Motorcycle: Overhead Valve Twins - Knucklehead, 1936 -1947
Date: Sunday, October 31 @ 14:53:39 UTC
Topic: Motorcycle, Motorcycles, Motor Cycle
Knucklehead, 1936 -1947
It's difficult to appreciate the impact Harley-Davidson's first overhead-valve roadster twin must have had on a motorcycling public that was emerging groggily from six long years of Depression. In fact, no Milwaukee model before or since was ever quite so comprehensively new. Even the latest Twin Cam, remember, is installed in largely familiar running gear.
Not so with the legendary Knuckle. Not only was the engine a major departure, but it sat in an equally new twin-cradle frame, with just the mudguards and generator remaining from the flathead VL. Its striking styling owed much to the art-deco innovations of the Depression years, capped by a teardrop-style tank, complete with an audacious white-faced Stewart-Warner speedometer calibrated to 120mph (193kph). Fanciful, perhaps, but factory had reported 100 mph (161kph) development. Little wonder that the owners queued in droves when 1st examples were shipped to in January 1936, despite the Knuckle costing $40 more than a temporary 80-inch twin.
Designed by William S. Harley and Lothar
A. Doerner (the latter tragically while testing a 1937 model), the in (989cc) twin underwent no fewer than 70,000 hours of testing, according to contemporary claims. As with previous mainstream models, the Knuckle was initially available in three guises: E (standard, discontinued after 1937 but restored during the Second World War), ES (sidecar) and EL (high-compression sport). All proved free-revving and eager, while the EL in particular — with 40 horsepower at 4,800rpm — offered a huge performance increase over the laggard side-valvers. Even the mighty 80-inch VL couldn't come close to an ohv twin.
Yet despite the time-consuming route to production - and not for the first time — there were initial problems.
Many changes were implemented during the first year of production, including frame reinforcing and revisions to the kick-start gears. The rocker assemblies became enclosed for 1938, by which time the patented tank-mounted instruments also included warning lights for low oil pressure and generator output.
But the 61's worst fault concerned its dry-sump lubrication. Some parts got too little oil while others - including the road underneath — got too much. A partial fix was introduced in 1937 but the glitch was not completely solved until the arrival of the centrifugally-controlled oil pump bypass of the 74-inch (Model F) Knuckle of 1941. The crankshaft main bearings had been up-rated in 1940, perhaps in anticipation of the extra loads on the bigger engine. The arrival of the more potent "74" also brought larger 8 1/2 in (216mm) diameter flywheels, revised crankcases, an up-rated seven-plate clutch and a carburettor choke increased to 1 1/8 in (28.6mm). By now, the two ohv models combined were comfortably out-selling Milwaukee's established range of side-valve twins. Given further development, the Knuckle could surely have become better still, but with the outbreak of war in December 1941, production concentrated almost exclusively on military side-valve twins and development of the ohv models all but ceased. Milwaukee lore has it that the very best of the big Knuckles were those built in that final pre-war year, but hostilities meant that relatively few Model Fs reached the road until 1947, when the line's reign was almost over. Not surprisingly, that first overhead-valve "74" remains one of the most prized Harley roadsters of all. By 1948, however, a new pretender was on the scene: the age of the Panhead had arrived. At last it was goodbye, finally, to oil leaks from the rocker box, which was a problem that had never been completely solved on the Knucklehead.
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