L-head, also known as a flat head refers to a type of cylinder head design commom up until the early 1950's. In an L-head engine, the entire valve train is located inside the engine block. The cylinder head contains no moving parts. This sort of engine has a low deck height, making for a compact engine, and for the time offered good torque. However, the design does not allow good air flow, as valve location mandates a shaped port after each valve. Those ports increase combustion chamber size, limiting compression ratio. the result is an engine of limited efficiency. The famed ford Flathead ford V-8 suffered from severe cooling problems.

In an L- or flat head, the valves parallel the cylinder. A traditional L-head designates an engine where all valves are on the same side of the head. When there is a second camshaft and valve train on each side of the head -- practical only in inline engines-- the engine is described as a T-head. Early Stutz engines were T-heads, which bring the advantages of a crossflow configuration. In both T and L heads part of the port is included in the combustion chamber. Today, L-head engines are made only for lawnmowers, where ultimate performance is readily sacrificed for simplicity. However, they were quite common up through the late fifties.