A flathead engine can be differentiated from many other internal combustion engine designs by the fact that the entire valve train is located within the block. This early engine design came with several different advantages and disadvantages, and has become much less common than overhead valve (OHV) and overhead cam (OHC) engines. Flatheads used in automobiles included inline four cylinder engines, straight eights, and even a large V12. They were also popular in motorcycles until they were replaced by panheads and other designs. Due to the location of the valves inside the engine block, the heads on a flathead engine are typically little more than solid pieces of metal.
Many internal combustion engines have their valves located within the cylinder head. Flathead engines have these valves in the block, next to the pistons. The primary advantage of this design is that the drive mechanism for the valves can be substantially simpler. OHV and OHC engines require components like rocker arms and pushrods to operate the valves, but this can be done more directly in a flathead engine. Another advantage, particularly in sidevalve engines, was that the driver could continue operating their vehicle without doing substantial damage to the engine if a valve happened to drop.
One of the biggest advantages offered by flatheads derives from the relatively small overlap that occurs when neither exhaust or intake valves are closed. Due to the manner in which the valves are opened by pressing directly on them, and the speed at which they can open and close, this overlap is typically shorter than in OHV or OHC engines. This may allow a flathead engine to perform better at lower revolutions per minute (RPMs), and gives the engines the reputation of being difficult to stall out.
Most of the drawbacks inherent in flathead engines are also related to the valve design. The method by which they take in the fuel and air mixture as well as expel the combustion gases can be less efficient than many OHC and OHV engines, resulting in poor performance at high speeds. Flatheads also have a lower maximum compression ratio than many other engine designs, which can contribute to lower efficiency.
Modern use of flathead engines is mainly limited to custom built and restored vehicles. In these applications, the use of an authentic flathead engine may be more important to the rodder or customizer than the potential drawbacks that are inherent in the design. They are also sometimes used in small engines such as lawnmowers, where the simplicity of the valve structure is useful and high speed performance unimportant.