Reviving the 1941 International TD-6 Dozer and Its Dual-Start Legacy

[MikePhua]

The Birth of the TD-6 and International Harvester’s Engineering Vision
The International TD-6 crawler tractor was introduced in the late 1930s by International Harvester, a company that had already made its mark in agricultural machinery and industrial equipment. By 1941, the TD-6 had become a staple in small-scale earthmoving, logging, and wartime infrastructure projects. With a rated drawbar horsepower of around 25 and an operating weight of roughly 8,000 lbs, the TD-6 was compact but capable, designed to serve contractors, farmers, and military engineers alike.
What set the TD-6 apart was its dual-engine starting system—a gasoline pony motor used to start the main diesel engine. This approach was common in pre-war diesel equipment, where electric starters lacked the torque and reliability needed to crank cold diesel engines. The TD-6’s design reflected a transitional era in heavy machinery, where mechanical ingenuity bridged the gap between simplicity and power.
Terminology Notes

• Pony Motor: A small gasoline engine used to start a larger diesel engine by spinning it up to cranking speed.
  
• Diesel Conversion: The process of switching from gasoline start to direct electric start or full diesel ignition.
  
• Magneto Ignition: A self-contained ignition system used in early gasoline engines, generating spark without a battery.
  
• Dry Clutch: A friction clutch not immersed in oil, used to engage the transmission.
  
• Crawler Tractor: A tracked machine designed for pushing, pulling, and grading tasks.
  

Starting Ritual and Mechanical Choreography
Starting a TD-6 was a multi-step process that required patience and precision:

• Open the decompression valves on the diesel engine
  
• Start the pony motor using a hand crank or electric starter
  
• Allow the pony motor to warm up and engage the diesel flywheel
  
• Once spinning, close the decompression valves and switch fuel delivery to diesel
  
• Monitor exhaust and engine tone for successful ignition
  

In one logging camp in British Columbia, a TD-6 was known to start reliably even in sub-zero temperatures, thanks to its pony motor and pre-heated intake manifold. The operator, nicknamed “Red,” claimed he could start the machine blindfolded—a testament to the ritualistic nature of early diesel operation.
Common Issues and Restoration Challenges
Restoring a TD-6 after decades of dormancy presents unique challenges:

• Pony motor magneto failure due to moisture or age
  
• Carburetor clogging from varnished fuel
  
• Diesel injector leakage or poor atomization
  
• Decompression valve sticking or misalignment
  
• Clutch plate glazing from long-term inactivity
  

Recommended solutions:

• Rebuild magneto with new coil and condenser
  
• Ultrasonic clean carburetor and replace float needle
  
• Bench-test injectors and replace with matched spray pattern units
  
• Lap decompression valves and verify seating
  
• Resurface clutch plates and adjust linkage tension
  

In one restoration project in Iowa, a retired mechanic rebuilt his TD-6 using parts sourced from three donor machines. He fabricated a new exhaust manifold from cast iron pipe and used a vintage magneto tester to revive the pony motor’s spark.
Hydraulics and Blade Control
Many TD-6 units were equipped with cable-operated blades, though later models featured hydraulic lift systems. The hydraulic pump was typically engine-driven and fed a single-acting cylinder for blade elevation.
Maintenance tips:

• Flush hydraulic fluid and replace with ISO 32 or equivalent
  
• Inspect hoses for dry rot and replace with modern braided lines
  
• Rebuild lift cylinder with new seals and wipers
  
• Clean control valve and verify spool movement
  
• Add inline filter to prevent contamination
  

In one grading job in Kentucky, a TD-6 with a hydraulic blade was used to level a barn foundation. The operator noted that the blade response was slow until the fluid warmed up, a common trait in early open-center systems.
Transmission and Undercarriage Wear
The TD-6 featured a 5-speed manual transmission with a dry clutch and mechanical steering clutches. Its undercarriage included track chains, rollers, and sprockets that were built to last but prone to wear if neglected.
Inspection checklist:

• Check track tension and adjust using recoil spring bolts
  
• Inspect rollers for flat spots or seized bearings
  
• Measure sprocket tooth wear and replace if hooked
  
• Clean clutch compartments and verify engagement
  
• Use SAE 90 gear oil in transmission and final drives
  

In one farm in Nebraska, a TD-6 was found buried under hay bales. After freeing the tracks and replacing the clutch discs, the machine was used to clear fence rows and dig irrigation ditches.
Preservation and Historical Significance
The TD-6 is more than a machine—it’s a mechanical artifact from a time when diesel power was still finding its footing. Its dual-start system, rugged frame, and simple controls make it a favorite among vintage equipment collectors and working restorers.
Preservation tips:

• Store indoors or under tarp to prevent rust
  
• Drain fuel and oil during long-term storage
  
• Keep battery disconnected to preserve wiring
  
• Document serial numbers and casting codes for historical records
  
• Join vintage tractor clubs for parts sourcing and community support
  

In one parade in Minnesota, a fully restored 1941 TD-6 led a convoy of antique equipment. The crowd applauded as the pony motor barked to life, followed by the deep rumble of the diesel engine—a sound that once built roads, cleared forests, and shaped the land.
Final Thoughts
The 1941 International TD-6 dozer is a testament to mechanical resilience and engineering creativity. Its dual-start system, robust drivetrain, and enduring legacy make it a machine worth preserving, not just for its function but for its story. In the rhythm of restoration, the TD-6 doesn’t just run—it reminds us how far we’ve come, and how much we still admire the machines that got us there.