Motorcycle: Twins - Model JD & FD Big Twins, 1921 - 1929
Date: Sunday, October 31 @ 23:00:55 UTC
Topic: Motorcycle, Motorcycles, Motor Cycle
Model JD & FD Big Twins, 1921 - 1929
Milwaukee's first 74-inch model, the so-called "Superpowered Twin" hit America's streets for the 1921 model year. Producing some 18 horsepower, the big engine was intended to compete with Indian's big twins and four cylinder models from Henderson and others. The "74" followed the practice of the "61" from which it evolved, employing inlet-over-exhaust valves (the F-head layout) driven by a single camshaft in a timing cover on the right side of the engine.
However, many major components -crankcases, cylinders and heads — were all new, and both bore and stroke were increased. The transmission was three-speed, the gears and shafts being located in a separate aluminium housing which was connected by enclosed chain to the left end of the crankshaft.
An "automatic" oil pump delivered lubrication for the engine's mixture of plain phosphor bronze, roller and ball bearings. As with almost every other motorcycle of the time, used oil was burned or dripped on to the ground. The multi-plate clutch, on the other hand, ran diy.
Although often referred to generically as the JD, up to 1925 the 74-inch twin might more helpfully be called the Model D, with the addition of essentially the same prefixes as contemporary 61-inch twins. "JD" indicated a complete electrically equipped version, while the magneto model was designated "FD". Where fitted, electrical equipment comprised a six-volt generator, battery, contact breaker points and coil, headlight, tail light and oil warning indicator light. An additional "S" in the model description indicated a sidecar model, with lower gearing and a 0.12m- (3mm-) thick steel spacer between cylinder and crankcase mouth to reduce the compression ratio. Other subsequent suffixes included such variables as piston material — A for aluminium, B for iron. The lighter weight and improved heat dissipation of the aluminium alloy pistons were particularly beneficial after their introduction in 1924.
A tubular steel, single-loop frame tied the whole thing together, the rider cushioned from the worst road shocks by a "Full-Floteing" Mesinger saddle. Front suspension comprised similar double-sprung girder forks to the smaller twin. Only a rear brake - a 7^in (186mm) drum actuated by a long steel rod from a pedal on the right footboard - was fitted until 1928 when a front drum brake also became standard. For markets in which a second brake was mandated by law, an additional "stopper" - essentially a parking brake - could also be fitted. Other changes over the "74"''s life span included the use of "Alemite" surfaces for all chassis bearings in 1924. The rider would periodically inject these with grease from a purpose-made high-pressure grease gun, which did much to extend component life, especially in wet or dusty conditions. Less practical, but canying echoes into the styling of Harleys today, 1926 found the big twin equipped with broad "balloon" tyres and a curvaceous "teardrop" fuel tank. Over its eight-year career, the "74" proved itself a brisk performer which enhanced Milwaukee's reputation for building dependable motorcycles.
Some indication of its virtues - and not least of Barley's tradition of sticking with what worked — was that, at 87 x 101.6mm, the "74'"s cylinder dimensions remained almost unchanged up to the Shovelhead models of relatively recent times.
Clearly, the machine that would take its place in the future would have to be at least as special as the present one. Yet to begin with, at least, it would be anything but.
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