In 1901, Hart-Parr began production of their first tractor. It was rated at 17-30. The 17-30 models were built until 1906. Production of the 22-40 began in 1903 and went through 1907. By 1908. it had evolved into the 22-45. This change was due to an increased compression ratio. Production of the 22-45 lasted until 1911.
In 1908, Hart-Parr built a 4-cylinder model rated at 40-80 horsepower. This machine was too large and complicated for the average customer. Only 10 of them were built, and none are known to exist today. The 15-30 was the first model built by Hart-Parr that was intended to be a smaller tractor. The first year the tractor was equipped with a horizontal engine and a single wheel. This was not a successful tractor, resulting in a recall. The next model used a vertical engine and was built until 1911. The 20-40 replaced the 15-30 in 1911 with some minor improvements. The only known example of this model resides in the Floyd County Historical Society in Charles City, Iowa. It was built from 1911-1914. The 30-60 was an updated model of the 22-45 and was so successful that it was nicknamed “Old Reliable." This model was built from 1911-1916. While the 30-60 was in production, Hart-Parr developed their largest model, the 60-100. This was also a 4-cylinder model, but it never went into production. It is believed there were only one or two built. The 12-27 was also a small model of the early Hart-Parrs, using a tricycle configuration but with two small front wheels. It used a single cylinder vertical engine. In 1914, the engine was bored out, and it became the 18-35. It was also known as the "Oil King." The Oil King was built until 1919.
In 1914, Hart-Parr made an unsuccessful attempt to build a completely redesigned small tractor. It was a 3-wheel unit with the single wheel being in the rear with the operator sitting off to the side of the wheel. It was known as the Hart-Parr Little Devil. It was rated as a 15-22. There were many problems with the tractor, and it was eventually recalled by the company. Only a few models still exist today, and one is in the Floyd County Historical Society in Charles City, Iowa.