The L-head inline six was far and away the engine of choice for the U.S. automakers for several decades.

Auburn, Kaiser-Frazer, Pontiac, Oldsmbile, Graham, Hupmobile, Willys-Overland, Packard, Studebaker, Nash, Hudson, Plymouth, Chrysler, Dodge, DeSoto. . . and we surely missed a few. What do all these automobile brands have in common? They were all offered with L-head, inline six-cylinder engines. In the ’30s, and ’40s, valve-in-block sixes were ubiquitous throughout the U.S. auto industry.

Indeed, the L-head aka flathead six, with its valve gear all contained within the cylinder block, was a familar feature in the Motor CIty for decades. The design first became commonplace in the ‘teens but its popularity lasted well into the ’50s, while American Motors offered one all the way though 1965 (read about it here).There were two notable exceptions to the movement, however: Chevrolet, which used a more advanced overhead-valve six, and Ford with its popular and powerful flathead V8. But even Ford gave in and introduced a flathead six in 1941 (see our feature here). America’s two biggest carmakers in production volume, they could afford to go their own way.

1933 Oldsmobile Six

Also remarkable is how similar all these sixes could be, even in their minor details. Displacements were typically in the neighborhood of 220 to 230 cubic inches, with rated outputs of roughly 80 to 95 horsepower. Peak power generally arrived at 3200 to 3500 rpm. A four-main crankshaft was the norm, as were undersquare stroke/bore ratios in the 3:4 range. Nearly all used a one-barrel downdraft carburetor and six-volt battery-coil ignition. The camshaft and valve gear, and the intake and exhaust manifolds as well, were invariably arranged on the right (passenger) side of the cylinder block, in part to make room for the steering gear on the left side of the chassis. The cylinder case and head were cast iron, of course, and compression ratios were typically below 7:1, as dictated by the relatively low-octane fuels of the era and the inefficiency of the L-head combustion chamber.

1930 Marquette showing lubrication circuit

Despite its limitations, the L-head six provided more than enough power and speed for the roads of the day. Many carmakers offered both inline sixes and eights in their lineups, but the six was the popular choice. For car buyers, It was smooth, rugged, and easy to own. And for the carmakers, the L-head six was simple and cheap to manufacture with a minimal component count. Where inline fours were once the standard, engine smoothness became more critical as enclosed body styles took over the industry, and sixes filled the bill.

In the ’50s, the flatheads were finally pushed to the side by more modern overhead-valve sixes and V8s. But there was one final day in the sun for the L-head with the Hudson Hornet Twin-H six, below. Introduced toward he end of the flathead era in 1951 and large for a six at 308 cubic inches, the Hornet was blessed with dual carburetors and a high-strength nickel-iron alloy block, and it dominated stock-car racing in the early ’50s.