Sidevalve

Sidevalve refers to an internal combustion engine design in which the intake and exhaust valves are located in the engine block beside the cylinder, rather than in the cylinder head. This arrangement, commonly called a flathead (L-head) design, uses a flat cylinder head and valves that open into ports in the block. The camshaft is typically placed in the block and operates the valves through lifters and pushrods to side-mounted rocker arms.

In operation, the combustion chamber is formed by the piston surface and the block, while the valves sit in side-valve seats cast into the block. Air-fuel mixture enters and exhaust gases exit via the side ports. The design is known for its simplicity and compact layout, which kept manufacturing costs low and suited early automotive and small-engine use.

Advantages include low production cost, simple construction, durability, and good low-end torque at modest rpm. Disadvantages include restricted airflow and less efficient combustion at higher engine speeds, which limits high-RPM power and overall efficiency compared with overhead-valve designs.

Historically, sidevalve engines were common in early automobiles, notably the Ford Model T, and widely used in motorcycles and small industrial engines. They were largely superseded by overhead-valve designs from the mid-20th century, though the term remains used for vintage engines and some low-cost or small-displacement applications.