The Sportster engine is a two-cylinder, four-cycle, air cooled, overhead-valve, V-type engine with 55 cu. in. displacement. It has three major component assemblies - cylinder, crankcase and gearcase.

Cylinder assemblies include cylinder head, valves, rocker arms and piston. Cylinders mount on the engine crankcase in a 45 degree "V," with both connecting rods connected to a single crank pin.

The reciprocating, linear motion of the piston in the cylinder is converted to circular motion in the crankcase. The built-up crankshaft consists of an offcenter crank pin interposed between two counter-weighted flywheels which rotate on two end shafts (pinion and sprocket shafts) supported by anti-friction roller bearings. The lower end of the rear cylinder connecting rod is forked to fit around the single-end front cylinder connecting rod, allowing a single connecting rod-crank pin connection to the flywheel.

Fly-wheel rotation is clockwise (viewing engine from right side). Using the front cylinder firing position as a starting point, the rear cylinder fires at 315 degrees rotation (360 degrees minus the 45 degrees between cylinders). The front fires in an additional 405 degrees (360 degrees plus the 45 degrees between cylinders), completing the 720 degrees of flywheel rotation necessary for the four piston strokes.

The gearcase is located on the right side of the crankcase and houses a gear train which operates and times the valves, ignition and crankcase breather. The generator is also driven from the gear train. The rotary crankcase breather valve is located be

tween crankcase and gearcase compartments and functions to relieve crankcase pressure caused by downstroke of pistons, and controls the flow of oil in the lubrication system.

A cam gear train consisting of four cam shafts with one cam lobe on each shaft is gear driven. The engine valves are opened and closed through the mechanical linkage of tappets, push rods and rocker arms. Tappets serve to transmit the cam action to the valve linkage. Valve and breather timing are obtained by meshing gearcase gears with timing marks aligned.

Ignition spark is produced by operation of circuit breaker, ignition coil and spark plugs. The breaking of a single set of breaker points by a double-lobe cam on the timer shaft determines the spark timing. The narrow lobe times the front cylinder. The wide lobe times the rear cylinder. Both spark plugs fire on each breaker point opening (twice per complete cycle of 720 degrees flywheel rotation since cam shaft operates at 1/2 engine speed). The valves are timed to produce combustion conditions in only one cylinder at a time so the spark in the other cylinder occurs ineffectually during its exhaust stroke.

Most other engine components function similar to usual internal combustion engine design. For further description of part function, see pertinent manual sections.


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