Before the advent of the shaft-drive four in late 1904, FN motorcycles were single cylinder machines with belt final drives. They were neat, well-made and purposeful, but not particularly innovative. One thing that was not lacking at FN, however, was development, and it's interesting to see just how the single cylinder engine evolved in the first three years of production.
The photo shows the three engines that powered production FN motorcycles in 1902, 1903 and 1904, rated at 1 1/4 h.p., 2 h.p. and 2 3/4 h.p., respectively.
On the left is the 133 cc engine that powered the first FN velo-moteur, of which 300 were made from late 1901 into 1902. It's only when you see it against the later offerings that its tiny size becomes obvious - it can be comfortably picked up in one hand. Unlike the later motors, the 133 cc crank case is split horizontally (as were motors in shaft drive fours and singles), and the flywheel (not shown here) is outside the crankcase.
The 188 cc 1903 and 300 cc 1904 motors are often confused. When viewed side-by side the difference in size is obvious, but since the 1904 motor is essentially "scaled up" in all dimensions they look remarkably similar when viewed individually. Externally the most obvious difference between the two relates to the position of the exhaust valve. On the earlier motor, it's tucked in close to the cylinder, while on the later motor the designers have made a conscious effort to separate the valve chest from the cylinder. Not only is the cast-in tappet block moved out from the base of the cylinder, but there is an air gap between the exhaust valve pocket and the top of the cylinder.
The later motors also have bosses at the lower rear of the crankcase where the FN patented oiling system was fitted. A magneto, driven by a set of cased gears from the camshaft on the drive side of the motor, was an option from 1904.
If your motor happens to be apart, it can be readily identified by its internal dimensions: the 1903 motor has bore and stroke of 57 x 74 mm, while the 1904-on motor is 70 x 80.
Here in Australia we have some interesting early FN hardware, including a rather nice 1903 188 cc model which is used regularly in rallies.
Another early belt drive FN was active in Victoria in the early 1980s. Here we see it at rest under the trees on a Maryborough Rally of the VMCC of Victoria.
The informed enthusiast could squat down beside the machine, study the gap between the exhaust valve and the cylinder, and identify the motor as the 1904-on 2 3/4 h.p. 300 cc unit.
Later the same bike found its way into the Otis Chandler collection in the USA, and was recently sold at auction when the collection was dispersed. In the auction catalogue it was listed as a 1903 2 h.p. 188 cc. It's an easy mistake to make, although there are a number of clues to a 1904 date in the cycle parts.
The 2 3/4 h.p. belt drive single remained in the catalogue into 1906, with very few changes from the 1904 model. The last FN belt drive model was the 1907-1908 FN lightweight, after which FN spent 15 years producing only shaft drive motorcycles.