The long frame option made the tractor more stable and comfortable but reduced maneuverability a little. Standard was the short frame (four rollers per side) and optional was the long frame (five rollers). The track roller frames hold the wheels upon which the tractor rides over the portion of the track on the ground. This reduced the amount of track on the ground and made the tractor more maneuverable, and reputedly reduced track wear, but a bit more prone to a “rocking horse” motion. This lifted the very front of the track a little off the ground. Agricultural crawlers most often had smaller idler wheels than units destined for construction use. The idler wheels are the front wheels around which the track rolls. Other track variations and options included the size of the idler wheels (19 vs 23 in.) and the track roller frames. A ’48-47 J-Series crawler with a 40 inch track gauge would have a 3J serial number prefix and a 50 inch unit would be a 5J. Track gauge is the center to center dimension of the tracks. The D2 came in two track gauges 40 and 50 inches. When it debuted, the D2 was one of Caterpillar’s most modern crawlers and helped cement Caterpillar into history. It cranked out a maximum of 34 flywheel horsepower at 1650 rpm (32 hp continuous at 1525 rpm). It had many heavy duty design characteristics as well, including wet sleeves and five main bearings. It displaced only 221 cubic inches but weighed a whopping 1,600 lbs. Small in displacement but not small in the physical sense. This was the smallest diesel Cat had made to that time. The early D2 crawler was powered by a new engine, the four-cylinder D3400 (see DW November 2017, “The Ship’s Cat” in Vintage Smoke column). One way to tell the D3400 apart from some of the later Cat fours is by the manifolds being both on one side versus the later cross fl ow head. There was a 24-volt direct start system available as an option but it’s rare and was ordered mostly in warm climates. There was a compression release on the engine as well, but letting the engine spin over and generate heat from compression with no injection was a good way to warm it up. Not only that, you could engage the clutch and spin the diesel over for as long as it took to warm it up, before engaging the injection pump and letting the diesel take over. The pony engine was plumbed into the cooling system of the tractor so, when running, it put some heat into the engine. In the days when batteries were weak, unreliable and diesels were very hard to start, the pony engine made a lot of sense. The pony engine came standard with a rope start but a 6-volt starter/generator was optional. You can see one cylinder head at the back of the engine. The D3400 was most often started by a 35.6 ci, 2-cylinder horizontally opposed gas pony engine making 10 hp at 3000 rpm. The Cat injection pump is larger than a Geo Metro engine and certainly heavier. It was introduced in 1938 and was built into 1947. The D3400 diesel was a burley brute, even if it only displaced 221 cubic inches. The round thing on the firewall is the rope-start flywheel on the pony motor. The short lever by the seat engages the rear PTO. In the center, up high, mounted to the gasoline tank for the pony engine, is the engine speed control. The two foot pedals are the steering brakes, which can be used in conjunction with the corresponding steering lever for turning. The two steering levers declutch the tracks left or right. The lever on the far left is the hand clutch. The operator’s station is a forest of levers. The area behind the seat is the 20 gallon fuel tank, which was enough for a full day of hard work. Ditto the optional PTO, which was used for everything from agricultural implements to winches. The standard swinging drawbar was useful in agriculture and excavation. A small diesel crawler didn’t come from Cat until 1938 in the form of the D2. One of the first new designs to come after the Holt/Best merger was the Caterpillar 10 of 1928, a compact crawler delivering 18.1 PTO hp from a 143 ci gas engine. The crawler gained the most favor on larger acreages out West and on the soft ground of the fertile Central Valley of California. In the days of small farms and small tractors, small diesel crawlers also had a following in the ag world. By they end of the 1930s, Cat had a vast array of diesel-powered crawlers, as well as a large line of diesel engines for industry of all types. A diesel powerplant was one of their earliest priorities and the D9900 diesel debuted in 1931 powering the legendary Caterpillar Model 60 (see Diesel World, July 2008 issue). Best were individually among the pioneers in the crawler realm when they merged in 1925 to form the Caterpillar Tractor Company. Crawlers could put more power into traction with less ground pressure than anything else. In the early days of motorized farming, crawler tractors had a big part to play in some parts of the country.
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