In the beginning of the 1960s the Husqvarna factory chief engineer Ruben Helmin had an idea of putting two 2-stroke engines together. It would result in a potent twin power plant with a displacement of half a litre. It wasn’t until in the autumn of 1967 that he began developing his thoughts into reality.
Helmin used two 250cc engines and integrated them into one power unit. The pistons were the same as the regular 250 with two rings on each piston as opposed to the later 504 version with only one ring for each piston. The result was a 492cc motor mounted into a 360 frame. The modifying consisted of other engine plates and used the gearbox and parts from the 420cc Grand Prix bike, then used in the world championship.
“The engine developed some 50 horsepower at 6,000 rpm,” said Ruben Helmin when the bike was introduced in 1968. The new Siamese twin racer was meant for the desert, but initially there were also hopes for a brand-new machine to compete with the big bore bikes widely used for motocross at this stage.
Motocross riders Torsten Hallman and Rolf Tibblin, representing the 250 and the 500cc class respectively, made test runs with the new beast, which had lots of power and a 100+ mph top speed.
After a further year of development, the giant Husqvarna beast was over-bored to a displacement of 504cc, which made it eligible for the international FIM cup during 1969. This machine now had two upper exhaust pipes as opposed to the prototype, which had one of its exhausts swept underneath the 360 frame. This did not give the bike sufficient ground clearance and the exhaust was soon changed to a two-into-one system. The two pipes were brought together just under the fuel tank, which also had to be adjusted for desert use. It could swallow up to 15 litres of fuel and was fastened with leather straps for a quick release. The full tank weight of the race-ready machine was 137 kilos.
Baja winner Gunnar Nilsson was given the job to ride the bike by the Husqvarna factory. In the five-race series, Gunnar went to beat the big-bores and won the FIM championship. Reliability and performance proved to be excellent. This win proved extremely popular and the marketing people now started to consider production of the twin racer. Despite the international success, the project was still only at an experimental stage.
Bench figures from 1968 showed that the power output was now 52hp at a healthy 6,800rpm. The top speed was clocked at 110mph using a rear sprocket of 47 instead of the commonly used 53 ratio. For carburation, the Amal 32mm concentric carb was handling the breathing. Two throttle cables were used from the carbs all the way up to the throttle grip. A Bosch ignition system was fitted and had a slightly altered performance as opposed to the standard version. The flywheel and the magneto originally came from a snowmobile, however still the same size as for the 400, but stronger for the twin.
With this development, the frame had to be strengthened to cope with the additional power resources. Basically, it was now the same as for the 400 Husqvarna. Heavy engine plates were used to match the big power plant. Two extra brackets were mounted underneath the engine to serve as skid-plates. They also supported the frame and the footpegs.
After success in the FIM series, a 492cc machine was shipped to the USA for participation in the Mexican Baja 1000 race. With the two ace riders Gunnar Nilsson and J.N. Roberts they managed to win this classic event by a good margin, despite a late crash in the dark. Gunnar Nilsson had to spend almost half an hour searching for his bike in total darkness. Luckily, the incident occurred close to the finish in La Paz.
The overall win was followed by a few other US races, but without much success. It was then decided not to continue the project and the bike was forgotten in storage until it was rediscovered in the 1990s.