Locating and Restoring the Thermostat Housing for a Caterpillar D2 Bulldozer

[MikePhua]

The D2 and Its Historical Significance
The Caterpillar D2 bulldozer, first introduced in 1938, was one of the smallest track-type tractors ever produced by Caterpillar. Designed for light grading, agricultural work, and small-scale earthmoving, the D2 became a symbol of post-war mechanization in rural America. With over 15,000 units produced across multiple series (3J, 4U, 5J, and 5U), the D2 remains a favorite among collectors and restoration enthusiasts.
Powered by a two-cylinder diesel engine with a gasoline pony motor for starting, the D2 was simple, rugged, and highly serviceable. Its cooling system relied on a mechanical water pump and a thermostat housed in a cast iron housing mounted atop the cylinder head. This housing regulated coolant flow to the radiator, ensuring optimal engine temperature during operation.
Terminology annotation:
- Thermostat Housing: A cast or machined component that holds the thermostat and connects the engine to the radiator via coolant passages.
- Pony Motor: A small gasoline engine used to start the main diesel engine, common in early Caterpillar machines.
- Coolant Regulation: The process of controlling engine temperature by modulating coolant flow based on thermostat position.
- Casting Number: A unique identifier stamped or molded into cast parts, used to match components during replacement.
Challenges in Sourcing Vintage Thermostat Housings
Finding a replacement thermostat housing for a D2 can be difficult due to the age of the machine and the limited availability of original parts. Many housings were cast in small batches and are no longer manufactured. Over time, corrosion, cracking, and impact damage have rendered many original housings unusable.
Common issues include:

• Cracked mounting flanges due to over-torquing or vibration
  
• Internal corrosion from untreated coolant or water use
  
• Warped mating surfaces causing coolant leaks
  
• Missing or damaged casting numbers complicating identification
  

Recommendations for sourcing:

• Search salvage yards specializing in antique Caterpillar equipment
  
• Contact vintage tractor clubs and restoration networks
  
• Look for casting numbers such as “4U-XXXX” or “5J-XXXX” to match series
  
• Consider fabricating a replica using 3D scanning and casting if originals are unavailable
  

One restorer in Wyoming found a usable housing by contacting a retired Cat dealer who had stored obsolete parts in a barn for decades. The part was cleaned, resurfaced, and installed with a new gasket—restoring coolant flow and engine temperature control.
Restoration and Installation Tips
Once a housing is located, proper preparation is essential to ensure long-term reliability. Even if the part appears intact, hidden cracks or corrosion may compromise performance.
Steps for restoration:

• Clean the housing thoroughly using a wire brush and solvent
  
• Pressure test the housing to check for leaks or cracks
  
• Resurface the mating flange to ensure a flat seal with the cylinder head
  
• Install a new thermostat rated for 160–180°F depending on climate
  
• Use a high-quality gasket and torque bolts evenly to spec (typically 25–30 ft-lbs)
  

If the housing is missing entirely, some restorers have adapted housings from similar-era Caterpillar engines, modifying bolt patterns and coolant passages to fit. This requires machining skill and careful measurement but can be a viable solution for rare models.
Preserving the Cooling System in Vintage Machines
The D2’s cooling system is simple but vulnerable to neglect. Many units were run with plain water, leading to scale buildup and corrosion. Modern coolants with rust inhibitors and proper flushing can extend the life of the radiator, pump, and housing.
Preventative measures:

• Flush the cooling system annually with a mild descaler
  
• Use a 50/50 mix of ethylene glycol coolant and distilled water
  
• Replace hoses and clamps with modern equivalents rated for vintage systems
  
• Monitor engine temperature with an aftermarket gauge if the original is missing
  

In one case, a collector installed a digital temperature sensor in the thermostat housing to monitor engine heat during parades and shows. This helped prevent overheating during slow-speed operation and preserved the original engine.
Conclusion
Locating and restoring a thermostat housing for a Caterpillar D2 bulldozer requires patience, resourcefulness, and a deep appreciation for mechanical history. Whether sourced from a salvage yard, a collector’s stash, or fabricated anew, the housing plays a vital role in regulating engine temperature and ensuring reliable operation. With careful installation and cooling system maintenance, even the oldest D2s can continue to run smoothly—honoring the legacy of one of Caterpillar’s most iconic machines.