1952 - 1966, 45 cubic inch (741cc), air-cooled, 45-degree, sidevalve V-twin
▲ Harley-Davidson KR. A racing 750cc sidevalve might not sound like much of a sporting proposition, but these simple and rugged bikes were once all-conquering and provided exciting racing at a time when US motorcycle sport was in decline. In the 1950s, the AMA "Equivalency Rules" in Class C restricted rival OHV bikes (i.e. foreign bikes) to just 500cc. Some cried "foul". But H-D and Indian had a point. A 500cc sidevalve is no match for 500 OHV machine. There had to be some kind of technical handicap. And so there was...
▲ AMA homologations rules allowed Joe Public to buy into exactly the same racing kit as the factory teams, and Joe Public occasionally won. These KRs were seriously quick for the age and good for around 125mph. Stopping wasn't much of a requirement for anyone, so brakes were dispensed with. Today, these bikes are still broadly "affordable" and accessible, and the competition history is now the stuff of racing legend. If you're riding a Harley-Sportster, you'll have no trouble recognising this ancestor. These K-series machines were the last H-D flatheads, and the best.
▲ Harley-Davidson Model WR for 1950. With one of these between its legs, Milwaukee's most famous son more or less wrote the book on sidevalve/flathead tuning for performance. It was a hugely successful bike that gave the wonderful Indian Scout plenty of bloody noses. By 1952, with the arrival of the civilian Model K and the racing variant, the KR, the hand-shift WR's days were done. However, on US classic racing scene, these bikes are still out there on the track reprising a golden age of AMA competition. Bonhams offered this bike for sale in April 2012 with an estimate of £16,000 - £22,000 (€18,000 - €25,000). It didn't sell, but the price was broadly right. Image courtesy of Bonhams.
▲ Harley-Davidson Model K for 1953. This was the second year of production for this new 30hp, 80mph, 750cc lightweight (or at least lighter-weight) Hog, but it still tipped the scales at around 400lbs (182kg). This motorcycle was in fact almost 200lbs (91kg) lighter than an FL Panhead, and it sold for $865 new. The Pan retailed at around $1,100. It was a new look for a new generation of post-war riders and was intended to give BSA, Triumph and Norton and hard run for their money. And so it did. The K morphed into the KK, the KH and the KHK. Eventually, this sporty bike became the Sportster.
1961 Harley-Davidson KR - Specifications
Ignition system: Magneto
Carburettor: Single Linkert
Maximum speed: 125mph
Primary drive: Single row chain
Rear suspension: Rigid
Rear wheel: 18-inch
Front brake: None
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.
Copyright Sump Publishing 2016