Serial Number Reference

Caterpillar D4 — Model Years & Serial Ranges

Track-type tractor / dozer · classic era 1936–1959, with later generations for reference

Post-War D4 — Cat D315 engine (1947–1959)

The two "U-series" machines most restorers work on. Numbers shown are the first serial of each year; a machine belongs to a year if its number falls between that year's start and the next year's start.

6U — Narrow gauge (44″)
Year	First serial
1947	6U1
1948	6U1166
1949	6U2309
1950	6U3783
1951	6U5811
1952	6U7131
1953	6U8256
1954	6U9699
1955	6U10532
1956	6U11305
1957	6U11888
1958	6U12393
1959	6U12504 – end

7U — Wide gauge (60″)
Year	First serial
1947	7U1
1948	7U1798
1949	7U4258
1950	7U8028
1951	7U11888
1952	7U16009
1953	7U20837
1954	7U26194
1955	7U30584
1956	7U35570
1957	7U39597
1958	7U42013
1959	7U42726 – end

Early D4 & RD4 — Cat D4400 engine (1936–1947)

Gas-start diesel machines. Per-year serial breaks were not published for the wartime/early prefixes; ranges shown are production spans.

4G 1936–1938

Original RD4; redesignated D4 in 1937. Retained the 4G series through the change.

7J 1939–1943

By year: 7J1 (1939) · 7J1755 (1940) · 7J4120 (1941) · 7J7545 (1942) · 7J9940 (1943).

2T 1943–1945

WWII-era production. Large final-drive hub adopted at 2T8270.

5T 1945–1947

Final D4400-engine series before the U-series changeover.

Later Generations — for reference

A different design era (revised engines, later hydraulics). Listed so the full "D4" model-year span is complete.

Model	Years	Notes
D4C	1959–1963	First of the new-generation D4 line
D4D	1963–1977	Built in US, France & Japan; many plant prefixes
D4E	1977–1984
D4H	1985–1996	Elevated-sprocket "High Drive"
D4C (II/III)	1987–1993	Name reused for a later model; prefix 1RJ / 83J / 82J

How to read your number: the plate/casting shows a prefix + number (e.g. 7U14023). Find the largest year-start at or below your number in that prefix's column — 7U14023 sits between 7U11888 (1951) and 7U16009 (1952), so it's a 1951 build. Prefix 6U = narrow-gauge (44″), 7U = wide-gauge (60″). Year-end serials: 6U ends ~6U12571, 7U ends ~7U42890.