The Diesel Sixty marked a significant milestone in the development history of the track type tractor in that it was the first such diesel engined machine to be put into full production by any heavy equipment company, and was the forerunner of tens of thousands of Caterpillar diesel tractors.
The Diesel Sixty was not Caterpillar’s first track type tractor by a long shot and was itself an adaptation of an existing machine that had already been in production for a number of years.
Originally designed by CL Best in 1918, and released for sale the following year, the Sixty was at the time, the largest track type tractor built by that company.
When Holt and Best merged in 1925 to form the Caterpillar Tractor Company, the Sixty was one of the CL Best models carried over into production with the new company becoming the Caterpillar Sixty.
At that stage in the Sixty’s career it was powered by a very large 4-cylinder gasoline engine running at 650rpm and producing 50 drawbar horsepower.
Weight of a gasoline Sixty was around 9.5 tons without any attachments.
It was manufactured at both the Stockton, California and Peroia, Illinois plants and was a very successful & reliable tractor.
In the 1920’s, the diesel engine was a large and cumbersome piece of equipment, usually placed in a fixed installation to power generators, pumps and the like, but it was also very economical to run.
Both Holt and Best had been looking at adapting the diesel engine to power their tractors but it wasn’t until after the merger of the two that development of the idea really began in earnest.
Other companies, notably International Harvester and Atlas, had attempted to produce a diesel-powered machine in 1928, but their prototypes were notoriously unreliable and were withdrawn very quickly.
It quickly became apparent that none of the existing diesel engines that were available would fill the needs of a track type tractor so Caterpillar set about designing their own.
This took up considerable R&D costs (over US$1 million) for a couple of years until the first experimental engine ran.
It was at this stage of development that the 2-cylinder pony engine starting system was also developed to pre-condition the diesel engine before starting.
This allowed the diesel to be started in all types of temperatures allowing oil circulation and coolant warming before putting the main engine under load.
(Love them or hate them these little starting engines were a standard fitment to Cat diesels for over 35 years).
With a workable diesel engine that Caterpillar called the D9900, the next task was a suitable chassis to put it in and the existing Sixty chassis ticked all the boxes.
Some modifications were of course necessary.
For a start, the chassis frame rails needed to be extended and increased in size due to the new engines heavier weight and castings were used in places that had formerly been fabricated.
A change was also made to the gearbox to accept the increased horsepower and torque the new diesel produced.
These modifications increased the machines weight by almost two tons.
With all these changes in place, the first Diesel Sixty, serial number 1C1, rolled off the line in September 1931 and things in the track type tractor world changed forever.
We should not forget that all of this took place during the Great Depression, so Caterpillar were taking quite a risk, one that could have potentially ruined the company.
However, the new diesel engine proved its worth right from the start as it had great lugging ability, was reliable, and extremely economical to run compared to a gasoline powered tractor.
A new market was quickly established for diesel-powered tractors.
Demand was such that Caterpillar began to design diesel engines to fit other tractors within their range, notably the D7700 and D8800.
The Diesel Sixty also featured in a number of endurance and economy tests to promote Caterpillar’s new form of power.
Probably the most famous of these Sixty’s is ‘Old Tusko’, a 1932 built machine (s/n 1C12) which took part in a non-stop plowing endurance test which set a world record not only for time but also for fuel economy.
The speed at which the acceptance of the new form of power occurred can be gauged by the fact that by the end of 1933, a scant 2 years after the first diesel Sixty tractor had appeared, over half the production of Caterpillar track type tractors were diesels.
This was not the only claim to fame for the Caterpillar model Sixty.
It was also the start of the lineage that includes the Diesel Sixty Five, Diesel Seventy, Diesel Seventy Five leading ultimately to the D8 which was introduced in 1935.
To the fledgling attachment industry, the Diesel Sixty represented a blank canvas and many manufacturers vied for the opportunity to outfit one of the new machines.
(Caterpillar did not begin to manufacture its own attachments until 1946).
So, as a consequence, it is not uncommon to see Sixty’s – both gasoline and diesel powered – with a great diversity of front and rear end equipment.
Favoured suppliers were LeTourneau, LaPlant-Choate, Isaacson, Allied, Euclid, Buckeye, Kay-Brunner, Wooldridge and Heil, all of whom manufactured dozer blades of one kind or another, with Hyster-Willamette and Carco providing logging winches.
Caterpillar also offered “retro-fit” engine kits to re-power existing gasoline engined Sixtys.
These included the diesel engine (initially a model D7700), strengthening parts, new engine mounts, new transmission shaft, radiator shroud and other components necessary to carry out the conversion. Later kits included a D8800 engine in place of the D7700.
Despite all of the accolades, the Diesel Sixty was only produced from 1931 through to the end of 1932 with a production run of just 157 machines. It was replaced by the improved Diesel Sixty Five, making the Diesel Sixty quite a rare tractor.
The first two Diesel Sixtys were built in Stockton CA, with the balance being manufactured in Peoria.
Caterpillar also sold around 100 conversion kits for re-powering former gasoline engined Sixtys.
There is one final claim to fame for the Diesel Sixty. It was the first Caterpillar track type tractor to be painted in the new ‘Hi-Way Yellow’ paint scheme, introduced December 7t, 1931 with only the two Stockton built machines leaving the assembly line in battleship grey.
For the Diecast Model Collector
For once you are in luck!
German diecast manufacturer NZG saw fit to produce a model of the Diesel Sixty in 1991 to commemorate 60 years of the diesel tractor.
This model is to 1:25 scale and was produced in a limited run of 10,000 and came with a certificate.
While detail is not on the high side, the model is accurate, nearly all metal and is a very good platform for super detailing.
As they are out of production, Ebay is your best source for one of these and you can expect to pay around US$250 – $350 for a good example.
The photos in this feature include a photo of a Diesel Sixty from my collection with a few added ‘extras’.