The XLR was Harley's production racer, designed to compete in scrambles or "TT" races, where the capacity limit was 900cc. The XLR was an unusual combination of a chassis from the KR model and a motor from the overhead valve Sportster. While the motor appeared to be similar to the production model, it had a shorter stroke and a larger bore for higher rpm, producing more power, although the capacity remained the same. Motor internals were pure racing components, and in this form the machine was competitive with its main adversary, the Triumph 650cc TT Special.
The XLR-TT roared on the scene in 1958 and remained in low-volume production until 1969, and is one of the rarer members of the extended Harley-Davidson Sportster family. Depending on which historian you listen to, there were between 200 and 500 XLRs made during the XRL's eleven years of production. Despite looking like a stripped XL Sportster, these were pure race bikes with no provision for lighting or other street amenities.
Because the 883 cc V-twin was too big for AMA sanctioned national-class races, the bikes ran in the unlimited open class or at so-called "outlaw" races that ignored AMA etiquette. Despite weighing 355 pounds, many XLRs ended up on scrambles tracks or "TT" races (the precursor to motocross) where the capacity limit was 900 cc. Others slogged through the rocks, roots, and rivers of enduro courses. Some sprouted fairings and sticky tires and went apex hunting on road courses: most notably the late Lance Weil who took an XLR-based roadracer to England and showed the tea-and-crumpet crowd that Yank riders knew how to do more put a foot down and turn left Other XLRs – or parts of them – went nitro drag racing, and there was an XLR-based engine, albeit heavily reworked, in the Manning/Riley/Rivera streamliner that took Cal Rayborn to a 265.492-mph (426.5 km/hr) world record in 1970 at Bonneville.
The XLR's performance came from its engine spec. While run-of-the-mill Sportsters went out into the world with their cams running in needle bearings and bushes, XLRs benefited from friction-reducing ball bearings. Likewise, their crankshafts ran in roller bearings. The bikes had specific flywheels, pistons and connecting rods. Different cylinder-head castings allowed larger valves and a bumped-up compression ratio. Hotter cams were installed as well as a lightened valve train. Ignition was via magneto, moved from the right side of the engine to a tucked-up position ahead of the front cylinder. Even the XLR frames were special, built with a better grade of steel so the walls could be thinner and the whole assembly lighter.
Engine: Air-cooled, V-twin four stroke
Ignition: Fairbanks-Morse magneto
Power Rating: 70 hp @ 6,500 rpm
Bore & stroke: 3.005" X 3 13/16"; pistons were cam ground solid skirt aluminum fitted with .006" clearance;
Displacement: 883 cc
Valves: Overhead, pushrod activated
Fuel System: Single Linkert carburetor
Transmission: Four-speed, close ration optional
Suspension: Front telescopic forks, rear twin shock
Brakes: Front and rear drum
Weight: 355 lbs
Top speed: 115 mph