Caterpillar RD6 Diesel Crawler Was a Milestone in Pre-War Earthmoving

[MikePhua]

The RD6 and Caterpillar’s Expansion into Diesel Power
The Caterpillar RD6 diesel crawler, produced between 1935 and 1941, marked a pivotal moment in the company’s transition from gasoline to diesel-powered machinery. Caterpillar Tractor Co., formed in 1925 through the merger of Holt and Best, had already established itself as a leader in track-type tractors. The RD6 was one of the first Caterpillar machines to feature a diesel engine as standard, reflecting the industry’s shift toward fuel efficiency and torque-heavy performance.
The RD6 was powered by the D6600 engine, a three-cylinder diesel that produced roughly 45 horsepower. While modest by today’s standards, this engine delivered exceptional torque and fuel economy, making it ideal for agricultural and construction tasks. Caterpillar sold thousands of RD6 units across North America, with strong adoption in farming, logging, and road-building sectors. Its success helped solidify diesel as the dominant power source for heavy equipment in the decades to follow.
Engine Behavior and White Smoke Diagnosis
Operators of surviving RD6 units often report white smoke from one cylinder during startup or idle. This symptom can be caused by several factors, including low compression, stuck piston rings, or faulty injectors. In older diesel engines like the D6600, white smoke typically indicates incomplete combustion—often due to cold temperatures, poor atomization, or valve issues.
Terminology:

• White Smoke: Exhaust containing unburned diesel fuel, often seen during cold starts or when combustion is incomplete.
  
• Compression Loss: A reduction in cylinder pressure due to worn rings, valves, or cylinder wall damage.
  
• Injector Fault: A malfunction in the fuel delivery nozzle that can cause uneven spray patterns or delayed ignition.
  

In one case, a collector inspecting two RD6 units found that both emitted white smoke from a single cylinder. One machine had a clearly burnt exhaust valve, while the other ran evenly but still smoked. This suggests that even well-running engines may exhibit minor injector imbalance or ring wear after decades of service.
Valve and Ring Wear in Long-Stored Machines
RD6 tractors left exposed to the elements often suffer from internal corrosion, especially if the exhaust stack is left uncovered. Moisture entering through the exhaust can settle in the cylinders, leading to rusted rings and pitted valve seats. Over time, this results in poor sealing, low compression, and misfires.
Preventive measures:

• Always cap the exhaust stack when storing the machine outdoors
  
• Rotate the engine periodically to prevent ring seizure
  
• Use fogging oil in cylinders during long-term storage
  
• Inspect valve lash and seating during annual maintenance
  

Terminology:

• Fogging Oil: A preservative oil sprayed into the intake or cylinders to prevent rust during storage.
  
• Valve Lash: The clearance between the valve and rocker arm; critical for proper timing and sealing.
  

A restorer in Oregon discovered that an RD6 stored for 15 years had one cylinder with zero compression. After removing the head, he found the rings fused to the piston and the exhaust valve heavily pitted. A full rebuild restored the engine to smooth operation, with no smoke under load.
Injector Performance and Fuel Atomization
The RD6’s mechanical injectors rely on precise nozzle geometry and spring tension to deliver atomized fuel into the combustion chamber. Over time, these components can wear or clog, leading to uneven spray and delayed ignition. Unlike modern common-rail systems, the RD6’s injectors are manually timed and require careful calibration.
Injector service tips:

• Clean nozzles using ultrasonic baths or solvent soak
  
• Replace worn springs and seals during rebuild
  
• Test spray pattern using a bench tester
  
• Avoid mixing injector parts from different engines
  

Terminology:

• Atomization: The process of breaking fuel into fine droplets for efficient combustion.
  
• Bench Tester: A device used to simulate injector operation and visualize spray patterns.
  

A technician in Alberta rebuilt the injectors on his RD6 using NOS (new old stock) parts and reported a dramatic reduction in startup smoke. He noted that the middle cylinder had previously shown delayed firing, which resolved after injector calibration.
The RD6’s Legacy and Collector Appeal
Today, the RD6 is prized by collectors and vintage equipment enthusiasts for its historical significance and mechanical simplicity. Its three-cylinder diesel is easy to work on, and parts are still available through specialty suppliers and salvage yards. Many RD6 units are restored for parades, demonstrations, or light-duty farm work.
Collector insights:

• Serial numbers can be used to trace production year and configuration
  
• Original paint schemes were typically Caterpillar yellow with black trim
  
• Some RD6s were converted to electric start, replacing the gasoline pony motor
  
• Restoration costs range from $5,000 to $15,000 depending on condition
  

Terminology:

• Pony Motor: A small gasoline engine used to start the main diesel engine; common in pre-electric start machines.
  
• NOS (New Old Stock): Unused parts manufactured during the original production period, often found in surplus inventories.
  

In 2023, a fully restored RD6 sold at auction in Iowa for $18,500, reflecting growing interest in pre-war diesel equipment. The buyer, a retired farmer, planned to use it for light grading and as a showpiece at local fairs.
Conclusion
The Caterpillar RD6 diesel crawler stands as a landmark in the evolution of heavy equipment. Its rugged design, efficient diesel engine, and mechanical accessibility make it a favorite among restorers and historians. While white smoke and compression issues are common in aging units, they are often solvable with careful diagnostics and respectful restoration. More than just a machine, the RD6 represents a time when diesel was new, engineering was bold, and every startup was a symphony of gears, valves, and grit.