Restoring a 1942 Caterpillar D4 Pony Motor: Blow-by Challenges and Mechanical Lessons

[MikePhua]

Introduction to the Pony Motor System
The Caterpillar D4 from the early 1940s featured a gasoline-powered pony motor used to start the main diesel engine. These compact starter engines were designed for reliability in remote conditions, but their age and mechanical simplicity present unique challenges during restoration. One such issue is excessive blow-by—combustion gases escaping past the piston rings into the crankcase—often mistaken for poor assembly or worn components.
Key Terminology

• Pony Motor: A small gasoline engine used to start a larger diesel engine via mechanical engagement.
  
• Blow-by: Leakage of combustion gases past piston rings into the crankcase, often visible as vapor or oil mist.
  
• Ring Gap: The space between the ends of a piston ring, critical for sealing and thermal expansion.
  
• Valve Lash: The clearance between the valve stem and rocker arm, affecting timing and compression.
  
• Magneto Shaft: A rotating shaft driving the ignition magneto, essential for spark generation.
  

Rebuild Overview and Observations
The restoration involved a full teardown of the pony motor after a thrown rod cap. Key steps included:

• Polishing the crankshaft and machining new bearings
  
• Replacing damaged rods and installing new rings and seals
  
• Honing cylinder bores with 80-grit stones
  
• Lapping valves and setting clearances
  
• Rebuilding the carburetor and adjusting ignition timing
  

Despite these efforts, the engine exhibited severe blow-by and oil loss during initial runs. The plugs showed a healthy tan color, indicating combustion was occurring, but oil was visibly expelled from the crankcase breather.
Possible Causes of Blow-by

• Ring Seating Issues
  New rings require time to seat properly against the cylinder walls. A rough hone can accelerate this process but may also cause premature wear if ring quality is poor.
  
• Subpar Ring Quality
  The rings used in the rebuild were described as “very below average,” possibly contributing to poor sealing and excessive gas leakage.
  
• Overfilled Crankcase
  Excess oil can increase crankcase pressure and exacerbate blow-by symptoms.
  
• Valve Clearance Errors
  One cylinder had zero lash before adjustment. Incorrect lash can reduce compression and affect ring sealing dynamics.
  
• Ventilation System Faults
  A blocked or poorly designed crankcase vent can trap pressure, forcing oil out through weak seals.
  

Field Anecdote: Lessons from a Military Engine Shop
The rebuilder, with experience in military engine shops, noted that despite meticulous assembly—including chamfering ring edges and gapping—the pony motor’s behavior was erratic. The engine ran better before the rebuild, suggesting that ring selection and bore finish played a larger role than expected. This echoes stories from WWII-era mechanics who often reused original rings if they showed better sealing than replacements.
Historical Context: The Pony Motor Legacy
Caterpillar’s pony motors were a hallmark of pre-electric start systems. In cold climates or remote operations, they offered unmatched reliability. However, their design—especially in early models—was unforgiving. Ring tolerances, valve geometry, and ignition timing had to be precise. The D4’s pony motor, with its side-valve layout and splash lubrication, was particularly sensitive to ring quality and bore finish.
In the 1950s, Caterpillar began phasing out pony motors in favor of direct electric starters, citing maintenance complexity and part scarcity. Yet many vintage equipment enthusiasts continue to restore these engines for historical preservation and operational authenticity.
Recent Developments and Restoration Tips

• Use of Break-In Oils
  Lucas 30-weight break-in oil was used in this case, which can help rings seat but may also highlight sealing flaws.
  
• Compression Testing
  A planned compression test can reveal cylinder-specific issues and guide further teardown decisions.
  
• Magneto Shaft Failure
  During testing, the magneto shaft snapped—an uncommon but critical failure that halted ignition. This underscores the importance of inspecting all rotating components during rebuilds.
  
• Valve Stem Grinding and Welding
  Valve lash was corrected by grinding stems, though later documentation suggested a tighter spec. Rewelding and regrinding may be necessary to restore proper clearance.
  

Recommended Diagnostic and Repair Steps

• Perform a compression test on all cylinders
  
• Inspect crankcase breather for blockage or poor routing
  
• Verify oil level and adjust to spec
  
• Recheck ring gaps and seating with bore light
  
• Replace rings with higher-quality alternatives if needed
  
• Inspect magneto shaft and gear lash for wear or misalignment
  
• Confirm valve lash against updated specifications
  

Preventive Maintenance Tips

• Use high-quality rings and verify bore finish compatibility
  
• Avoid overfilling oil during break-in
  
• Monitor blow-by and oil loss during early runs
  
• Bench-test pony motors before installation
  
• Maintain crankcase ventilation and inspect breather systems regularly
  

Conclusion
Restoring a 1942 Caterpillar D4 pony motor is a rewarding but technically demanding endeavor. Blow-by, while common during break-in, can signal deeper issues with ring quality, bore finish, or assembly technique. This case highlights the importance of matching parts to vintage tolerances and respecting the quirks of early engine design. With patience, precision, and a touch of old-school ingenuity, even a stubborn pony motor can be coaxed back to life—ready to spin its diesel counterpart into action once more.