Motorcycle: Hardware - F Heads & Flatheads
Date: Sunday, October 31 @ 13:56:58 UTC
Topic: Motorcycle, Motorcycles, Motor Cycle
F Heads & Flatheads
A halfway house between flathead and overhead valve design was the inlet-over-exhaust (ioe) layout. Also called the "F-head", as the name suggests, this configuration combined an overhead inlet valve with a side exhaust valve.
All early Harleys followed this design, with one important additional characteristic. For reasons of simplicity, the inlet valve was of the "automatic" or induction type, in which the descending piston simply "sucks" the valve open only for it to be returned to its seat by a conventional spring. This spring was of necessity light, which precluded high engine revs. One major advantage of the system was that by removing the inlet valve housing, both valves could be removed easily for servicing — a considerable advantage on these relatively primitive engines. It was also found that since the valves directly faced each other, the incoming fuel charge helped cool the exhaust valve, extending its life. As well as the early singles, the first V-engines adopted the same layout - to their cost, for it proved an untimely failure with the twin cylinder design.
Both the revised twin and the 5-35 series singles of 1913-18 moved to a far more positive ioe arrangement with a conventional, mechanically-operated overhead inlet valve. The same design was grafted on to the 74-inch Model J of 1922, also known as the "Super-powered Twin". This 18 horsepower engine was also the first Milwaukee twin to feature lightweight aluminium pistons. When Model J production ended in 1929, Harley twins remained exclusively side-valvers for the next six years.
"Flathead" was the unflattering generic title given to any side-valve engine, whether made by Harley-Davidson or not. The reason for the nickname is obvious: with all the valve gear, including the valves themselves, located below the level of the piston at the top of its stroke, the heads were flat. The first side-valve Harleys were the Series A and B singles of 1926. All roadster twins from the end of the 1920s until the arrival of the Knucklehead in 1936 were flatheads as well. Milwaukee's catalogues continued to feature side-valve models until 1951 and even into the 1970s.
Side-valve engines were popular because they were relatively cheap to make and maintain, as well as much more compact than F-head and overhead valve designs. Although valve control was usually quite good - the camshaft and valves were in close proximity -the side-valve layout inevitably produced an elongated combustion chamber with tortuous gas-flow characteristics and poor valve cooling. This placed a severe limitation on the power such designs could produce. Side-valvers were renowned as dependable plodders, such as the seemingly unstoppable WL45.
Harley's side-valve years threw up one notable exception, however - the overhead valve 21-inch (346cc) single, produced from 1926 until the eve of the Knuckle in 1935. Designed with the help of Harry Ricardo, this single was not only successful in its own right but gave rise to the celebrated "Peashooter" racer. Five years earlier, Ricardo created the Model R, Triumph's first four-valve engine. His company contributed to the development of the latest Triumph triples and Aprilia's new V-twin, the RSV Mille.
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