Harley-Davidson Two-Stroke Engine History

Harley-Davidson Two-Cycle Engine

When you think about Harley-Davidson motorcycles, it’s most often about the V-Twin engines, the retro-styling and the inescapable sound.

Many forget that the motor company manufactured a lightweight two-stroke engine and runabout motorcycle for 15-years.

In 1947 as a 1948 model, if you purchased an entry level runabout motorcycle it came with a two-stroke 125 cc single piston motor.  There were two motorcycles engines built — the Model 125 or S-125 (eventually called the ST-125).  The Model 165 or ST-165 replaced the ST-125 in 1953 when the engine size was increased to 165 cc. The ST models were the motor companies idea of how America motorcycle riding should be accomplished after WWII.

The Hummer

So how did Harley-Davidson develop or get the 2-stroke design?

The name “DKW” comes from a two-stroke engine built in 1919 by the Danish engineer Jørgen Skafte Rasmussen, in Saxony, Germany. It was a small engine, which Rasmussen called Das Kliene Wunder (the little marvel) that gave DWK its start in the motorcycle industry.

As WWII drew to a close in 1945, DKW’s factories had either been damaged or occupied by the Red Army. The Soviets took DKW plans, tools, and personnel back to Moscow where copies of the 125 were soon produced. The Soviet version of the 125 was first released in 1946 as the Moskva M1A and later as the K-125.

AMF Merger – 1969

As part of Germany’s war reparations, Harley-Davidson acquired the rights to the German DKW three-speed, two-stroke 125 cc Single.  Harley product shipments began in 1948 and thousands were manufactured in various incarnation until production ceased in 1966.

An updated model called the Hummer was added to Harley’s lineup in 1955, and subsequently all Harley single-cylinder two-strokes built between 1948 and 1966 incorrectly have come to be known as Hummers. The Hummer was named after Dean Hummer, a Harley-Davidson dealer in Omaha, Nebraska who led national Harley two-stroke sales.  The Hummer was very basic — it had magneto ignition and was sold without battery, electric horn, turn signals, or a brake light.

The Topper Scooter

In 1960, Harley-Davidson consolidated the Model 165 and Hummer lines into the Super-10, introduced the Topper scooter, and bought fifty percent of Aermacchi’s motorcycle division. Importation of Aermacchi’s 250 cc horizontal single began in 1961. The motorcycle had Harley-Davidson badges and was marketed as the Harley-Davidson Sprint. The engine of the Sprint was increased to 350 cc in 1969 and would remain that size until 1974, when the four-stroke Sprint was discontinued.

In 1962, Harley-Davidson built the Ranger, an off-road motorcycle without lights, made only for a year.  It had an extra-low final-drive ratio of 7.0:1 (12-tooth countershaft gear and 84-tooth rear sprocket) had neither a lighting system or front fender. Speculation was this motorcycle was built to consume the motor company supply of 165 cc engines, which would not be needed for any other models.

Aermacchi-built Harley-Davidson — The  Sprint

After the Pacer and Scat models were discontinued at the end of 1965, the Bobcat became the last of Harley-Davidson’s American-made two-stroke motorcycles. The Bobcat was the last of the 125-based Harley’s and manufactured only in the 1966 model year.  It was also the only 125-based Harley with a standard dual seat.

In 1969, American Machine and Foundry (AMF) bought Harley-Davidson, streamlined production, and slashed the workforce. The tactic resulted in a labor strike and lower-quality bikes.  Sales and quality declined, and the company nearly went bankrupt.

Harley-Davidson replaced their American-made lightweight two-stroke motorcycles with the Aermacchi-built two-stroke powered M-65, M-65S, and Rapido. The M-65 had a semi-step-through frame and tank. The M-65S was a M-65 with a larger tank that eliminated the step-through feature. The Rapido was a larger bike with a 125 cc engine. The Aermacchi-built Harley-Davidsons became entirely two-stroke powered when the 250 cc two-stroke SS-250 replaced the four-stroke 350 cc Sprint in 1974.

Harley-Davidson purchased full control of Aermacchi’s motorcycle production in 1974 and continued making two-stroke motorcycles there until 1978, when they sold the facility to Cagiva and ending it’s run of two-stroke engines.

Photos courtesy of and taken at Harley-Davidson Museum

For additional Harley-Davidson V-Twin Engine History see this page.

Sources:
Craig Hammitt LinkedIN Article
Wikipedia
Cycle World Article (1993) Article

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The Buzz on Harley Hummer’s

A bit of nostalgia on this rain/snow mixed day.  

After WWII the allies engaged in a scramble to pinch Nazi Germany’s technology and this included motorcycles.  The Harley “Hummer” (Model 125) design was an adaptation of the German DKW motorcycles whose engineering designs were forfeited to the Allies as a part of War Reparations.  The simple DKW 125cc was reproduced as the British BSA Bantam, the “Hummer”, the Russian Moskva M1-A and the Yamaha YA-1 in Japan. 

The Hummer was added to Harley-Davidson’s model line in 1948. The “Hummer” is the unofficial nomenclature as its official listing is the American Lightweight and referenced that way in the Enthusiast Magazine.  The Hummer was a stripped-down basic model using a redesigned “B-model” engine with the old 125 cc capacity. It was named after Dean Hummer, a Harley dealer in Omaha, Nebraska Omaha, who led national Harley two-stroke sales. The Hummer was as basic as it could have been, and was sold without battery, electric horn, turn signals, or brake lights. 

However, one company went on to innovate the DKW 125.  IMME (a German word for bee and the tank logo) was made in Immenstadt in Bavaria from 1949-51.  The Imme’s list of innovation is impressive in today’s standards let alone in the 50’s.  It had a single sided front fork, a rear swing-arm which doubled as exhaust, rear monoshock, and quick release wheels that leave chain and sprocket in place and a twist grip 3-speed gear box.  

The designer was Norbert Riedel whose claim to fame was designing 2-stroke motors to jump start German jet fighters. Production started in 1949, but only 80 motorcycles were made the first year.   Production numbers reached 400 a month later, and in 1950 up to 1000 a month. The bikes could be sold, but some financial (and warranty) problems occurred and the IMME AG went out of business.   Riedel had designed a new egg-shaped twin 150cc engine and planned a comeback with an improved version of the IMME and a 150cc scooter, but in 1951 the factory was shut down again.  Later Norbert Riedel worked for Triumph in Nürnberg and Victoria. He died in 1963 in an avalanche accident.  

Photo courtesy of Harley Hummer Club.

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