side valve
A type of valve in an internal combustion engine, and thus also (metonymic) the type of valvetrain design, the type of engine that uses it, or an instance of such an engine; this design has the inlet and exhaust valves in the cylinder block beside the piston. The cylinder head is then a simple dome extending across the valves and piston.
side valve: valves in the block, not the head
A side valve engine positions both inlet and exhaust valves in the cylinder block, parallel to the cylinder bore, rather than overhead in the cylinder head. The valves sit in a chamber cast into the side of the block, angled to direct flow past the piston. The simple flat or domed cylinder head merely seals the combustion chamber. This layout was dominant in automotive and stationary engines from the 1900s through the 1950s, and remains common in small air-cooled and industrial engines today.
The cam operates the valves directly or through short pushrods that run within the block. Because the valves lie beside the piston rather than above it, the flame front in the combustion chamber travels a longer path, and combustion tends to be slower and less complete than in overhead designs. Port passages must curve sharply inside the block to channel mixture into and exhaust out of the lateral valve chamber. These passages are relatively restrictive, limiting maximum engine speed and peak power output.
Why and where side valves persist
Side valve design remains attractive for low-cost, low-speed applications because the block is simpler to cast and machine, the cylinder head is cheap, and the entire assembly is robust and forgiving of poor maintenance. Motorcycle engines, garden equipment, pumps, and generators commonly use side valve layouts. The mechanism is inherently compact in the vertical direction and tolerates lateral vibration without symptomatic valve train chatter, a real advantage in rough-duty service.
The main drawback is efficiency. Higher compression ratios and faster combustion are harder to achieve, so fuel consumption per brake horsepower is worse than equivalent overhead-valve or overhead-cam engines. The term 'side valve' is sometimes used interchangeably with 'flat head' or 'L-head,' though L-head technically refers only to the specific block shape.
In industrial settings, side valve engines dominate small-displacement stationary power units where initial cost and simplicity outweigh fuel cost. A side valve engine running at 1800 rpm will outlast a higher-revving design under intermittent load, and replacement parts remain inexpensive because the design is still manufactured worldwide in small volumes for utility applications.