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.
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.
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.
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.
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.
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.
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
Weight: 15,642 lbs.
Fuel Capacity: 45 gals.
* As Rated by Caterpillar