Classic MachinesContractor

Birth of a legend, the early Caterpillar D8

A true classic tractor in every sense of the word, the Caterpillar D8 has been around in various forms since 1935. This month we will take a look back in time through the earliest iterations of what was to become a classic track type tractor. By RICHARD CAMPBELL

The Caterpillar D8 was originally developed from an earlier machine, the Caterpillar Diesel Seventy-Five which was first introduced in 1933.

Powered by a 98 horsepower Caterpillar D11000 six-cylinder diesel engine, the Diesel Seventy-Five was itself a development of Caterpillar’s first diesel tractor, the model Sixty, which first appeared in 1931, and was the world’s first diesel powered track type tractor.

Replacing the Diesel Seventy-Five, the first Caterpillar D8 as such was the model RD8-5E series which was introduced in 1935 and discontinued the same year after only 35 examples had been manufactured. It was replaced by the RD8-1H series the same year.

Featuring Caterpillar’s new model D13000 diesel engine, the RD8-1H produced 110 flywheel horsepower and featuring a six-speed forward, two reverse transmission.

In its bare state, an RD8 weighed approximately 15 tons.

There has been much debate over the origin of the “RD” prefix applied to the D8 between 1935 and 1937.

Your author understands that it was in part, homage to the diesel engine’s original inventor, Rudolph Diesel, but any other plausible reasons have been lost over the passing years.

Reference to the RD8 was dropped midway through 1937 and has not reappeared since.

Caterpillar manufactured just under 8,000 of the D8-1H series before WW2 came along and the company went onto a wartime footing.

The armed services required track type tractors in abundance, so Caterpillar introduced the next D8 in the series, the D8-8R which was a somewhat more austere version of the previous 1H, utilizing some non-critical materials in its construction, and offering less optional extras.

There was a war on after all!

Output of the D13000 engine was increased to 130 flywheel horsepower by use of a newly developed, smaller, and more efficient fuel pump.

The machine retained the former model 1H’s six-speed transmission.

Bare weight of the tractor rose to 15½ tons.

The D8-8R series was manufactured for the duration of America’s involvement in WW2 (1941-1945) with some 9,900 being manufactured.

A good number of D8-8R’s made their way to New Zealand after the war, being uplifted from the Islands by savvy scrap dealers and sold to contractors throughout New Zealand who had been starved of equipment during the war.

Ex-US Military D8-8R’s can be identified by the numerals “US7” stamped into the machines serial number plate.

Anticipating a boom in construction following WW2, Caterpillar engineers began design work in 1944 on what was to become the most prolific of the D13000 engined variants of the D8, the D8-2U series which was officially released in mid-1945.

The D8-2U was somewhat of a different beast than previous versions of the tractor.

Changes included another horsepower increase, this time up to 148 flywheel horsepower, and a new 5-speed constant mesh transmission featuring a forward and reverse lever.

Also, for the first time, a transmission oil filter was standard equipment, previous versions of the tractor only having a drain screen in the transmission.

D8-2Us also featured Caterpillar’s new oil clutch, a wonderful invention which extended clutch plate life and helped to cool this important piece of equipment, often subjected to heavy loads.

Weight of a bare D8-2U was approximately 16¼ tons.

Production of the D8-2U lasted from 1945 through to 1953 and amounted to over 27,000 machines, making it a very important type.

The D8-2U is also notable in having a couple of sub-variants of the type manufactured, the D8 Pusher, and the D8 Hi-Output.

Both of these D8 sub-types were chosen at random off the production line and customized with an additional 1900lbs of extra counterweight, heavy duty track guards, different transmission ratios, and more air induction capability to accommodate the increased 180 flywheel horsepower output of the diesel engine (which also had a flywheel damper installed).

The units were sold as a package with a Caterpillar 8S dozer blade, No.25 double drum PCU and some units featured a rear C-frame with a push block for tandem pushing scrapers.

It is not known how many of these hybrids were manufactured, but the machines became available from 1953 until the end of D8-2U production.

Serial numbers of these units started at 2U21513 and up.

Heaviest of all the D13000 engined D8’s, bare weight was around 22½ tons.

The final D8 to use the Caterpillar D13000 engine was the D8-13A variant, introduced in 1953 and manufactured concurrently with the 2U series.

Now at the end of its developmental life, the D13000 engines output was again raised, this time  to 185 flywheel horsepower.

The D8-13A retained the 5-speed transmission and oil clutch of the previous D8-2U and featured a larger L-shaped fuel tank and single operators’ seat (replacing the former bench type of previous versions), making it a good spotting feature of this particular model.

Weight of a bare D8-13A was just over 16½ tons.

Only 3510 of the D8-13A version were built between 1953 and 1955 when the machine was replaced by the all-new D8E, F and G series machines (14A & 15A), but that as they say, is another story and will be covered in an upcoming article.

Engine starting

All the Caterpillar D8 variants mentioned in this article shared one thing in common – they all utilized a 2-cylinder petrol starting engine to “pre-condition” the main diesel engine (the main diesel engine being decompressed for this operation).

The little motor could be quite a cantankerous beast at times and was normally hand cranked.

A backfire could send you over the bonnet if you were silly enough to keep holding on to the crank handle!

Around 1938 a direct electric starting option was made available, complete with 6v battery and generator, to start the starting engine.

This option was not inexpensive but certainly saved many an operator’s knuckles over the years!

Blades and other work tools

When the Caterpillar RD8 was first introduced, bulldozer blades, cable controls, towed scrapers and rippers were supplied by outside manufacturers such as LeTourneau, LaPlant-Choate, Bucyrus-Erie, and others under several ‘preferred supplier’ agreements.

By far the predominant supplier however was LeTourneau.

Caterpillar did not begin to manufacture and supply its own attachments until 1946.

For the model collector.

Regrettably for such a famous tractor, models of the early versions of the Caterpillar RD8/D8 are practically non-existent.

There is a resin kitset of the D8-2U to an undetermined scale issued by Model Transport, but this is fairly crude and demands some skill to assemble.

Really the only game in town is the elderly 1:25 scale Caterpillar RD8 issued by NZG of Germany back in the 1980s.

Unfortunately, while big, this is also a crude model but can be vastly improved by the addition of white metal aftermarket parts which used to be available from Sherwood Models of Canada.

The bad news is that you will have to perform some rather drastic surgery on your model and even worse, Sherwood are now out of business and these parts are getting harder to find.

Your author has so far converted 14 examples of the NZG model using Sherwood parts (and a few of his own) to create a fleet of pre-1955 Caterpillar D8s.

A photo of ‘Retro Contractors Auckland’ spread of vintage D8s accompanies this article.


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