Flathead motorcycles
Flathead motorcycles are a type of bike that was a standard for pre-war motorcycles, in particular US V-twins such as Harley-Davidson and Indian, some British singles, BMW flat twins and Russian copies thereof.
Flathead motorcycles have side-valves contained within the engine block, instead of in the cylinder head, as in an overhead valve engine. This early engine design has mostly fallen into disuse.
In 1925 Cleveland Motorcycle Manufacturing Company released a motorcycle with a T-head four-cylinder engine designed by L. E. Fowler.
Harley-Davidson flatheads
Model W, singles, and 45s (DL, RL, and WL)
The flathead engine saw service in Harley-Davidson motorcycles beginning with the Model WS flat-twin, produced from 1919 to 1923, and continuing in 1924 with single-cylinder export-model and singles and continued in Servi-Cars until 1973. In the domestic U.S. market, the D model and its technical descendant, the R model, started Harley's side-valve tradition in the 45-cubic-inch displacement class. The D and R models featured a total-loss oiling system and were succeeded in 1937 by the W 45, which had recirculating oil lubrication. The WLA went on to serve in World War II as the U.S. and Canadian Army's WLC primary two-wheeled mount and subsequently as a civilian middleweight through 1952. The engine continued virtually unchanged with 2.745" bore and 3.8125" stroke with various G-based designations in the three-wheeled Servi-Car until production ceased in 1973.K-series and developments
In 1952, the K series flatheads was introduced with the same bore and stroke, selling in parallel with the W series, designed to compete with British sporting motorcycles of the time. The K models featured a 750cc unit construction engine and transmission case, right side foot shift and left side foot brake. From 1954 to 1956, the KH received an increase in stroke to 4-9/16" to bring displacement to 888cc.The K series was replaced by the overhead valve Sportster series in the retail market in 1957.
The American Motorcycle Association class C rules of 1952 allowed sidevalves of 750cc to compete against 500cc overhead valve bikes. The 750cc KR factory racer was highly competitive in dirt track and road racing, and was produced in limited numbers until 1969, when the AMA changed the rules by increasing the Class C displacement limit to 750 cc. Without the displacement advantage, the KR flatheads were not competitive against 750 cc overhead valve bikes.