Reviving a 1959 Caterpillar 955H: Restoration, Maintenance, and Field Wisdom

[MikePhua]

Introduction to the 955H Legacy
The Caterpillar 955H track loader, introduced in the late 1950s, remains a symbol of rugged reliability in earthmoving and forestry operations. With its mechanical simplicity and robust build, it continues to serve in niche applications like land clearing, animal rescue operations, and rural development. However, restoring and maintaining such a vintage machine requires a blend of historical knowledge, mechanical intuition, and practical improvisation.
Terminology Notes

• Track Loader: A crawler-type machine with a front-mounted bucket used for digging, loading, and grading.
  
• Power Shift Transmission: A hydraulic transmission allowing gear changes without clutching.
  
• Torque Converter: A fluid coupling that transmits and multiplies engine torque to the transmission.
  
• Fuel Tower: A vertical housing containing multiple fuel filters and bleed screws.
  
• Scavenge Pump: A pump that removes excess oil from the flywheel housing to prevent overfilling.
  
• Serial Number (S/N): A unique identifier for the machine, crucial for sourcing correct parts.
  

Initial Assessment and Engine Identification
The machine in question was acquired for use on a 65-acre animal rescue property. Though labeled as a 1959 model, the serial number (60A00799) suggests it was manufactured in 1960. Interestingly, the engine bore a 43A prefix, indicating it originated from a 944 wheel loader. This transplant raises concerns about parts compatibility, especially since the original 955H engine was a D330 without an air compressor, while the 944 variant included one.
Fuel System Challenges and Priming Techniques
During transport, a deteriorated rubber fuel line ruptured, causing the engine to run dry. After replacing the line, priming the fuel system proved difficult. Operators shared several effective methods:

• Gravity Bleed Method
  Fill the fuel tank completely so the fuel level sits above the injection pump. Loosen bleeder screws at the filter tower and injectors to allow gravity-fed fuel to purge air.
  
• Inner Tube Pressurization
  Clamp a partially inflated inner tube over the fuel tank fill neck. The gentle pressure pushes fuel through the system without damaging seals.
  
• Manual Bleeding
  Loosen the large thumbscrew on the filter tower (identified as a needle valve) to bleed filters. Use a 5/16" six-point socket to crack injector bleeders if a Cat wrench is unavailable.
  

Maintenance Recommendations and Fluid Specs
Given the unknown service history, a full fluid and filter change was advised. Key recommendations include:

• Transmission and Torque Converter
  Use TDTO (Transmission Drive Train Oil) or SAE 30 engine oil. Check for signs of fuel dilution in engine oil—a common issue with worn fuel transfer pumps.
  
• Final Drives
  Use SAE 80/90 gear oil for optimal protection, especially in warmer climates.
  
• Flywheel Housing
  Maintain oil level below the torque converter to prevent overheating. Approximately 2.5 gallons of engine oil is typical.
  
• Steering Clutches
  If equipped with hydraulic assist, ensure the booster pump delivers 9 GPM at 1940 RPM and 350 PSI. Relief pressure should be 550–650 PSI.
  

Real-World Anecdotes and Operator Wisdom

• A retired quarry mechanic emphasized cutting open used filters to inspect for metal shavings—a low-cost diagnostic method.
  
• One operator noted that his machine ran well once warmed up, but experienced transmission slippage on steep inclines. This was resolved by topping off fluids and replacing filters.
  
• Another shared that his fuel tower had four filters, and the phenolic rods inside were fragile—requiring careful handling during replacement.
  

Historical Context and Broader Lessons
The 955H was part of Caterpillar’s transition from cable-operated loaders to hydraulic systems. Its simplicity made it popular among forestry crews and rural contractors. Unlike modern machines, it lacks electronic diagnostics, relying instead on mechanical feedback and operator intuition.
In the 1960s, Caterpillar standardized serial numbers across machines and engines. However, engine swaps were common, especially in remote areas where parts availability dictated improvisation. This practice, while practical, complicates restoration efforts today.
Conclusion: Breathing Life into Iron
Restoring a 1959 Caterpillar 955H is more than a mechanical task—it’s a tribute to enduring engineering. From fuel system quirks to transmission tuning, each challenge offers a lesson in patience and problem-solving. As one operator put it, “She’s tired, but she’s still got fight. You just have to listen to her.”