Identifying the Year of a Caterpillar D4 Dozer

[MikePhua]

The D4 and Its Historical Significance
The Caterpillar D4 is one of the most enduring models in the company’s lineup of track-type tractors. First introduced in the 1930s, the D4 was designed as a mid-size dozer for agricultural, construction, and military use. Over the decades, it evolved through multiple series—each marked by changes in engine design, transmission type, and frame configuration. From the early 2T and 4G series to the post-war 6U and 7U models, and later the D4C, D4D, and D4H, the machine has remained a symbol of rugged simplicity.
Caterpillar’s serial number system is the key to identifying the year of manufacture. Each machine carries a stamped serial prefix followed by a unique number, typically located on the left rear frame or engine block. This prefix corresponds to a production series, which can be cross-referenced with factory records or published guides.
Serial Number Prefixes and Year Ranges
Some common D4 prefixes include:

• 2T and 4G: early 1930s to mid-1940s
  
• 6U and 7U: late 1940s to mid-1950s
  
• 2T: often military surplus units from WWII
  
• D4C: 1960s to 1980s, with multiple sub-variants
  
• D4D: late 1970s to early 1980s
  
• D4H: mid-1980s to early 1990s, often with powershift transmission
  
• D4E and D4G: later models with improved hydraulics and emissions compliance
  

For example, a D4 with a 7U prefix and a serial number in the 30,000 range likely dates to the early 1950s. A D4H with a 1RJ prefix would be from the late 1980s.
Where to Locate the Serial Number
To find the serial number:

• Check the left rear of the engine block
  
• Inspect the top of the transmission housing
  
• Look near the operator’s seat on the frame rail
  
• Examine the data plate on the firewall or dashboard
  
• Review any stamped numbers on the final drive housing
  

If the plate is missing, the stamped number may still be visible under paint or rust. Use a wire brush and flashlight to reveal hidden digits. A restorer in Montana uncovered a 2T serial number beneath five layers of paint on a WWII-era D4 used for airfield construction.
Using Serial Numbers to Identify Year
Once the prefix and number are located, cross-reference with published Caterpillar serial number guides or dealer archives. Some online databases and printed manuals list year ranges by prefix and serial sequence. Dealers may also assist if the machine is still in their legacy system.
For example:

• 7U00101 to 7U99999: 1947–1959
  
• D4C 90J series: 1963–1977
  
• D4H 1RJ series: 1985–1990
  
• D4G prefix: early 2000s
  

A collector in Georgia used a 90J prefix to confirm his D4C was built in 1965, matching the engine casting date and transmission tag.
Additional Clues from Components
Other indicators of age include:

• Engine model and casting numbers
  
• Transmission type (dry clutch vs powershift)
  
• Hydraulic system layout and pump design
  
• Track frame style and roller configuration
  
• Electrical system (6V, 12V, or 24V)
  
• Paint color and decal style
  

Older D4s often used pony motors for starting, while later models switched to direct electric start. The presence of a torque converter or ROPS (Roll-Over Protective Structure) can also narrow the production window.
Restoration and Documentation Tips
For owners restoring or documenting a D4:

• Photograph all serial plates and stamped numbers
  
• Record casting dates on engine and transmission housings
  
• Compare parts to known diagrams from service manuals
  
• Join vintage Caterpillar forums or clubs for peer verification
  
• Contact the Antique Caterpillar Machinery Owners Club (ACMOC) for archival support
  

A restorer in Australia rebuilt a 6U D4 using original parts sourced from three donor machines, confirming the year through engine casting and transmission tag alignment.
Conclusion
Identifying the year of a Caterpillar D4 dozer is a blend of detective work and mechanical familiarity. With serial number prefixes, casting codes, and component clues, owners can trace their machine’s origin and place it within Caterpillar’s rich manufacturing history. Whether for restoration, resale, or pure curiosity, knowing the year adds depth to the story of a machine that helped shape the land—and still does.