The Flat-Head Motorcycle Engine- Kevin Cameron's Top Dead Center | Cycle World

Looking Back: The Flathead Engine

Harley gave it up in 1970, but...

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Technical Editor Kevin Cameron shares his wealth of motorcycle knowledge, experiences, insights, history, and much more.

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Modern engines have their valves directly above the piston (OHV = overhead valve), giving a compact, fast-burning combustion chamber of minimum volume, allowing use of torque-boosting high compression ratios. It wasn’t always so—major automakers built millions of flathead (a.k.a. “side-valve”) engines through the 1940s, with Chrysler (revered by hot-rodders for its OHV “Hemi” V8s, beginning in 1951) building its six that way until 1959. Until the easy availability of gasolines of higher Octane Number, permitting OHV designs to use high compression, the flathead did pretty much as good a job as needed doing.


In a flathead, the valves are placed beside the cylinder, stems down, with the plane of their heads in the same plane as the piston’s crown at top dead center. They were called flatheads because without valves in the head, the head could be what the name says—flat—and little more than an inch thick.

The central appeal of the flathead idea was simplicity and durability. With valve stems pointing down, the tappets and camshaft(s) could be barely higher than the crankshaft itself. This allowed easy enclosure and lubrication, whereas OHV, whether by pushrods-and-rockers or by overhead cam (OHC), required placing the valves and their mechanism on the cylinder head, up too high to be conveniently lubricated. Valve gear of early OHV motorcycle racing engines of the 1920s had to be lubricated by hand or by grease cup, and only by about 1937 did manufacturers master the complexity of circulating oil up to cylinder head level to lubricate valve gear.

Another important advantage of flathead valve operation was its lightness and minimum number of moving parts, making high rpm reachable even with modest valve spring pressure. There was the cam lobe, the tappet or lever that followed it, and the valve itself, lifted directly by the tappet. When the extra weight of pushrod and rocker was added, valve spring pressure had to essentially double (and many manufacturers added separate tappet and rocker springs to help provide it). This added pressure on cam and follower surfaces forced much redesign.

The negatives of the flathead are:

  1. Cylinder head and combustion chamber surface area must increase by the amount necessary to enclose the valves in their beside-the-piston position. This robs combustion gas of its heat.
  2. Where does that lost heat go? Into the cylinder itself, distorting it and leading to further loss from combustion gas leakage.
  3. A battle is fought between the need for high compression (which requires minimum combustion chamber volume) and the need for generous flow paths for engine intake and exhaust flow (important parts of which are in the head).

At first it looks like a fourth negative should be the length and awkwardness of the flathead’s combustion chamber—a kind of shallow inverted bathtub that must cover both the piston and its valves. That looks like a long, slow way for the flame to travel from just one spark plug!

But at least three innovators saw a way to turn this drawback into advantage. One was Charles B. Franklin, an Irish racer/builder with extensive experience at Brooklands, England’s great speedway. He later became Indian’s chief engineer in the US, where his Powerplus-based Indian flathead racers often defeated the "more modern" OHVs of rival makers. How?

A motionless correct mixture of gasoline vapor and air burns quite slowly—too slowly to make the IC engine practicable. The key ingredient is mixture turbulence, which stretches, tears, and whirls the spark plug’s flame kernel quickly to all parts of the combustion chamber.

What Franklin discovered—almost certainly by accident—was that if part of the head were located very close to the piston crown at top dead center (TDC), the mixture between the two would be rapidly “squished” out as the piston approached TDC, vigorously agitating the mixture in the rest of the chamber. With squish, Franklin’s flatheads burned so fast that they didn’t have time to develop knock (the abnormal form of combustion that can quickly destroy pistons). Almost certainly, Franklin’s flatheads were so fast because, well-defended as they were against knock (a.k.a. detonation) by squish, he could safely raise their compression ratios higher than could tuners running supposedly “more modern” OHVs like Harley-Davidson’s eight-valve racers.

Near the end of the reign of Harley’s postwar K-model-based 750 flathead racer, a sociable afternoon at the shop of California tuning legend C.R. Axtell resulted in a power boost for that engine so considerable that Triumph’s previously dominant OHV 500 twin was made obsolete overnight. What Neil Keen, Axtell, and a couple of others stumbled upon and refined was that Harley’s design emphasized compression ratio over flow. When the part of the head through which flow from the valves to the descending piston was enlarged, more power tumbled forth (the 1968 and 1969 KRs therefore ran on absurdly low compression ratios just under 6:1. Modern OHC sportbike engines typically run 12:1).

Just this morning, reading on a flathead forum, I learned that a clear thinker identified only as "Frank" had made an admirable related discovery while racing 5-hp Briggs & Stratton flatheads in a class full of rules. He reasoned that with the valves as large as they were and the required stock head so close to them, flow was being limited by valve masking. Leaving the head stock, he moved it toward the valves by reducing the shank diameter of the head bolts. Then he cut the valve diameters down until they seated only at their edges. Both of these changes provided more flow area around the valves and so made more power. As he says on his forum post, “These two things helped us get suspended from racing and asked to never return.”

For racers, having your innovation banned is the highest praise!

Innovation comes from having a problem to solve, hardware in your hands, and ideas turning over constantly in your head. What if we…?