Restoring a Caterpillar D2 5U Series Dozer from Dormancy

[MikePhua]

The Legacy of the Caterpillar D2
The Caterpillar D2 was introduced in 1938 as one of the smallest diesel-powered track-type tractors in Cat’s lineup. Designed for farming, grading, and light construction, the D2 became a symbol of rugged simplicity. The 5U series, produced in the early 1950s, featured the D311 four-cylinder diesel engine with a 4-inch bore, paired with a two-cylinder gasoline pony motor for starting. With an operating weight around 8,000 lbs and a drawbar horsepower of roughly 30 hp, the D2 was compact yet capable, especially in tight or soft terrain.
Over 20,000 units of the D2 were built across its production run, and many remain in service or restoration today. The 5U08580 serial number places this particular unit near the end of the 5U series, making it a valuable candidate for preservation.
Initial Condition and Missing Components
The machine in question had been parked for nearly a decade. When last inspected, it ran but suffered from steering issues and had several components removed. Notably:

• The left-side cylinder head of the pony motor was missing
  
• The exhaust manifold had been removed
  
• Steering clutches were reportedly stuck
  
• The hood was absent, though the idlers appeared solid
  

These symptoms suggest a combination of mechanical neglect and environmental exposure. The pony motor, which is critical for cold starts, shares its short block design with the D4 but differs in bolt-on accessories. Without the head and manifold, starting the diesel engine becomes nearly impossible.
Diagnosing Steering Clutch Seizure
Stuck steering on a D2 typically points to seized clutch packs or rusted linkage. The D2 uses dry steering clutches housed in compartments beside the transmission. Common causes of seizure include:

• Moisture ingress through missing covers
  
• Lack of use leading to rust on clutch discs
  
• Hardened grease or oil contamination
  
• Broken return springs or linkage misalignment
  

To diagnose:

• Remove the clutch inspection covers
  
• Attempt manual actuation of the clutch arms
  
• Inspect for rust flakes or oil residue
  
• Check spring tension and lever travel
  

If the clutches are seized, disassembly and cleaning may be required. In extreme cases, clutch packs must be removed and rebuilt with new friction discs and springs.
Sourcing Parts and Technical Documentation
Finding parts for a 5U-series D2 requires persistence. Recommended sources include:

• Vintage tractor salvage yards
  
• Online auctions for original manuals (e.g., 5J3501 and up)
  
• Enthusiast groups specializing in pre-1960 Caterpillar machines
  
• Cross-referencing D2 and D4 pony motor components
  

The D2 3J and 5J series used the D3400 engine with a 3¾-inch bore, while the 4U and 5U series used the D311 with a 4-inch bore. This distinction affects cylinder head compatibility and gasket selection.
Some restorers have successfully adapted D4 pony motor heads to D2 blocks with minor machining. Others fabricate exhaust manifolds using cast blanks and welding techniques.
Recommended Restoration Sequence
To bring the machine back to life:

• Clean and inspect the fuel system, including tank, lines, and filters
  
• Rebuild or replace the pony motor head and exhaust manifold
  
• Free up the steering clutches and verify transmission engagement
  
• Check undercarriage wear, especially track tension and roller condition
  
• Replace fluids and seals throughout the drivetrain
  
• Test electrical system and install new battery and cables
  
• Prime the diesel injection system and verify compression
  

If the pony motor is beyond repair, some restorers retrofit electric starters or use external cranking methods. However, preserving the original start system adds historical value.
A Story from the Field
In 2019, a retired millwright in British Columbia restored a 5U-series D2 that had sat in a barn since 1987. The pony motor was missing its carburetor and magneto, and the steering clutches were frozen solid. After sourcing parts from a dismantled D4 and rebuilding the clutch packs, he got the machine running and used it to grade his driveway. He later displayed it at a vintage equipment show, where it drew admiration for its original paint and functional pony motor.
Conclusion
Restoring a Caterpillar D2 5U is a journey into mechanical history. With patience, resourcefulness, and respect for the machine’s design, even a long-dormant unit can return to life. The D2 was built to endure, and when its engine fires again, it speaks not just of torque and steel—but of the hands that brought it back. In the world of old iron, every bolt tells a story.