In the mid-'50s, Detroit auto makers were transitioning from Flathead to overhead valve (OHV) technology, so Harley engineers decided to follow suit and designed the new model with overhead valves while incorporating engineering of the W-model and retaining the K-model's integrated engine/trans and four-cam design. As such, the Sportster XL was born with cast-iron OHV cylinder heads and aluminum rocker boxes that eventually led to the Shovelhead engine design in 1966. Rumor has it that the genesis for the XL moniker was spawn from the name "Xperimental Lightweight."
The objective for using an overhead valve design on the Sporty was to improve cylinder filling and power. With OHVs, the valves are located in the cylinder head and positioned above the cylinder. A valve opens by moving downward toward the cylinder, providing the air/fuel mixture a rather direct and downward path into the cylinder for improved filling. On the other hand, with a Flathead (also called side-valve) engine, the valves are not in the cylinder head; instead, they are located beside or directly to one side and parallel to the cylinder and move upward during opening. This arrangement requires the air/fuel mixture to make two 90-degree turns to either fill or exit the cylinder, which is much less efficient than with an OHV engine.
Introduced in 1952, K-models...
Introduced in 1952, K-models included a 45 ci (750cc) side-valve engine. The 4-cam engine was integrated with a four-speed transmission for compactness. It included brass 1-1/2-inch bore Linkert carburetor and right-side foot shift. The 45ci engine was rated at 30 HP. In 1954, the 45ci engine was stroked to increase displacement to 53.9ci or 883cc. - Harley-Davidson Archives Copyright H-D
K-model engines came equipped...
K-model engines came equipped with the popular brass 1-1/2-inch bore Linkert carburetor, which was introduced in 1933 and used until the end of the Panhead era in 1965. The K-model's Model M-53 Linkert included a solid brass body, round float bowl located underneath and bombsight in its throat for improved fuel atomization. - The Author
The 55ci OHV Sportster engine...
The 55ci OHV Sportster engine was introduced in 1957 and evolved from the K model engine. The K's integrated engine/trans design and four cams were retained. Cylinder heads and cylinders were cast iron and fuel was fed by a Linkert DC carb with front-located float bowl. - Harley-Davidson Archives Copyright H-D
Although the new OHV Sportster engine and older side-valve KH engine were both rated at 883cc or 53.9 cubic inches, Factory engineers destroked the Sporty from the KH's 4-9/16 inch stroke to 3-13/16 inch. To maintain the same displacement with a shorter stroke necessitated increasing the Sporty's bore to 3.00-inch from the KH's previous 2.75-inch. The Sporty's shorter stroke and larger bore allowed the use of bigger valves while producing a higher revving engine with lower piston speed at any given rpm.