Harley-Davidson Panhead V-Twin Motorcycles - HISTORY OF THE BIG TWIN

The Harley-Davidson Panhead V-twin was introduced in 1947 in 61- and 74-inch versions. Here’s a look at the technical evolution of the engine and chassis.

Harley-Davidson Panhead V-twin
With Harley-Davidson's Panhead motor, head cooling was boosted by switching material from cast-iron to aluminum. Notice the large one-piece valve covers—the “pans”—which gave this engine its name.Courtesy of Harley-Davidson

Fall 1947 The new "61" and "74" engines are announced for 1948, to replace the Knucklehead. The 61's bore and stroke are 3.3125 x 3.5 inches = 60.3 cu.-in., and the 74's was 3.4375 x 3.96875 in. = 73.7 cu.-in., making claimed 55 horsepower. OHV had overcome much of the 74- and 80-inch flatheads' overheating problem. The new Panhead engine used the former bottom end.

To give the new engine the ability to handle the heat of higher-power operation, head cooling was greatly boosted by switching material from cast-iron to aluminum, with three times iron’s heat conductivity. Because even high-strength aluminum is too soft to act as a valve seating, valves were seated on shrunk-in hard seat rings. Because of the higher heat expansion of the aluminum heads, it was decided to incorporate hydraulic clearance adjusters into the tops of the four pushrods. This was hoped to eliminate need for traditional manual adjustment of valve clearances, but particulates blocked ports in the hydraulic adjusters, sometimes causing collapse and valve noise. The cylinders remained cast-iron. New aluminum heads were cast from the same high-hot-strength alloys used in the engines that powered the B-17, P-47, and F4F Hellcat combat aircraft.

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The large one-piece valve covers—the “pans” that gave this engine its name sought to simplify the problem of oil containment but was not completely successful. Panheads still leaked oil from the top end. A felt blanket was placed over the rocker gear in hope of quieting valve noise and providing another source of oil for it. Rocker arms were redesigned. Formerly they rotated on stationary shafts but now turned in bolted-together trunnions. These hollow rockers sometimes caused oil pressure loss when their end plugs came out. Oil was pumped through drillings in the cylinders, into the heads, and into the bolted-on rocker supports. Oil returned through other cylinder drillings to grooves on the cylinder mating surfaces of the crankcase. Some oil also returned via pushrod tubes.

Harley-Davidson Panhead V-twin
Harley-Davidson had thought that the one-piece valve covers would simplify the problem of oil containment, but that was not completely the case, and Panheads still leaked oil from the top end.Courtesy of Harley-Davidson

A new cam is designed. The double-gear oil pump is “outdoors,” bolting to the rear of the crankcase on the timing side as before, is enlarged to circulate oil faster.

1949 Front brake drum is enlarged to 8-inch. A new telescopic fork gave more than twice the previous 2 inches of the springer fork's front wheel travel. This change to telescopic fork was necessary to compete with the many tele-equipped British bikes now being imported. Some of the fork parts are manufactured by Menasco, who had supplied telescopic landing-gear struts for aircraft during World War II.

1951 Cam lobes are given clearance ramps, chrome-plated piston rings are adopted (these had become essential in wartime air-cooled aircraft engines, as without them, engines running on high power could wear their rings to half their radial thickness in 30 flight hours), new one-piece rockers. Late this year a foot shift option is announced.

1952 Harley-Davidson's 61E model is dropped. Foot shift introduced on FLF, valves are Parko-Lubrized (black phosphate protective coating) for oil retention. This is the last year for pushrods with valve clearance adjusters at their tops. Production of 80 cu.-in. flathead ceases. Hydraulic valve clearance adjusters moved to bottom of pushrods. There are port and cam changes which add 10 percent more power.

The K-Model flathead replaces the 750 WR as the basis for Harley’s race bikes.

Harley-Davidson Panhead V-twin
On the Panhead engine, oil was pumped through drillings in the cylinders, into the heads, and into the bolted-on rocker supports. Oil returned through other cylinder drillings to grooves on the cylinder mating surfaces of the crankcase. Some oil also returned via pushrod tubes.Courtesy of Harley-Davidson

1953 Valve roto caps adopted to prevent hot-spotting of exhaust valves by constant slow rotation. Later, the same effect would be obtained by proper contouring of the valve operating tip of the rocker-arm. From this year the oil return grooves on the crankcase/cylinder-mating surface were discontinued and oil drained directly to the case. New aluminum tappet guides and tappets (there had been problems with the lifters). In this year the clearance adjusters were relocated into the roller tappets. With the pushrod-mounted clearance adjuster there had been problems with dirt leading to collapsed lifters, valve noise, and need for service. To keep particulates out of the adjusters an oil screen was added to the feed line. By moving the lifter closer to the source of oil pressure, consistent operation was achieved.

1955 FLH is given 60 hp (H stood for High compression). Hycar rubber-impregnated asbestos base gasket. People were running their H-Ds hard enough to need a new bottom end under the Panhead top end – everything in the same places but new cases with stronger main bearings. Left crankcase is strengthened to carry Timken tapered roller bearings on an enlarged sprocket shaft, in place of previous cylindrical rollers. Both crankpin and sprocket mainshaft are now 4620 steel (good strength, high machinability, tough). Panhead intake Y-manifold is upgraded with O-ring ports. Valve covers become 6-bolt for a time, then revert to 12-bolt type.

1956 "Victory" cam on FLH increases hp 12 percent on FL at 4800 rpm. Intake ports are polished.

Harley-Davidson Panhead V-twin
In 1958, Harley updated the suspension, and thus name, to “Duo-Glide,” a name owed to the angled rear suspension units that added around 80 pounds of weight.Courtesy of Harley-Davidson

1958 Cooling fin depth on cylinder heads is increased to one inch. Exhaust valve guides in FL and FLH become alloy steel instead of bronze. There is a new exhaust valve material. Only FLH gets heavier inner and outer valve springs. Pinion mainshaft is increased in diameter by .250 inch and its bearing becomes two retainers carrying 14 ½-inch long rollers, replaces single retainer with 13 3/16-inch rollers. Crankcase side cam bearing becomes Torrington full complement needle. There is a new crankcase breather. Rear brake becomes hydraulically operated.

Although it appears at first that a narrow crank like these could be adequately supported by just two main bearings, when engines turn faster, parts that seem rigid to us begin to flex. Flywheels vibrate on the crankpin “like elephant ears flapping,” and the sprocket and pinion mainshafts wobble. Many makers of higher-rpm singles tried to control this wobble by placing two main bearings on each side, but there has never been full agreement on a best way to keep everything in alignment.

A big change is adoption of rear suspension this year, allowing the “Hydra-Glide” name (which referred to the hydraulic damping in the new telescopic fork) to be replaced by “Duo-Glide” (Sportster had rear suspension from its onset in 1957). This adds a significant 80 pounds of weight. Angled rear suspension units fit well with traditional parallelogram-shaped bags or carriers.

Harley-Davidson Panhead V-twin
The long-established ritual of kick-starting was replaced by electric starting in 1965, giving rise to the new name “Electra-Glide.” With the starter came a 12-volt electrical system and a larger battery in a case on the right-hand side.Courtesy of Harley-Davidson

1959 Number of plates in dry clutch increases from three to five.

1960 Higher lift Victory cam increases horsepower to 60. Crowned rollers are adopted for pinion-side main bearing (because all crankshafts must flex in operation, mainshafts wobble slightly rather than rotating accurately around their own centerlines. Because this slight wobble overloads the ends of bearing rollers, giving them a slightly barrel-shape—called "crowing"—extends the life of the rollers. Rollers and balls flatten slightly under load.

1963 An external oil line tees from between the tappet blocks to overhead rocker assembly on both heads, replacing harder-to-manufacture oilways drilled through cylinders and heads.

1964 Valves are now Nimonic-80A, a nickel-based heat-resisting alloy originally made for jet engine turbine blades.

1965 In another momentous change, the long-established ritual of kick-starting is replaced by electric starting, giving rise to the new name "Electra-Glide." With the starter came a 12-volt electrical system and a larger battery in a case on the right-hand side. The starter is located behind the engine and above the gearbox, its small drive pinion engaging a ring gear on the clutch. These and other changes add 75 pounds to total weight. Engine power is up from 65 by use of new cam, higher compression ratio. A larger crankpin was fitted, and a heavier drive chain. Public stock offerings this year and next failed to attract needed capital. Accumulated vehicle weight increases need more power.