Restoring the D318 Starting Motor on Vintage Caterpillar Equipment

[MikePhua]

The Legacy of the D318 Diesel and Its Pony Motor System
The Caterpillar D318 diesel engine was widely used in mid-20th-century track loaders, dozers, and industrial power units. Known for its rugged construction and long service life, the D318 powered machines like the CAT 977 and other 9U-series dozers. One of its most distinctive features was the use of a pony motor—also called a pup motor—as a mechanical starter.
Before electric starters became standard, Caterpillar employed small gasoline engines to crank over large diesels. These pony motors were mounted directly to the flywheel housing and engaged via a clutch and gear system. Once the pony motor reached sufficient RPM, it would spin the diesel engine until combustion took over. This system was reliable in remote areas and cold climates, where battery power was limited or inconsistent.
Common Issues with Seized Pony Motors
Many surviving D318 units suffer from seized pony motors due to long periods of inactivity. These small engines, often two-cylinder horizontally opposed designs, are vulnerable to moisture intrusion, carbon buildup, and valve sticking. Symptoms of seizure include:

• Starter rope or electric starter unable to turn the motor
  
• No compression or resistance during manual rotation
  
• Rusted cylinder walls or stuck valves
  

Fortunately, the pony motor on the D318 is relatively accessible. The cylinder heads can be removed in place, and the head gaskets are reusable if handled carefully. Stuck valves can often be freed by tapping with a wooden dowel and applying penetrating oil. Compression as low as 30 PSI may still allow the pony motor to start and crank the diesel.
Identifying the Correct Manuals and Serial Numbers
Restoration begins with identifying the correct model and serial number. The D318 engine may carry a power unit tag such as 5V16667, indicating its industrial designation. Machines like the CAT 977 track loader often have spray-painted identifiers but require confirmation via stamped serial plates.
For accurate documentation, owners should seek manuals for the 977 20Axxxx series or the D318 industrial engine. These manuals include exploded diagrams, torque specs, and service procedures for both the diesel and pony motor systems. Sources include vintage equipment dealers, online auctions, and collector forums.
Starting System Variants and Operator Positioning
Not all pony motor setups are identical. Some configurations allow the operator to start the pony motor from the seat using a mechanical linkage or electric starter. Others require manual engagement from the side of the engine. This variation depends on the machine’s original application and whether it was retrofitted during its service life.
Operators should inspect the linkage, starter motor, and fuel system of the pony motor. Common upgrades include converting to electric start using a 12V automotive-style starter and solenoid. This modification improves reliability and reduces physical strain, especially in cold weather.
Anecdote from Northern Ontario
A restorer in Thunder Bay acquired a CAT 977 with a D318 engine and a seized pony motor. After removing the heads and soaking the cylinders with ATF and acetone, he freed the pistons and valves over several days. He reused the original gaskets and fabricated a new starter rope guide. The pony motor fired up on the third pull, and the diesel engine started shortly after. He now uses the machine for trail clearing and keeps a logbook of all maintenance steps.
Preservation Tips for Vintage Starting Systems
To maintain a pony motor in working condition:

• Run the motor monthly to prevent internal corrosion
  
• Drain fuel between seasons and use stabilizers
  
• Lubricate valve stems and cylinder walls with fogging oil
  
• Store the machine under cover to reduce moisture exposure
  
• Keep spare spark plugs, carburetor gaskets, and starter rope on hand
  

For long-term preservation, consider converting to electric start while retaining the original pony motor for authenticity. This hybrid approach balances reliability with historical value.
Conclusion
The D318 pony motor system is a testament to Caterpillar’s engineering during an era when mechanical solutions were favored over electronics. While these starting motors may seize after decades of dormancy, they are surprisingly resilient and can be restored with basic tools and patience. For collectors and operators of vintage CAT equipment, reviving the pony motor is both a technical challenge and a tribute to the machines that built the modern world. With proper care, the D318 and its pup motor will continue to roar to life for years to come.