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Caterpillar first introduced the RD4 in 1936 as the diesel follow on to the successful CAT 30 gas model. The RD4 originally weighed in at 10,000 pounds, and used Caterpillar's D4400 engine, an inline four-cylinders, with a 4.25x5.50 inch bore and stroke. In 1935 Caterpillar had started the naming convention of "RD" for diesel or "R" for regular gasoline, followed by a number to indicate the relative engine size. In 1937 the "R" was dropped, and just a "D" was used for the diesel versions. At the time of introduction of the RD4, the D4400 engine produced about 43 HP at the drawbar so the 4 indicated the relative engine power. The D4 U series was fitted with the more powerful D315 engine. The later D4 series engines quickly increased in power, so the number "4" just became a figure of merit rather than indicating actual engine power.
- 6U = Narrow gauge 44"
- 7U = Wide Gauge 60"
- Caterpillar RD4 - 1936-37 (4G) with 4-cylinder D4400 engine
- Caterpillar D4 - 1937-39 (4G) with 4-cylinder D4400 engine
- Caterpillar D4 - 1939-43 (7J) with 4-cylinder D4400 engine
- Caterpillar D4 - 1943-45 (2T) with 4-cylinder D4400 engine
- Caterpillar D4 - 1945-47 (5T) with 4-cylinder D4400 engine
- Caterpillar D4 - 1947-59 6U1 > 6U12781, 7U1-7U44307 Cat 4-cylinder D315 engine.
- Caterpillar D4C - 1959-63
- Caterpillar D4D - 1963-77
- Caterpillar D4E - 1977-84
- Caterpillar D4H - 1985-96
- Caterpillar D4C SII -1990-93
- Caterpillar D4C SIII - 1993
- Caterpillar D5
- Caterpillar D3
- Caterpillar D2
- Heavy equipment
- G-numbers Army cats.
- TM 9-2800 military vehicles
- SNL G151 light tractor
- Orleman, Eric C. (2006) Caterpillar Motorbooks, St. Paul, Minnesota, page 38, ISBN 0-7603-2553-7
- Leffingwell, Randy (1996) Classic Farm Tractors: History of the Farm Tractor Motorbooks International, Osceola, Wisconsin, page 70, ISBN 0-7603-0246-4
- Caterpillar Chronicle by Eric C.Orlemann, published by MBI
- Caterpillar Pocket Guide, by Bob LaVoie
- Caterpillar by Eric Orlemann, pub by MBI, ISBN 0-7603-0529-3