Harley-Davidson motorcycle flathead engines

The flathead engine saw service Harley-Davidson motorcycles beginning with the "sport" model opposed twin produced from 1919-1923, and continuing in 1924 with single cylinder export 350cc and 500cc singles and continued in the Servicars until the 1970s. In the domestic U.S. market, the DL model, 1929-36, started Harley's side valve tradition in the 45 cubic inch displacement. The DLs featured a total loss oiling system (oil's always clean!), and were succeeded in 1937 by the WL, which had recirculating oil. The WL went on to serve in WWII as the U.S. and Canadian Army's primary two wheeled mount and subsequently as a civilian middleweight through 1952. The engine continued virtually unchanged with the GA designation in the three wheeled Servicar until production ceased in 1976.

In 1952, the W series was supplanted by the K series flatheads, designed to compete with British sporting motorcycles of the time. The K models featured unit construction engine and transmission cases, right side foot shift and left side foot brake, and evolved from 45 cubic inches to 55 cubic inches over the short life of the retail market run. The K series was replaced by the overhead valve Sportster series in the retail market in 1957. However, racing versions of the K model continued to be produced in very limited numbers for some time after, winning both roadraces and dirt track through 1969, when the American Motorcycle Association decided to change the rules and make the venerable flatheads uncompetitive. The K racers were replaced first by the iron XR 750cc overhead valve engine, and two years later by the alloy head XR, which continues in service in flat track racing to this day.

In 1930, the 74 cubic inch VL flathead replaced the JD, which featured intake over exhaust (IoE) valve configuration. The VL had a single downtube frame and total loss oiling, culminating in an 80 cubic inch version in 1936. In 1937, the engine was given the U designation and went into the same frame and running gear configuration as the overhead valve Knucklehead, which originated in 1936. The U continued to be produced, in varying configurations as both a 74 cubic inch and 80 cubic inch motor through 1948. By that time, the first year of the Panhead, it had been thoroughly superseded and outsold in the marketplace by the superior performance of the overhead valve model big twins.


Harley-Davidson engine timeline

Flathead: 1929–1936 *Big twins until 1948 commercial market as models UL and ULH, WL 45 cubic inch until 1952-some literature indicating existence up to 1952 engine serial numbers displayed at motorcycle shows, old dealer catalog showed as optional engine until 1952 for limited market- sidecar use or military/ government contract completion . K models 1956 - was model prior to OHV CH /sportster. KRs made/raced even later. Servicar/'trike' noted preceding sentence to 1970s. RN*

Knucklehead: 1936–1947

Panhead: 1948–1965

Shovelhead: 1966–1985

Evolution: 1984–1999

Twin Cam 88: 1999—

Revolution: 2002—