Harley-Davidson KR

When Harley-Davidson introduced the sporting KR in 1952, the sidevalve technology that underpinned this performance motorcycle was already a decade or so past its sell-by date.

Rival Triumph, for instance, had introduced the 500cc OHV Speed Twin in 1938. BSA had introduced the 500cc OHV A7 in 1946. Norton had fielded the 500cc OHV Dominator in 1949. And most of the other major motorcycle factories had relegated sidevalves to their "B" teams; i.e. worthy enough bikes for general plodding utilitarian purposes, but certainly not suitable for life in the fast lane.

The Model K

Harley-Davidson, of course, knew all about overhead valve engines. It had, after all, introduced the successful OHV Knucklehead in 1936 and "The Knuck's" successor, the OHV FL Panhead, which arrived in 1948. But the K series bikes, from which the racing KR was developed, were a new direction for the firm and a significant leap forward.

The Model K is a unit construction engine with a right-side foot shift, a four-speed transmission, and a hand-operated clutch.

The displacement is 45 cubic inches which is derived from a  2-3/4-inch bore and a 3-13/16-inch stroke. The compression ratio is 6.5:1.  A 1-1/2-inch Linkert "Bombsight" carburetor Model M-53 (unusually for H-D mounted on the left side) feeds the fuel to the 30hp engine.

Forks are telescopic, and swinging-arm suspension is deployed at the rear. It was the first time an H-D was hydraulically sprung at both ends.

This ancestor of the Harley-Davidson Sportster was aimed at the US street market as an American antidote to the British invasion that began post-war as part of a desperate effort to repay US loans granted in 1946 and which were paid off only in 2006.

The street-oriented Model K was a great motorcycle. It introduced a new dimension to US biking, not least due to its lighter weight and more compact dimensions. Consequently, it was inevitable that US racers would also be attracted to the model, and even more inevitable that the factory would pre-empt and accommodate their requirements.

Indian Scout & the Harley-Davidson WR

Since the late 1930s, Harley-Davidson had been fielding a racing team in the AMA C Class. Racing successes, it was long understood, invariably boosted showroom profits and generally increased corporate prestige. And with the impact of the Great Depression still taking its toll, H-D was desperate for growth.

At that time, the only two significant AMA racing contenders in the USA were Harley-Davidson and Indian. Indian was fielding the highly competent and well-respected 750cc Scout. H-D's response was the 750cc WR; a 45-degree sidevalve derived from the Harley Model WL (aka the "45").

This Class C competition was fierce, and neither side were prepared to take any prisoners. But by 1952, the age of the WR had come to a natural end, and with the introduction of the 30hp civilian Model K sidevalve, which led directly to the Model KR, the game was about to step up a notch.

Harley-Davidson flathead tuning

Both Harley-Davidson, and Harley-Davidson privateers, had gained a huge amount of experience in WR sidevalve tuning, so much so that these pioneers practically wrote the book on how to screw every last surge of power from a flathead engine. Valve angles, valve sizes, intact throats, carburettor jetting, compression ratios, exhaust bore & length, component lightening techniques & tricks, etc; it all came under the most intense scrutiny and was tested not so much on the bench as in hard-nosed competition.

Consequently, with its simple construction, smooth power delivery and predictable manners, the incoming KR received much the same treatment making it a formidable new force on short tracks, long tracks and on pretty much any AMA Grand National competition circuit.

And it was, perhaps, no co-incidence that the gear lever had appeared on the right hand side of the Model K civilian bikes. Dirt track racing, after all, travels counter-clockwise, and the engineers and management at Milwaukee had  prepared themselves for a new assault on the trophies that for a long time had been hard to touch, let along hold.

But why didn't Harley-Davidson simply bite the bullet and shift to OHV technology with the K? Largely because of domestic motorcycle demands for simple and rugged motors coupled with the poor fuel quality of the day which mitigated against high compression engines. And a low-compression OHV engine is largely a waste of engineering expertise and development. Moreover, the Model K was cheaper to manufacturer, and racing homologation rules demanded a race-derived bike that was bought by the general public.

In 1956, the KR won every C Class AMA race, bar none. From 1959 to 1965, 12 of the 15 AMA Grand National Championships were also captured by these motorcycles.

Model K production

Most of the KRs were built with a rigid frame. But swinging-arm examples were also prepared and reserved for circuit tracks such as the Daytona 200.

So how many KRs were built? Exact figures aren't forthcoming. But it's generally reckoned that not less than 236 bikes were assembled, and not more than 500.

Today, these racing KRs are highly sought after, but prices are, paradoxically, surprisingly low. We've seen a few of these in recent years (2015 -2016) sell for around $25,000, and that's not a lot of money for a motorcycle that put one of the world's greatest motorcycle manufacturers back on the racing map and helped blaze a trail for thousands of Sportster derivatives and the famed XR-750.