A legendary tractor – the Caterpillar D8

There are not too many tractors that can be considered truly legendary. However, the Caterpillar D8 is one of those machines, and in this issue, and I will examine those machines produced in the first 20 years that establish the legend of the D8. Richard Campbell.

The D8 was not Caterpillar’s first large track type tractor, nor it’s first diesel powered machine.

That honour belongs to the model Sixty, the first diesel variant of which appeared in 1931 powered by a Caterpillar model D9900 engine.

This was followed closely by the Diesel Sixty-Five (1933) and the short-lived Diesel- Seventy (both also powered by the D9900 engine), then the D8’s grand-daddy, the Diesel Seventy-Five which was manufactured from 1934 through to 1935 and used a new Caterpillar engine, the model D11000.

The very first Caterpillar D8 (or RD8 as it was originally called) appeared in 1935 and was known as the 5E series, but only 34 of these were built before production switched to the RD8-1H series which was fitted with Caterpillars new D13000 series diesel engine.

The RD designation

There are a few stories attached to the RD prefix that was used on the models D4, D6, D7 and D8.

One story was that it was an homage to Rudolph Diesel, inventor of the diesel engine. The other (and most likely story) is that it had its roots in President Franklin D Roosevelts ’New Deal’ plan, designed to get the USA working again after the Great Depression with the ‘R’ standing for Roosevelt.

In any case, the ‘RD’ designator only lasted until 1938 after which time common sense prevailed, and the machines reverted to D4, D6, D7 et cetera.

The RD8-1H/D8-1H

Introduced the same year as the RD8-5E, the D8-1H series proved to be a very popular tractor, with Caterpillar manufacturing almost 10,000 of them by 1941, at which time production switched to a slightly upgraded version known as the D8-8R series.

The D8-1H was powered by a six-cylinder Caterpillar D13000 diesel engine that produced 110-flywheel horsepower.

A six-speed direct drive transmission with a single reverse gear was connected to the engine via a manually operated dry type clutch and gave the RD8-1H a top speed of around 5.4mph in sixth range.

Early machines had fabricated front idlers that had a spoke pattern, but these proved to be troublesome in service and were replaced with cast, spoked units.

Several other upgrades were made on the production line before war-time necessities required a new model tractor. A bare RD8-1H series machine weighed approximately 16 and a half tons.

The D8-8R series

Manufactured from 1941 through to 1945, the D8-8R was Caterpillars “austerity tractor” and featured a lot fewer options due to War Production Board restrictions.

The vast majority of the 8R series machines manufactured were supplied to the US and other Allied nations armed services, and in this capacity, they served worldwide, with quite a few ending up in New Zealand post-war.

Lauded as the tractor that could “just about do anything”, the D8-8R featured the same Caterpillar D13000 diesel engine of the earlier 1H series, but with its output increased to 131-flywheel horsepower.

The transmission fitted to the 8R series was slightly different in that it incorporated a hi-lo reverse range for speedier backups, these alterations also lowered top speed slightly to 4.6 mph.

Drum style front idlers also made an appearance, replacing the older spoked type.

The last D8-8R (8R9999) left the line in January 1946. Shipped from the Peoria manufacturing facility, a bare D8-8R weighed just over 17 tons.

Post-war, the D8-2U series

By far the most numerous of all the D13000-engined variants of the D8 was the D8-2U series, with over 27,000 of them being manufactured between 1946 and 1953.

The D8-2U featured a host of improvements over its predecessors including Caterpillars famous oil type clutch (on later versions).

Engine output was raised to 148-flywheel horsepower, and a new transmission was installed, featuring 5-forward and 3-reverse ranges, making the tractor an even more flexible tool.

Special production variants also began to appear ex-factory such as the D8 Pusher, and D8 Hi-Output machines, both basically designed as scraper push tractors for Caterpillar’s newly introduced line of motor scrapers.

These featured increased output from the D13000 engine, which had its high idle speed increased as well as a change in fuel rack setting to achieve the required horsepower, and a higher ratio transmission to better keep up with the new scrapers in the cut.

The D8-2U was also the first of the D8s to be offered from the factory with Caterpillar’s own range of blades, cable controls, towed scrapers, and hydraulic systems.

Previously, Caterpillar had relied on a range of outside manufacturers such as LeTourneau, LaPlant-Choate, Isaacson, Wooldridge and others for these items, but now, the entire machine could be supplied fully equipped ‘in-house’.

A bare Caterpillar D8-2U tipped the scales at just over 18 tons.

The D8-13A, last of the D13000 powered D8s

Introduced in 1953, the 13A series Caterpillar D8 was the last of the D13000-engined D8 variants.

Engine output was increased to 180-flywheel horsepower, but by now the D13000 engine was really at the very end of its development potential.

Caterpillar did, however, have another spectacularly good diesel engine waiting in the wings, the model D342, but that is another story.

Production of the D8-13A only lasted for two years before it was replaced by the all new D8-14A and D8-15A series machines using the new D342 engine.

Despite a short production period, Caterpillar did manage to manufacture 3,510 D8-13As before production switched to the new types.

A bare Caterpillar D8-13A weighed 18.5 tons before any attachments were added.

For the model collector

When it comes to models of early Caterpillar D8s, there is very little to be had.

At one stage back in the mid-1980s, German manufacturer NZG issued a 1:25 scale RD8 to celebrate “50 years of the Cat D8”.

This model, now hard to find, was produced in a limited run of 10,000 models, all serial numbered. It also had horrible tracks and limited detailing despite the large scale.

It is possible however, to create a museum-quality model from one of these NZG models with some replacement parts, good references and a considerable amount of elbow grease.

I have chopped up 15 of them so far and I am very pleased with the results!

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