The iconic Caterpillar D7 has been in continuous production in various versions since 1935. It has fought in three major conflicts – WWII, Korea & Vietnam – and is strong testament to a product that really was “built for it” By RICHARD CAMPBELL
The D7 owes its origins to the Caterpillar Diesel Fifty which was introduced in 1933.
While only in production for two years, the design proved its viability and in 1935 an improved version with a new name was introduced, the RD-7 5E series, powered by a 76 horsepower Caterpillar D8800 four-cylinder diesel engine. Very few RD-7 5E machines were built before Caterpillar came out with another version of the machine, the RD-7 9G series.
Also powered by the D8800 engine, now rated at 80 flywheel horsepower, the RD-7 9G was available in narrow gauge (60”) or wide gauge (74”).
Caterpillar dropped the “RD” designation at machine number 9G6501 with the tractor now just becoming the D7.
Production ended in 1940 with more than 7000 machines manufactured.
World War 2
As America entered a war footing, a new version of the D7 was introduced, primarily intended for military service, but a great many were also sold into commercial use.
This was the D7 7M series which was manufactured from 1940 through 1944.
Also powered by the D8800 engine, this had been uprated to 93 flywheel horsepower and the machine was only available in 74” gauge, Caterpillar having dropped the 60” option due to very low sales demand.
Some 8100 examples were manufactured before the model was supplanted in production by the improved D7 3T, D7 4T and D7 6T.
Wartime D7’s acquitted themselves very well and were used on all fronts, earning a respected reputation for ruggedness and reliability.
Post War Variants
The D7 3T, D7 4T and D7 6T were introduced in mid-1944.
The machine’s incorporated several improvements that Caterpillar engineers had developed but were unable to introduce due to the necessity of maintaining production of the existing 7M for WWII requirements.
As the urgent demand for tractors subsided these improvements made their way into series production.
The D7 4T was built solely for the US Army while the D7 6T was built for the US Navy.
This enabled Caterpillar to fulfill military contracts that had been let but not completed.
Just over 7500 D7 4Ts were manufactured before the type was discontinued in 1948, and an unknown number of D7 6T’s.
The fate of many of these military D7s is quite sad as many were pushed into vast pits of surplus equipment and buried while others were just driven into the jungle and left there, some with very few hours on the clock.
This wholesale “jettisoning” of unwanted equipment is what makes the D7 4T a somewhat rare type these days and the D7 6T even rarer.
There is hardly any difference between a D7 3T, 4T or 6T other than the optional external attachments applied to it.
A highly successful variant.
Between 1944 and 1955, Caterpillar manufactured over 28,000 D7 3T models.
With the trusty Caterpillar D8800 engine initially providing 93 flywheel horsepower, this was increased to 108 flywheel horsepower around 1952.
One of the new features Caterpillar had introduced was a forward/reverse lever into the transmission meaning the operator didn’t have to repeatedly shift gears all the time when changing direction.
Other improvements added along the way was Caterpillar’s famous oil clutch and a two-position front idler which better set the machine up for trailed or pushed attachments.
D7 3T’s can still be found doing occasional work some sixty years after the last one left the production line.
The next step
With the D8800 diesel engine now at the upper reaches of its development potential, Caterpillar introduced a new version of the D7 in 1955.
This was the legendary D7 17A series, (also known as the D7C).
This featured the all-new Caterpillar D339 four-cylinder diesel engine rated at 128 flywheel horsepower and incorporating all the better features of the previous 3T model including oil clutch, forward/reverse lever plus the addition of the newly-developed hydraulic track adjusters, a great labour-saving device which most people take for granted these days.
Halfway through the machine’s extensive production term, Caterpillar made two crucial changes to the machine.
At serial number 17A11981, a turbocharger was added to the engine, boosting output to 140 flywheel horsepower and allowing the machine to work at higher altitudes without engine de-rating.
Caterpillar also fitted oil-cooled steering clutches and brakes which greatly extended the service life of these components.
Although Caterpillar did not change the serial number prefix of the new improved D7 17A, it did get a new designation of D7D.
These later D7Ds have a distinctive engine note which sets it apart from all other D7’s.
Production of the D7C/D7D had exceeded 19,000 machines when the last one came off the line in 1961.
Into the 1960s
Two new D7 models were introduced in 1961 – The D7E 47A series and D7E 48A series.
Difference between the two models was simple – the 47A was a direct drive machine with an oil clutch and the 48A had the newly developed Caterpillar three-speed powershift transmission.
Both utilised the turbocharged Caterpillar D339T diesel rated at 160 flywheel horsepower.
The engine output was increased to 180 flywheel horsepower during the machine’s production run.
Manufacturing span of the D7E was from 1961 to 1969 encompassing over 10,000 machines, and they are notable in being the last D7’s solely manufactured in the USA.
The post-1970 models D7F, D7G and upwards will be covered in a forthcoming article.
What to hang off your D7
Until 1946, Caterpillar did not manufacture any attachments for its track type tractors.
These three companies manufactured everything you needed to bolt onto, hang off of, or pull behind your D7.
This all changed in 1946 when Caterpillar began to build its own attachments tailored to its tractors with the exception of Hyster who still supplied the majority of logging winches to Caterpillar.
The New Zealand Connection
The story of the early Caterpillar D7 in NZ is also the story of New Zealand logging, roading, quarrying and the Public Works Department (later Ministry of Works).
Many ex-wartime D7’s were imported into New Zealand and served extensively throughout the country in Government and private ownership.
D7 17A’s ruled every major skid site in the North Island for a great many years and can still be seen plying their trade in this work occasionally.
They are a very hard tractor to kill!
The number of early Cat D7’s (1935-1970) imported into New Zealand would easily exceed 1500 units.
For the Model Collector
There are models of earlier D7s available.
At around 1:160 there is the old Matchbox #8 which represents a non-turbo D7-17A.
Its size and lack of detail really preclude it from the serious collector.
Regrettably there is nothing available in the more common 1:50 size.
Both represent the D7 7M and are excellent representations.
However, as with all kitsets, time and patience are absolutely essential to achieve a good result.
In 1:25 there are a couple of interesting models.
During the mid-1950s, US company Cruver produced a plastic D7 3T/7M (its difficult to tell) which is a darn nice model.
Nothing works, it just looks good, but it does have a fatal flaw in that some examples have warped over time due to the type of plastic used.
These can be found on Ebay for not too serious dollars.
The last is the NZG/Conrad D7 4T which has been issued a couple of times.
It comes with a LeTourneau CK-7 blade, double drum rear mounted PCU and military paint job.
What could have been a fantastic model due to its size is spoiled by shortcuts taken in design and manufacture.
The model has hideous tracks that no self-respecting D7 would have ever worn, a track frame that is far too long, poor engine detail and a badly functioning cable control that has too few sheaves.
Your author took to his example with drill, saws and files plus a heap of replacement parts and turned it into a far more pleasing replica of the real thing.