There cannot be a true motoring enthusiast alive who is not aware of the ingenious Riley valve-gear of the pre-war years, wherein inclined overhead valves in hemispherical heads were operated by two camshafts set high in the crankcase so that notably short push-rods could be used.
It has become a classic means of combining the advantages of twin overhead-camshafts with those of simpler push-rod operation. So effective was this valve-gear that it sufficed for the supercharged ERA racing engine, and was used for later Lea-Francis cars and others. All praise to Percy Riley for the design, which surfaced around 1925.
However, at the back of my mind was the thought that somewhere, years ago, I had seen (possibly in the Science Museum) an engine with similar valve-gear. Memory told me it was a Dorman engine.
Doing some research, I find that in 1919 H Dorman & Co of Stafford did have such an engine, which was also innovative in having an aluminium cylinder block with “wet” cast-iron liners, regarded as advanced at the time. Its valve-gear was very like the Riley’s: camshafts on either side of the crankcase, set sufficiently high for the push-rods they operated to be two-thirds the length of ordinary push-rods. Thus, as with later Riley engines, not only were inclined overhead valves possible without the complication of overhead-camshaft valve-gear, but the low valve-gear inertia-factor of a twin-overhead-camshaft engine was achieved, at least in part, by those short, light, push-rods.
It seems both Percy Riley and John Dorman realised that if driving their high’ camshafts might absorb a little more power than would turning one conventional camshaft beside the crankshaft, this should be more than offset by a gain in combustion efficiency, which floating valves (caused by valve-gear inertia) could undermine.
So Dorman seems to have led Riley! Except that in the Dorman engine the push-rods engaged the outer extremities of the overhead-rockers, so the valves were only slightly inclined in a pent-roof head; whereas Percy Riley let his short push-rods engage the inner ends of the rockers, and thus it was easily possible to incline the valves at an acute angle, giving proper hemi-heads. Here is another interesting example, however, that very little in engineering history is entirely new.
In fact, it could be said that using camshafts on either side of the crankshaft to actuate oh-valves, albeit with long push-rods, was but the logical development of the earlier T-head side-valve engine; that is how Pipe, Benz and Fiat probably regarded it when they wanted both inlet and exhaust valves “upstairs”.
Mechanical simplicity came by having only one base-chamber camshaft, and it was only when the power-race called for better breathing, via inclined oh-valves, and better-formed combustion-chambers, that it became necessary either to put the camshaft(s) up top or, like Dorman and Riley, devise more ingenious valve-gear. WB