The Sportster's new cast-iron heads were designed with deep hemispherical combustion chambers, similar to WWII aircraft engines. The hemi-style chambers allowed for big, high-flowing valves, but also required heavy high-dome pistons for high compression. Heavy pistons make balancing an engine more difficult, while high piston domes impede cylinder airflow during valve overlap and interfere with flame travel during combustion. Note that in the '80s, both the Evolution Sportster and Big Twin engines were designed with lightweight flattop pistons and shallower combustion chambers to minimize the inherent drawbacks in the original Ironhead Sporty. The '57 Sportster XL was shipped with a rather conservative 7.5:1 static compression ratio (for an OHV engine). Two years later, the compression was upped to a more reasonable 9:1 ratio.
The XL's aluminum pistons connected to Harley's typical "knife-and-fork" design, connecting rods riding on a single-throw crank with heavy counterweights. The four camshafts were a carryover from the K-model and provided straighter pushrod angles (and more accurate valve timing) than with a single-cam design. Solid roller tappets rode on the cam lobes and transmitted valve timing to the rocker arms via steel-tipped aluminum pushrods. The '57 Sporty cylinder heads shipped with relatively small valves, but valve sizes were increased in '58.
This four-cam design is used...
This four-cam design is used in both the K model and Sportster engines. - Andrews Products, Inc.
This is a nice example of...
This is a nice example of a 1957 Sportster XL, photographed at a local vintage motorcycle show. '57 Sportsters came with solo saddle, 4.4-gallon fuel tank, 2-into-1 exhaust, speedometer, and distributor ignition. The saddlebags were an option. - The Author
1957 Sportster XLs shipped...
1957 Sportster XLs shipped with a large chrome headlight and fork nacelle with integrated speedometer. Also seen is the round hand-adjusted fork dampener at the top of the forks. - The Author
The integrated engine/transmission design made for a very compact yet rigid drivetrain that was several years ahead of the British bikes. A robust triplex primary chain connected the crank to a dry clutch, which was a carryover from the K-model, as was the four-speed transmission, right-side secondary drive, and right-side foot shift. A heavy cradle-style frame rolled on narrow 18-inch spoked wheels, while narrow drum brakes served as binders for stopping.
A Linkert DC carb with a front-mounted float bowl (not the brass M-53 Linkert on the K) was fed gas from a 4.4-gallon fuel tank. Instruments were sparse on '57 XLs, including only a nacelle-mounted speedometer. However, a hand-actuated fork dampener for minimizing wheel wobbles was standard equipment on the front fork.