Features

THE 1972 ALLOY XR750

THE STANDARD

By Matt Cuddy

In 1970 Harley needed a new dirt track motor.

From 1954 to 1967, Harley mounted riders had won the AMA Grand National flat track series twelve out of thirteen times. That was done on the KR flathead 750, the formula of that time being 750cc side valve, or 500cc overhead valve. The KR model would stick around 'till 1969, when the AMA formula changed in 1968, that allowed overhead valve motors of the same 750cc displacement.


Mert Lawill's 1969 KR750. 44 horsepower at 7000 rpm.

In 1967 & 1968 Gary Nixon won the series on a Triumph, and the Brit bikes began to rain on Harley’s parade. As a result Harley won the event just once between 1967 and 1971. The race designers at HD started sweating bullets, and searched for a formula that would once again dominate the series.

In 1970 Harley introduced a new XR750, based on the Sportster iron-barreled street bike motor. If anything this was out of desperation, as HD didn’t have any new design on the books to put an end to the Brit bike onslaught. The iron barreled XR750 had some overheating problems, due to the iron heads, and suffered a large amount of DNF’s, making you wonder at the time if the dominance of HD was over in dirt track racing. It put out 73 horsepower at 8000 rpm, and was of an oversquare design, that had a quad camshaft arrangement for max tuning possibilities.


1970 XR750 Iron Barrel. Oversquare with 4 cams.

Then, in 1972 Harley introduced the “alloy” XR750, that consisted of a modified 1970 Sporty motor fitted with alloy barrels & heads. It was an instant success. To drive that point home, in 1972 HD factory racer Mark Brelsford campaigned his Alloy XR in 15 AMA dirt-track races, winning three, and finishing in the top five 11 times. By the end of the series, Brelsford had clinched the Grand National Championship


1972 Alloy XR750. 90 horses at 8000 rpm. Spindly frame seemed to work despite itself. Konis too.

In the 28 seasons since then, alloy XR engines have won AMA championships 20 times, with only eight titles left for the rest of the industry (Honda & Yamaha). And if you go to a Grand National Dirt Track Series race today, chances are the winner will be riding an XR Harley. In the world of racing engines, 28 years is an incredibly long time to be “on top”.


1986 XR750. Not a whole lot has changed since 1972. Brake fade was non-existant.