Four-cylinder FN motors

In 1914, FN introduced the 7 h.p. four alongside the long-established 5 h.p. . The photo below shows why the earlier machine was called the "lightweight" four! The 8 h.p. motor introduced in 1921 was taller again, with a deeper sump courtesy of an extension plate, and the overhead inlet valves.

Late and early FN motors

Inside, the differences were just as marked. The early motor used a 5-main-bearing crank, running in plain bearings. The lubrication system looks impressive with the individual feeds below each big end, but in reality it was nothing more than a total-loss all splash affair, fed from either a hand pump or drip feed from the oil tank. Four windows in the sump allowed the oil level to be monitored. Even as late as 1913, the inlet valves were automatic.

1913 four motor

Inside the 7 and 8 h.p. motors is a very substantial 2-main-bearing crank, running in large ball races. The big ends are bronze bushes. Notice that the 1922 8 h.p. motor here uses the same ZFN magneto as the 1913 model above. The following year, a Swiss Scintilla magneto with fixed ignition was used.

1922 8 h.p. motor

Despite the mechanical oil pump on the post-1914 four, the lubrication system remained primitive. Basically, the sump carries an amount of oil, which is lifted up by the pump to a tray containing four dippers, one for each big end. Everything from here relies on splash, with the oil draining through the straining mesh into the sump before starting again. When the oil level in the sump falls, it can be replenished from the oil tank. There are three windows in the system; two in the sump and the other on top of the oil pump.

Lubrication system, 1922

Surprisingly even the chain-drive M50 four cylinder, introduced for 1924, retained this lubrication system.

Copyright Leon Mitchell 1999

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