The Fordson was a mass-produced general-purpose tractor made by Henry Ford and Son from 1917 until 1920, when it merged into the Ford Motor Company, and the first Fordson Model F was completed in 1916 as the world’s first small, lightweight, mass-produced affordable tractor, making it possible for the average farmer to own a tractor for the first time. It went into mass production in 1917 and sold for $750.00. The original Fordson used a 20 horsepower, four-cylinder vaporizing oil engine, a three-speed spur gear transmission with three forward speeds ranging from approximately 2 1/4 to 6 1/4 mph, and a worm gear reduction set in the differential. It could pull discs and plows that would require at least four mules and could work all day if the radiator was continually filled, the fuel replenished, and the water in the air filter tank changed, though on dry days mud would build up in the water tank after a few hours of operation. The Fordson was cheaper to maintain than horses, and a government test concluded that farmers spent $.95 per acre plowing with a Fordson compared to feeding eight horses for a year and paying two drivers, which cost $1.46 pr acre. The Fordson Model F also had flaws, including lack of weight that allowed wheel slippage in some conditions and a habit of rearing over backwards if the plow encountered an obstruction. Its engines were difficult to start, especially in cold weather when oil congealed on the cylinder walls and clutch plates, so it had to be hand cranked with great effort, sometimes by strong men taking turns, and farmers sometimes built a fire under the tractor to warm the crankcase and gear boxes. While kerosene fueled the tractor in use, gasoline was required to start it, and getting it into motion was difficult because the gears had to be shifted and the clutch would not disengage fully from the engine to allow gear change, so the hand lever had to be rammed into position and the clutch pedal ridden until the oil warmed up and the clutch released. By 1918, more than 6,000 Fordson tractors were in use in Britain, Canada and the United States. From 1921 until 1927 the Soviet Union purchased over 25,000 Fordsons. The Fordson Model N replaced the Fordson Model F in 1927, production ended in 1928, and later Irish- and English-built Fordsons were imported to the U.S. until that arrangement ended in 1939 with the introduction of the line of Ford tractors made in the U.S. for domestic sales.