Caterpillar's D7 crawler by Richard Campbell

The D7 owes it’s origins to the Caterpillar Diesel Fifty which was introduced in 1933.
While only in production for two years, the design proved it’s 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 machines 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 4T’s were manufactured before the type was discontinued in 1948, and an unknown number of D7 6T’s.
The fate of many of these military D7’s 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 it’s 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 machines 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 D7D’s have a distinctive engine note which sets them 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.



Thanks for checking out NZ Contractor magazine's clips.
Our slideshows are made from images and captions first published in the magazine.

свернуть