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. Because the ports are located to the inside in V-8 engines cylinder head exhaust gasses must flow across the head before exiting through the exhaust manifold.The famed ford Flathead ford V-8 suffered from severe cooling problems for that reason. Lincoln V-12 engines from the late forties/early 1950's were even worse as the engine was was two cylinders larger, reducing the ability of the engine to shed heat on its own. Lincoln V-12s were famed for head gasket problems, because if you weren't super careful with the oil and coolant levels the motor would overheat, which often warped the heads.

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. Nor is cooling a problem with a single cylinder. However, L-head engines were quite common up through the middle 1950s.

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