1935 Caterpillar Diesel Forty

By the mid-1930s, Caterpillar had only been a company for ten years but was dominating the crawler market. They had also become a leading innovator of diesel engine technology. Soon after developing their own diesel, Caterpillar Tractor Company began reducing their production of gasoline-powered equipment and increasing the number of diesel being sold. The market was moving that way on its own but the diesel-related infrastructure was moving more slowly than Cat wanted, so instead of waiting for it to catch up, they pushed it along by helping to develop cures for some of the issues such as lubricants (see https://www.dieselworldmag.com/diesel-tractors/the-engine-that-answered-questions/) and fuels. Cat re-laid their foundation on diesel power and doubled down on the bet.

Gene Pock’s 1935 Cat Diesel Forty was apparently built as a drawbar crawler and used by a now forgotten municipality in the Midwest. The distinctions between drawbar and agricultural crawlers are generally slight and might come down to certain options. This tractor clearly has never had a dozer attachment on it, so it wasn’t used that way. The canopy is a factory Cat accessory and original to the unit.

Cat’s first diesel, the D9900, appeared in 1931 but was supplemented by new designs in 1933 based around a 5.25 x 8.00-inch bore and stroke cylinder. The new engines came in three, four, and six-cylinder configurations. The D6100 was a three-cylinder that displaced 520 cubic inches and was used in the ‘33-34 Diesel Thirty-Five and the ‘34-36 Diesel Forty crawlers. The D7700 was a four that displaced 692 cubic inches and was used in the ‘33-36 Diesel Fifty. The D11000 was a six that displaced 1039 cubic inches and was used in the ‘33-35 Diesel Seventy-Five crawler. The engines also saw use as stationary powerplants at various times, as well as powering other mobile equipment like the legendary Auto Patrol road grader. The fours and sixes were used in marine power applications starting in 1938.

The Diesel Forty was the smallest diesel-powered tractor in the Cat lineup at the time it was built. There were smaller gasoline or distillate-powered units but not diesel. Diesel was a Godsend to the big crawler market because the gasoline versions used fuel at a rate that made owners blanch, even with gasoline under twenty cents per gallon. By the end of 1937, the lineup was joined by the D2, which was a small diesel (https://www.dieselworldmag.com/diesel-tractors/the-mighty-little-caterpillar-d2/). The RD-4 (later known as the D-4) also joined the lineup at the time the Diesel Forty was fading into the RD-6. It had the new 312 ci D4400 four-cylinder engine.

A couple of things to note start with model designations. From the early days, it was thought spelling out the model number … “Forty”  for example … was classier than using numerals. Ok, fine, but this would begin to go away after 1935. Second, note  “Diesel” in the model name. Cat offered both diesel and gasoline models. In the case of the Forty, a unit without “Diesel” in the model name had a gas engine. Further, the serial number prefix for a gasoline Forty was “5G” and the Diesel Forty was “3G.”

The engine designations have no discernable relation to power, displacement, or configuration. The expert’s best guesses are that they came from engineering and were related to design or project numbers. Thus far, researchers have not found any relevant information that sheds light.

A typical vintage crawler set of controls, including left and right brake pedals and clutch levers, four-speed gearshift, and engine speed control on the firewall. Instrumentation was minimal, with oil pressure and fuel pressure. There was an hour meter on the engine but the fuel level was checked by a dipstick.

The Cat Makes Forty

The Caterpillar Diesel Forty supplanted the Diesel Thirty-Five late in 1934. The Diesel Forty had some advancements and a few more horsepower than the Diesel Thirty-Five but was largely the same crawler. Powering both units was the D6100 three-cylinder. There are no Nebraska Tractor Tests of a Diesel Forty with the D6100 engine but a Diesel Thirty-Five was tested in October of 1933 and yielded a rated 39.29 belt horsepower at 850 rpm, and a maximum of 44.72 belt horsepower. Rated drawbar power was 29.98 horsepower at 850 rpm and a maximum of 39.53 horses at 850 rpm. Caterpillar advertised the drawbar power of the Diesel Thirty-Five at 40 horsepower. According to old materials on the D6100 powered Diesel Forty, Cat advertised it at 44 horsepower on the drawbar, a few more than the Diesel Thirty-Five, but that was just pushing the number a little for advertising, and the two tractors were really pretty close in performance.

Got three cylinders and uses ‘em all! Basically, this is half of the D11000 six. Or, you could say the D11000 was a double three-cylinder. The D11000 used two separate cylinder heads so either way, both engines were relatively easy to manufacture and because the three engines in this series used the same parts, you could call them “modular.” Sharing a 5.25 x 8-inch bore and stroke, the engines made approximately 15 rated horsepower per cylinder at 850 rpm and about 100 lbs-ft per cylinder at 600 rpm. They were started via a gasoline-fueled, two-cylinder inline pony engine that was hand-started via a crank at the front of the crawler. The 47 horsepower (maximum at the flywheel) D6100 triple lasted into 1936 and was replaced by the similar 60 horsepower (maximum at the flywheel) D6600 featuring a 5.75-inch bore. Those maximum flywheel numbers were not the ratings used in crawlers, but rather a rating used with power units. The whole 5.25 x 8-inch line was updated to a 5.75-inch bore. The small bore four-cylinder D7700 and six-cylinder D11000 outlasted the D6100 by a few years as stationary powerplants. This engine design would remain a strong seller through WWII and into the 1940s, before gradually being replaced by more modern engines. The last of the 5.75 x 8 engines would be gone by the end of the 1950s. The three cylinders were gone by 1941.

The Diesel Forty came in two track gauges, 56 and 74 inches. Track gauge is the center-to-center measurement of the tracks. The narrow gauge units came standard with 16-inch wide shoes and the wide gauge came standard with 20-inch. Other sizes and 13 types of grouser cleats were optional. The unit could be outfitted as a dozer as a drawbar or an ag crawler from the factory. An orchard version, with a low seat, was also offered. If a dozer, a LeTourneau blade was used in this era.

Other options included heavy-duty track roller frame guards, brush guard, front bumper, muffler and spark arrester, odometer, wooden cab, steel canopy (as shown in the pictures), generator and lights, front PTO, several seating options, and a few cold weather options.

A belt pulley was usually an agricultural attachment, though not always if a construction outfit had equipment that needed separate power. Back in the ‘30s, there were rock crushers and such that were driven by belts. The swinging drawbar did the heavy work. There were a number of PTO options, including the shown belt pulley, driven at 540 rpm, a direct-drive tapered splined shaft, or a standard 10 spine shaft. There were also a couple of drawbar options, including a raised attachment.

By Any Other Name

Starting in 1935, the Diesel Forty was supplanted by the RD-6. Cat went to a new system of naming crawlers that we won’t try to explain. Even the most die-hard Cat experts can’t explain it, with the meaning of “RD” unclear, not documented, and hotly debated. With the RD-6 came new 5.75-inch bore engines. The RD-6 essentially replaced the Diesel Forty in the lineup. The changeover wasn’t exactly linear and there was some overlap, but in ‘36 the Diesel Forty was no more.

In 1937, someone at Cat started taking their meds again and rationalized the model line into the now-familiar “D” nomenclature. The RD-6 became the D-6 and other legacy models in the lineup followed suit. The D-6 remained largely the same unit as the Diesel Forty and RD-6  and lasted through 1940. It was replaced by a completely updated D6 with a new six-cylinder D4600 engine.

The D6100 was started by a gasoline-fueled starting engine. By the ‘30s, electric starting systems were nothing new but starting a gasoline engine in a car or truck with a 4.5:1 compression ratio using the weak batteries of the day was one thing. Starting a monstrous diesel with a 16:1 compression ratio, in cold weather, was another. There were several answers to this, including engines that started on gasoline and switched to diesel (see https://www.dieselworldmag.com/diesel-engines/starting-on-gas/) but Cat chose the starting engine. Also known as a “pony motor,” the starting engine could be coupled to the diesel, and its cooling system was also connected with the diesel so while it ran, it was putting heat into the diesel. In many applications, the exhaust was also ducted through a passage in the diesel intake manifold to warm it up. The diesel had a compression release so at first, you could spin the engine up easily to build oil pressure and warm the oil a little. Then, you could drop the compression back in and lock out the injection pump so no fuel was being supplied. Spinning the engine with full compression built heat in the combustion chambers. Once the engine was nice and toasty, you engaged the injection and the diesel started and was about halfway warmed up. The starting engine was then uncoupled and shut down. Many of the Cat engines of the era shared starting engines and in the case of the Diesel Forty, it was an 83 cubic inch two-cylinder (3.625 x 4-in bore and stroke) that made 24 horsepower at 2700 rpm.

A Stepping Stone

The Diesel Forty can be regarded as one of the stepping stones Caterpillar used to succeed in the crawler market. It was a “just right” size for many and its legacy in the lineup still fills a popular size niche in the market. A total of 1,971 Diesel Forty tractors were made. And if you wonder whether Cat’s evolution to diesel wasn’t right on the money, note that only 584 gasoline-powered Forty crawlers were sold over the same production period.


1935 Caterpillar Diesel Forty

Engine: 3-cylinder, IDI, Cat D6100
Displacement: 520 ci
Bore & Stroke: 5.25 x 8 in.
*Rated Drawbar Power: 40 hp @ 850 rpm
*Maximum Flywheel Power: 47 hp @ 850 rpm
*Maximum Flywheel Torque: 320 lbs-ft @ 600 rpm
Compression Ratio: 15.5:1
Transmission: 4-speed
Weight: 15,642 lbs.
Fuel Capacity: 45 gals.
* As Rated by Caterpillar

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