The early Caterpillar D6 tractors – long-term survivors

For over 80 years, the Caterpillar D6 has been the backbone of the medium size Cat track type tractor range. Here is an overview of how things got started. By Richard Campbell.

The Caterpillar D6 can trace its origins right back to the Diesel 35 of 1933 when it was one of a trio of Caterpillar diesel tractors introduced during that year (the other machines were the Diesel 50 and the Diesel 70).

Although it was in production for only one year, it sowed the seeds for what was to come.

The following year with a slight horsepower tweak, the machine was re-introduced as the Diesel 40, and in 1935, in order to comply with Caterpillar’s new machine identification system, it became the RD6.

With a track gauge of 60 inches and a 5-roller track frame, the RD6 was powered by a 3-cylinder Caterpillar D6600 diesel which put out 52 flywheel horsepower.

In production for six years as the 2H series, the RD6 lost the “RD” from its name about 1938.

A favourite on the jobsite, farm or forest, many thousands of this type were manufactured.

An improved D6 was introduced in 1941 just in time for World War 2 and for the first time the machine was also available in a wider 74 inch track gauge due to customer requests.

Both the 60 inch gauge D6 (4R series) and 74 inch gauge D6 (5R series) were powered by the new Caterpillar D468 6-cylinder diesel engine which produced 72 flywheel horsepower.

These were very popular tractors and sold extremely well.

In this configuration, the D6 was manufactured up until 1947.

Caterpillar did some serious redesigning during the war and the result of this was probably the most successful of all the D6 models – the 8U and 9U series machines.

The 8U was a 60 inch gauge machine and the 9U was a 74 inch gauge machine, both having six track rollers, an increase of one over the former machine’s five.

Another new Caterpillar engine was chosen for the type, this time the 80 horsepower Caterpillar D318 (which was actually just the 4.25 inch bore D468 bored out to 4.5 inches with the addition of a new fuel pump).

Both of these machines were incredibly popular with the 8U selling over 11,000 examples and production of the 9U variant well over a staggering 28,000 machines.

Along the way various improvements made their way onto the production line including the Caterpillar oil clutch, a forward/reverse lever to cut down on gear shifting and an increase in the fuel pump rack setting to boost engine output to 93 flywheel horsepower.

The pair remained in production until 1959 when they were replaced by the D6B model.

Caterpillar D6Bs were also produced in the two 60 inch and 74 inch track gauges but for the first time, were also available manufactured outside of Peoria, USA.

All D6Bs were powered by the same D318 engine used in the earlier 8U and 9U series machines with a similar horsepower rating.

Most of the differences were cosmetic, making the tractors easier to mass produce and maintain in the field plus the operator’s area was considerably refined making the machines more user friendly.

As mentioned, manufacture of the D6B was undertaken outside of the USA with machines being produced in Japan (under a licence agreement with Mitsubishi) and also in Australia at Caterpillar’s plant in Melbourne.

The different variants stacked up as follows:

USA built D6Bs were the 37A series (60″) and 44A series (74″).

Australian D6Bs were the 56A series (60″) and 57A series (74″).

Caterpillar Mitsubishi D6Bs included the 37H series (74″) and 38H series, an 81 inch gauge low ground pressure model specifically designed for the Asian market and poor underfoot conditions.

All of the above machines featured the Cat oil clutch and a direct drive transmission.

US production of the D6B ceased in 1963 when it was replaced by the new turbocharged D6C model but the D6B remained in production in Japan up until 1967.

There was also a specialised pipe laying version of the D6 which rates a mention.

This first appeared as an attachment manufactured by Trackson for D6-9Us in the early 1950s and with the advent of the D6B (and Caterpillar’s acquisition of Trackson) became a standard production machine known as the 561B.

Trackson also had a hand in the manufacture of another D6-9U derivative, a machine called the No 6 shovel, introduced in 1953.

This used the chassis and engine of a D6 with a completely purpose-built loader frame, hydraulics and a bucket and was not an “add-on” as had been the case with earlier Cat Traxcavators.

The No 6 shovel eventually became the 977 Traxcavator in 1955.

Attachments for the D6

Prior to 1946, Caterpillar did not manufacture its own attachments for use with the D6 outside of cabs, guards and other small bits and pieces.

The job of building blades, PCUs, winches and other ancillary equipment was left up to outside suppliers.

Most prominent among those was LeTourneau which built a whole industry around tractor attachments.

Other major suppliers producing work tools for the D6 included LaPlant-Choate, Buckeye, Isaacson, GarWood, Carco, Hyster and Fleco just to name a few.

Just about any brand of 8-10 yard scraper could be hitched behind a D6 be it hydraulic or cable controlled.

From 1946 onwards Caterpillar offered its own work tools including the 6S and 6A bulldozer blades in cable or hydraulic configuration, No 24 and No 25 PCUs (cable controls) and the No 46 hydraulic control.

There is a great deal more to tell of the D6 story which we will examine in an upcoming issue of Contractor.

The New Zealand connection

From the very first Diesel 35s the type has been a popular seller for NZ Caterpillar dealer Goughs and importation of the machine (in its various models) would be in the thousands with the D6-9U model being particularly well represented.

The Caterpillar D6 has figured very prominently in the history of New Zealand road building, forestry and farming as it was an ideal size – not too small and not too large.

There were very few jobs a Kiwi could not do with a D6 and even fewer contractors who did not own or have a D6 of some type on their job in the 1950s.

Even the 561B pipe layer and No 6 shovel have made an appearance!

A number of older D6s have been saved and are being restored or are in preservation.

For the model collector

With a reputation as big as the Caterpillar D6 has you would think that there would be models available.

Sadly no.

While there are models in several scales of later Cat D6s (D6C upwards) the earlier variants have been completely overlooked.

There is one model of a Caterpillar D6-8U out there produced by American toymaker Dopeke in the early 1950s to about 1:16 scale.

While basically accurate, it was originally intended for the young contractor’s sandpit.

They are now ‘collectable’ and as such, fetch ridiculous prices at auction.

A very limited production 1:50 scale D6B was offered by Black Rat of the UK but only 50 were made and they are very expensive if you can locate one.

As this is an overview there are no brief specifications

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