1970 Harley-Davidson XR-750
I’m talking about the Harley-Davidson XR-750, which last month marked the 50th Anniversary since it’s debut.
From 1953 to 1969, Harley-Davidson manufactured the KR750, the backbone of American dirt track racing.
The motorcycle engine was an air cooled, side valve 4-stroke 45° V-twin (flathead), 45.125 cu in (739.47 cc) displacement built for racing. Unique for Harley’s at that time, the KR model shifted on the right, like a British bike, which worked great for dirt track. It wasn’t until 1975, when DOT specified that all motorcycles sold in the U.S. had to have a standardized, left-side gear shift.
Harley-Davidson Flat Track Racing
Prior to 1969, the American Motorcyclist Association (AMA) rules for the Grand National Championship were structured to favor “sidevalvers” (side-valve engines) rather than overhead-valve (OHV) engines. The intent was to deliberately favor American made bikes such as those from Harley-Davidson with their side-valve engines, and disadvantage the competition which was mainly from British manufactures i.e., Triumph, BSA, and Norton. The AMA rules allowed side-valve engines of up to 750cc capacity but OHV engines were limited to 500cc. The 50% engine size advantage stacked the odd in the favor of the side-valve bikes.
As you might imagine, the British manufactures complained…about everything—the rules, the manufactures, the officials, the drivers, the races, and the racing itself.
Harley-Davidson XR-750
As a result, in 1969, in prep for the 1970 race season, the AMA approved that all professional Amateur & Expert dirt track and speedway engines could be 750cc, regardless of configuration or valve style. This rule change eliminated the 250cc penalty for OHV engines that had existed since 1933. Many British manufacturers begin designing and developing OHV 750cc engines for competition. To be approved by the AMA (for Class C competition), a motorcycle must be a standard catalogued production model and at least 200 units of the same model with identical engines and transmissions and must be available for inspection and/or purchase within the United States.
XR-750 Engine – Ported and Polished Heads
Unhappy with the decision, but with a desire to continue the racing legacy, Harley-Davidson set out to create a new overhead-valve racing motorcycle. The Milwaukee motor company leveraged their OHV V-twin racing engine based on the Sportster XLR. However, it’s engine had a displacement of 900cc (55 cu. in.) and would need to have its capacity reduced to 750cc. Harley-Davidson engineers accomplished this by decreasing the engine’s stroke from 3.81″ to 2.983″ and increased the bore from 3.0″ to 3.2″ bringing the engine in just under the 750cc maximum.
The AMA approved the Harley-Davidson “XR”, a 750cc V-twin overhead valve engine, for Class C competition on Feburary 27, 1970. It had been tentatively approved in late-1969 as the “750 Sportster”, but the motor company lacked having 200 units available for inspection at the time. The motorcycle is dubbed the “iron XR“, or “Iron Head,” due to its steel cylinders and heads.
Even Knievel
For the 1972 race season, there were a number of changes. A vote was taken on November 18, 1971, and the AMA Competition Congress voted to allow qualified women to compete in all forms of AMA Racing.
The Water-Cooling was approved, as long as it is an integral part of a production motorcycle. Titanium frames were outlawed from all AMA competition. The AMA approved the Harley-Davidson XR-750, an updated version of the XR, for Class C competition on April 12, 1972. The updated engine used aluminum cylinders and heads to address the overheating issues that plagued the XR model. However, due to delays in getting all 200 units completed, it wasn’t approved in time for the Daytona 200, but debuts at the Colorado Springs national on April 30, 1972.
Speaking of aluminum heads, they were made, then shipped to Jerry Branch of Branch Flowmetrics in Long Beach, California to be ported and assembled. The new cylinder head design included larger valves. The cylinder heads were then shipped back to Harley-Davidson’s factory in Milwaukee for fitting to the new engines. This V-twin engine was not quite of the same dimensions as the Iron Head. The bore was increased to 3.1” and the stroke reduced to 3”. Carburetors were 36mm Mikuni, one for each cylinder. The exhaust systems were mounted high on the left side of the motorcycle well away from the carburetors. Power was 82hp at 7,700rpm giving the bike a top speed of around 115mph.
Harley-Davidson Flat Track Racing Team
Branch wrote engineering books on his engine air flow work and eventually sold Branch Flowmetrics to Mikuni in the late 1990s. Branch was the only independent company to ever supply Harley-Davidson ported and polished heads!
In 1972, Harley-Davidson was the first-ever “Grand National Manufacturer’s Championship“, which compiles the highest finish of each brand at every Grand National Championship event. Between 1972 and 2008, the XR-750 won 28 of 37 AMA Grand National Championships. The XR-750 racked up more wins than any other motorcycle in AMA racing history and earned the description of being the “most successful race bike of all time.” In addition, the XR-750 became a cultural icon with legendary stuntman Evel Knievel at the handlebars. Evel Knievel began jumping an XR-750 at the height of his career from 1970 to 1976.
Harley-Davidson created one of the greatest bikes in the history of American motorcycling.
For 2020, the Harley-Davidson Factory Flat Track team will use the updated Harley-Davidson® XG750R flat tracker, powered by the liquid-cooled, fuel-injected and race-tuned 750cc Harley-Davidson® Revolution X™ V-Twin designed for the Street 750 motorcycle.
Full details on the Flat Track team can be found HERE. Flat Track racing news is HERE.
Photos courtesy of Harley-Davidson
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