The XLR was Harley's production racer, designed to compete in AMA Professional Class C or "TT" races, where the capacity limit was 900cc. The XLR was an unusual combination of a KRTT chassis and a motor from the overhead valve Sportster. The motor was similar to the production model, with the same stroke and a bore; but the 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 1971, and is one of the rarer members of the extended Harley-Davidson Sportster family. Though there are no accurate records available, there likely were between 200 and 500 XLRs made during the XRL's fourteen 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 was too larger for AMA sanctioned races, the XLRTT was built specifically for AMA Grand National TT races where the capacity limit was 80 cubic inches (1310cc) until 1963 when the limit changed to 55 cubic inches (900cc) until 1973. XLR were found in several forms of racing. Some ran on circular tracks while others went nitro drag racing. The late Lance Weil took and XRL-based roadster to England and showed the tea-and crumpet crowd that Yank riders knew how to do more than put a foot down and turn left. The Manning/Riley/Rivera streamliner that took Cal Rayborn to a 265.492-mph (426.5 km/hr) world record in 1970 at Bonneville used a similar-based but highly reworked engine.
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 and connecting rods. Cylinder-head castings were Sportster-based but were ported and polished with larger valves and were machined for a long reach sparkplug. The the XLR used standard Sportster pistons. 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. The frame was slightly lighter due to less bracketry.
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 1958-1965; Tillotsen 1966-1971
Transmission: Four-speed, close ratio optional
Suspension: Front telescopic forks, rear twin shock
Brakes: Front and rear drum
Weight: 355 lbs
Top speed: 115 mph