What Is a Flathead Engine?

A flathead engine refers to a vintage type of gasoline engine where the intake and exhaust valves are located in the engine block right next to their corresponding cylinders, and actually open into a side recess in the cylinder head to the side of the cylinder. Since the cylinder head has no valve operating mechanism, such as in overhead valve and overhead cam designs, the head is virtually a flat cast plate with only tapped holes for the spark plugs, and thus the flathead name. This engine is also referred to as an L-head or side valve engine.

History

While it disappeared from automotive use over 50 years ago due to performance constraints, the flathead still found later acceptance as a smaller engine in tractors, lawnmowers and some motorcycles because of its relative simplicity, reliability and ease of manufacture.

Famous Flatheads

The most iconic flathead automotive engine ever produced was the Ford flathead V-8 produced from 1932 through 1953. Its ruggedness made it a favorite for reworking for hot rods. Military Jeeps used the flathead design for decades for both its four and six cylinder engines. Harley-Davidson made its flathead V-shaped motorcycle engine famous from 1929 through 1974. Legendary Briggs and Stratton lawnmower engines produced for nearly 100 years continually demonstrated the reliability of the design.

Performance Limitations

While this engine proved to be a relentless workhorse, the side valve orientation predicated performance limitations that were eliminated by more advanced overhead valve and overhead cam designs. The incoming fuel air charge had to take two 90 degree turns as it flowed into the cylinder, as did the exhaust as it left. The engine layout complicated modifications.

Heat Limitations

On the Ford V-8s, valving in the middle of the V required the hot exhaust to flow to the exhaust manifold outside the V through passages cast in the engine block, increasing the burden to the cooling system. Under sustained high load conditions, such as lugging up a long hill, these engines were susceptible to overheating.

Living Legends

The flathead engine forever forged its way into Americana with incredible legends about what its owners were able to do with it. Some Ford V-8 owners demonstrated virtually silent idling at 300 RPM ,which is an astounding claim for any engine. Today there are still flathead engine clubs and societies to herald this amazing engine.