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Harley-Davidson XR-750 Stop-Gap Racer

The iron-clad Harley-Davidson XR-750 was only a stop-gap model, but proved to be a great investment following the success of the alloy version.

ironhead-engine
courtsey Harley-Davidson Museum Archives
The first XR-750 bowed in 1970. Initially the Ironhead engine used a single carburetor and straight pipe exhausts.

Flathead race engines finally became extinct November 1968, when the AMA Class C rules committee legalized overhead-valve 750cc engines for Nationals competition. Previously, overhead-valve engines were limited to 500cc displacement, while sidevalves could be 750cc. The new rules caught Harley-Davidson race team with its valves down, and Milwaukee’s juggernaut team entered the 1969 race season outgunned. Or so it seemed.

Despite running essentially obsolete equipment, H-D team rider Mert Lawwill answered the call with his trusty sidevalve KR-750 to win the coveted Number One plate. But 1969 was the KR’s swan song. Gene Romero, riding new Triumph OHV twins and triples, won The Plate for 1970.

Meanwhile Harley’s engineers readied their own overhead-valve engine for racing. Their immediate response was the XR-750, an engine with iron cylinders and heads — a distinctly different engine than the alloy-based XR-750 that was scheduled for 1972. The all-new alloy engine was intended for one purpose only — to crush the competition on the flat tracks of America, which it ultimately did, dominating dirt track ovals across America for the coming decades.

But the fascinating story is found in that first-generation XR-750 using an engine essentially jerry-rigged for racing based on Harley’s XLR, itself a 13-year-old design originally developed for customer 883cc Sportsters.

No surprise, Harley scrambled to make the XLR legal for Class C racing. New and lighter flywheels shortened the stroke from 82mm to 79.5mm, ratcheting down displacement to 750cc. A multitude of other modifications followed, including beefing up the right-side main bearing, replacing camshaft needle bearings with ball bearings, and so on.

cal-rayborn

Image courtsey Harley-Davidson Museum Archives

Cal Rayborn aboard the first-gen XRTT-750.

Harley speed guru C.R. Axtell was tasked with reconfiguring the cylinder head ports for racing, and larger valves and an oversize carburetor improved breathing. That combination proved sorrowful, though, so a second carb joined the party. To do that Axtell rotated the rear cylinder to face the intake port forward on the engine’s left side, then designed two equal-length runners for both carbs.

As the new iron engine was readied for the 1971 race season, lessons learned were applied towards the alloy engine being developed simultaneously. As reported in Motorcycle Sport Quarterly magazine, “Whenever possible the ‘iron’ XR would serve as a test bed for the alloy version, but not at the expense of the [ironhead’s] development time. The first phase had to be completed on schedule, the second phase could wait.” By mid-1971 the phase-one XR-750 was competitive — to a point — but it looked like an abomination. Those iron heads retained insurmountable heat, causing detonation, so four oil coolers sprouted beneath the fiberglass gas tank.

The stop-gap engine won a few flat track races, but no championship. However, the XRTT road racer variant enjoyed historical success in the hands of pavement specialist Cal Rayborn, when, in 1972 he took his ironhead racer to the Match Series in England. There he won three of the six races, creating a legacy for an otherwise dark moment in Harley racing history. The following year he helped christen the new alloy XR-750, winning AMA National road races at Indianapolis and Laguna Seca.

No doubt, the iron-clad XR-750 was only a stop-gap model. But given the enduring success of the legendary alloy XR-750 that followed, the patchwork model proved to be a great investment by The Motor Company. — Dain Gingerelli

Published on Apr 10, 2020

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