Seized “Pup” or Pony Motor: Diagnosis, Remedies & Real‑World Insights

[MikePhua]

Introduction: Understanding the Pony Motor (aka Pup Motor)
The pony motor—also known as a pup motor or cranking engine—is a small gasoline-powered engine traditionally used to start larger two-cylinder diesel engines, especially in vintage tractors and early Caterpillar equipment. It engages a pinion gear into the main engine’s flywheel and spins it until the diesel engine builds oil pressure and starts.
Why the Pony Motor Was Used

• Vital for cold starts—its warmed exhaust helped bring the main engine to operating temperature in freezing conditions
  
• Provided longer pre-crank time than an electric starter, allowing safe buildup of oil pressure
  
• Allowed operators to check the machine and warm systems before full startup
  

How the Pony Motor Works

• Mounted under the flywheel housing or beside the main engine block
  
• Typically a two‑cylinder V‑4 engine with a clutch and gear engagement system:
  • One lever engages the pinion into the flywheel
    
  • A second lever engages the clutch to spin the shaft
    
  • Some systems included a gear selector for faster spin or slower torque
    

What Happens When the Pony Motor Seizes

• Rusted valves or stuck pistons due to inactivity or moisture ingress
  
• Horizontal opposed twins (like in early Caterpillar models D2, D4, D6) are especially prone to valve seizure
  
• Causes include long storage, water ingress through the exhaust, or corrosion from inactivity
  

Methods for Freeing a Seized Pony Motor

1. Penetrating Lubricants
   Apply Kroil, acetone mix, Rislone additive, or specialist penetrants into the cylinders. Let them soak to loosen rusted parts.
   
2. Hydraulic Pressure Method
   Remove spark plugs or valve covers, attach a hydraulic fitting to the spark plug hole, hook to hydraulic pressure from a loader or tractor, and apply pressure to force pistons to budge. Effective if the piston isn’t at bottom dead center.
   
3. Manual Impact Technique
   Remove the head; use a wooden block and sledge hammer to tap stuck pistons, often working in engines frozen for decades.
   

Preventive Insight: Avoiding Seizure

• Regularly start the pony motor, especially in winter or after long storage
  
• Keep valves free, and circulate oil
  
• Drain or purge water from exhaust passages; corrosion is the main cause of freeze
  

Real‑World Example
A 1970s two-cylinder diesel with a pony motor was seized after sitting unused. After soaking cylinders with an acetone-based penetrant, the engine freed up and started reliably—yet still consumed more oil than normal. Another case involved seeds falling into the exhaust, locking valves until mechanically cleared and vacuumed—once cleaned, the pup engine resumed operation.
Glossary of Terms

• Pony Motor / Pup Engine / Cranking Engine: Small auxiliary engine used to start large diesel engines
  
• Flywheel Housing: Enclosure around the main flywheel where pinion gear engages
  
• Clutch & Drive Pinion: Mechanisms enabling the pup engine to spin the main engine
  
• Valve Seizure: Stalled engine due to stuck valves, usually from rust or corrosion
  
• Penetrating Oil / Rust Remover: Fluid used to break down rust and free mechanical parts
  

Conclusion
While largely obsolete today, pony motors were ingeniously engineered solutions to cold‑starting large diesels before electric starters became universal. When a pony motor seizes, creative methods—including penetrating oils, hydraulic pressure, and direct mechanical intervention—can often restore function. Keeping the engine exercised and dry remains the best prevention. In rehabilitating old machines, patience, lubrication, and mechanical know-how can bring these vintage starting systems back to life.