The essential Norton Manx engine was developed in 1927 by Walter Moore, Norton’s chief design engineer. At that time it was a 500 cc single-overhead camshaft motor, good enough to win the 1927 Isle of Man TT for Joe Craig, the newly appointed factory team manager.
The 350 cc and 500 cc “cammy” engines were fitted to the Norton International (known as the “Inter”) introduced in 1932, and while Norton competed in every Isle of Man TT from 1907-70, the Manx name didn’t actually appear on a bike until 1938. At that time it was applied to the Manx Grand Prix model, a race version of the Inter with telescopic forks, “Garden Gate” plunger rear suspension, and no provision for lights. Gradual development left the 500 cc engine’s bore and stroke at 79 mm x 100 mm until 1938, when the “double knocker” DOHC head was fitted. After that, shorter stroke engines were tried to increase horsepower without raising piston speed.
The Grand Prix part of the Manx name was dropped in 1947, and the Manx factory racers received the famous McCandless double-loop “featherbed” frame in 1950. Geoff Duke led Norton to six world championships and numerous TT victories in the 1950s and Norton went 1-2-3 in both the 350 cc Junior and 500 cc Senior TT races in 1950. Two years later Norton was 1-2 in both events. Short stroke engines were standardized in 1954, the same year Norton quit road racing and Joe Craig retired. The Manx remained competitive far beyond the end of production in 1962, and the last important win was by Godfrey Nash in the 1969 Yugoslav Grand Prix.
The Manx remains a force to be reckoned with in vintage racing, partly because it has outlasted all its competition. The 50 bhp Manx motor was enormously strong, and the deep crankcase had remarkable rigidity. The cylinder was cast with extremely deep cooling fins, which could not be trimmed without causing distortion, and the mainshafts were forged in one piece with the flywheels. The massive rod was polished, and the slipper piston very heavy. These days the aluminum head holds sodium-filled valves, but the hairpin springs are still exposed.
Colin Seeley bought the Manx spares and tools in 1966, and sold them to John Tickle in 1969. Tickle sold to Unit Equipe in the late 1970s, which passed the Manx brand on to Andy Molnar in 1994. Molnar produced parts, then built his own complete Molar Manx models.
Norton Inter production continued after WWII, with the bikes gradually receiving more and more Manx parts. The Manx still had the DOHC engine and four leading shoe front brake, though, while the Inter did not. The success of the BSA Gold Star reduced Inter demand to the point that it was only available to special order from 1955. The last Inter left the works in 1958.
Manx Nortons were only ever produced in small numbers for racing, but a number of Inters have been upgraded to Manx specifications over the years, and a wise buyer will insist that a knowledgeable independent expert inspect any potential purchase.