam ring would hit the edge of the tappet rather than its face). That made roller tappets a necessity on radial engines. Wright or its tappet supplier having done the R&D to make the things reliable would make them attractive to other users—possibly such as Harley.

The above is an entirely circumstantial argument, suggesting that those Harley execs may have seen the trendy AJS and considered its design as they contemplated what sort of 350 to build back in Milwaukee. And when The Motor Company decided to build a new “45” V-twin in 1929, features of those singles offered themselves as models for the cylinders of that new twin. Or the choice of two separate camshafts for the Harley A and B-model singles may have been entirely independent, the result of an unnamed engineer’s preference. Unless someone out there knows for sure, that may be as far as we can get with our quest for the origins of the Harley Sportster’s four separate camshafts.

Somehow, the Sportster and its V-twin predecessors ended up with parallel pushrods on each cylinder, each with its own roller tappet and camshaft directly beneath it. And all four camshafts are geared together. The house I grew up in burned down. The high school I attended is now a call center. But a few things in life can be relied upon to remain constant—always the same—like the Sportster’s four separate camshafts.