Replacing the Pony Motor on a Caterpillar D2 with a Modern Gas Engine

[MikePhua]

The Legacy of the Caterpillar D2 and Its Starting System
The Caterpillar D2 crawler tractor, introduced in 1938, was designed for small-scale farming, grading, and light industrial work. With over 25,000 units produced through the mid-1950s, the D2 became a symbol of rugged simplicity. Its diesel engine was started using a gasoline-powered pony motor—a compact two-cylinder engine mounted on the side of the main block. The pony motor spun the diesel engine until compression and heat allowed ignition.
While effective in its time, pony motors are now a source of frustration for many owners. Parts are scarce, ignition systems degrade, and carburetors clog easily. Cold starts in winter can be especially challenging. As a result, some restorers and operators have opted to replace the pony motor with a modern gas engine or electric starter system.
Challenges of Pony Motor Maintenance
Common issues with pony motors include:

• Worn magnetos and weak spark
  
• Gummed-up carburetors from ethanol fuel
  
• Exhaust leaks into the diesel intake
  
• Starter clutch slippage
  
• Difficulty sourcing parts like coils, points, and gaskets
  

One operator described spending hours trying to coax his pony motor to life, only to have it stall mid-crank. After rebuilding the carburetor and replacing the magneto, the motor ran—but only under ideal conditions. In cold weather or after long idle periods, reliability dropped sharply.
Modern Conversion Options and Considerations
Replacing the pony motor with a new gas engine requires careful planning. The goal is to replicate the torque and RPM needed to spin the diesel engine through its compression stroke. Most conversions use small horizontal-shaft engines such as:

• Honda GX390 (13 hp)
  
• Briggs & Stratton Vanguard V-Twin (16–23 hp)
  
• Kohler Command Pro (14–20 hp)
  

These engines offer electric start, reliable ignition, and modern fuel systems. Key considerations include:

• Shaft alignment with the diesel flywheel
  
• Mounting brackets and vibration isolation
  
• Belt or chain drive configuration
  
• Safety shielding and throttle control
  
• Exhaust routing to avoid heat buildup
  

Some conversions retain the original clutch housing, adapting the new engine to engage the diesel flywheel through the existing mechanism. Others use direct coupling with a flexible shaft or torque converter.
Fabrication and Field Testing
One successful conversion involved a Honda GX390 mounted on a custom steel plate bolted to the D2’s frame. The operator used a double V-belt system to drive the flywheel, with a spring-loaded tensioner to engage the crank. After initial testing, he added a solenoid starter and remote choke cable for cold starts.
The result was a reliable, one-button start system that eliminated the need for manual cranking or pony motor maintenance. The diesel engine fired within 10 seconds, even in sub-freezing temperatures.
Historical Preservation vs Practical Upgrades
Purists argue that replacing the pony motor compromises the historical integrity of the D2. For museum pieces and parade tractors, restoration of the original system is preferred. However, for working machines used in grading, logging, or hobby farming, practicality often wins.
Some owners preserve the pony motor cosmetically while disabling it internally, using a hidden electric starter or auxiliary engine for daily use. This hybrid approach maintains the tractor’s appearance while improving reliability.
Safety and Operational Improvements
Modern engines offer several safety advantages:

• Automatic low-oil shutdown
  
• Enclosed ignition systems
  
• Consistent RPM control
  
• Reduced fire risk from fuel leaks
  

Operators should install kill switches, shielding around belts and pulleys, and proper grounding. Fuel tanks must be securely mounted and vented away from heat sources.
Recommendations for Conversion Projects
Before beginning a pony motor replacement:

• Measure available space and shaft alignment
  
• Choose an engine with sufficient torque and RPM
  
• Design a mounting system that allows easy removal
  
• Test drive ratios to ensure smooth diesel cranking
  
• Document all changes for future service and resale
  

Consult local fabricators or small engine specialists for bracket design and welding. Use high-quality belts and pulleys rated for continuous duty.
Conclusion
Replacing the pony motor on a Caterpillar D2 with a modern gas engine is a practical solution for owners seeking reliability and ease of use. While it may diverge from historical restoration, it allows these iconic machines to continue working in the field. With careful planning and fabrication, the conversion can preserve the spirit of the D2 while embracing the convenience of modern technology.