The Hart-Parr 10-20 B began production late in 1920. It was almost a clone of its big brother the 15-30 A, except the weight and dimensions were less. It still had the tin radiator and open bull gears as the 15-30 A but the water pump was located on top of the crankcase, and the push rods and rocker arms were enclosed as the 15-30 C. The engine was mounted on the cast steel frame just as the 15-30A. The engine rpm's ran a little faster at 800 rpm. The engine was equiped with the Kerosene Shunt for preheating the fuel before it went into the engine and it had a Madison Kipp lubricator for engine lubrication. The rear wheels were only 46 inches in diameter and angle lugs were standard equipment. The 10-20 B was tested at Nebraska in June of 1921.

The Hart-Parr 10-20 C is the same tractor as the 10-20 B with the exception of a few improvements. The frame of the tractor was now made of two pieces of channel iron instead of the cast steel as used on the 10-20 B.The engine remained the same, however, the bore of the engine was increased to 5.50 inches. The radiator now used cast iron top and bottom tanks with name HART-PARR prominently cast into the top tank. The transmission still had two forward speeds but the miles per hour was increased slightly. The tractor still used the band type clutch and open bull gears.