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The Clark 666C and Its Legacy in Timber Work
The Clark 666C skidder is a rugged, purpose-built forestry machine designed for log skidding and heavy-duty terrain navigation. Manufactured by Clark Equipment Company, which began producing industrial machinery in the early 20th century, the 666C was part of a lineage that emphasized mechanical simplicity and brute strength. With thousands of units deployed across North America, Australia, and New Zealand, the 666C became a staple in timber operations, often powered by the Detroit Diesel 4-53 two-stroke engine.
This engine, known for its distinctive sound and high-revving nature, delivers reliable torque in steep and muddy conditions. However, like many older machines, the 666C is prone to age-related issues—coolant leaks being one of the more elusive and frustrating problems.
Terminology Note
Operators have reported a peculiar leak pattern: coolant only escapes when the machine is climbing uphill under load. On flat ground or downhill, the leak disappears. This suggests a pressure-related issue rather than a continuous mechanical failure. The most likely culprit is the radiator cap relief tube, which may be venting excess pressure due to thermal expansion or cap fatigue.
When ascending a slope, the engine works harder, generating more heat. If the radiator cap is old or improperly rated, it may open prematurely, allowing coolant to escape. The fan orientation also plays a role—if set to blow rather than pull, it may redirect escaping coolant, making the leak harder to trace.
Engine Temperature and Cap Pressure Considerations
The Detroit 4-53 engine typically operates between 85°C and 95°C under load. In the reported case, the temperature climbs into the high 90s Celsius when pulling uphill, which is within acceptable limits but close to the threshold for pressure relief. If the radiator cap lacks markings or has degraded over time, it may vent at a lower pressure than intended.
Recommendations include:
While head gasket failure is a common concern in older machines, the absence of performance loss, white smoke, or coolant in the oil suggests the gasket is intact. However, if the leak worsens or coolant loss becomes chronic, a compression test or chemical block test may be warranted.
In forestry environments, head gaskets are often stressed by rapid temperature changes and heavy loads. Preventive measures include:
In 2018, a logging crew in British Columbia noticed their Clark 666C leaking coolant only during uphill skids. After replacing the radiator cap and reversing the fan direction, the issue disappeared. The operator had previously assumed a cracked head, but the fix cost under $40 and restored full functionality. The machine went on to complete a 1,200-hour season without further cooling issues.
Maintenance and Inspection Strategy
To manage coolant leaks effectively:
Clark Equipment Company, founded in 1916, was a pioneer in material handling and forestry machinery. The 666C was one of its most successful skidders, often paired with Detroit Diesel engines due to their simplicity and field serviceability. The 4-53 engine, introduced in the 1950s, remains in use today in marine, industrial, and military applications.
Despite its reputation for oil consumption and noise, the 4-53 is valued for its durability and ease of rebuild. Many operators continue to run these engines well past 10,000 hours with proper maintenance.
Conclusion
Coolant leaks in the Clark 666C are often terrain-dependent and pressure-related rather than catastrophic. By understanding the interplay between engine load, radiator cap function, and fan orientation, operators can resolve these issues without major repairs. The 666C remains a workhorse in the woods, and with a few targeted adjustments, it can continue pulling timber with confidence and minimal downtime.
The Clark 666C skidder is a rugged, purpose-built forestry machine designed for log skidding and heavy-duty terrain navigation. Manufactured by Clark Equipment Company, which began producing industrial machinery in the early 20th century, the 666C was part of a lineage that emphasized mechanical simplicity and brute strength. With thousands of units deployed across North America, Australia, and New Zealand, the 666C became a staple in timber operations, often powered by the Detroit Diesel 4-53 two-stroke engine.
This engine, known for its distinctive sound and high-revving nature, delivers reliable torque in steep and muddy conditions. However, like many older machines, the 666C is prone to age-related issues—coolant leaks being one of the more elusive and frustrating problems.
Terminology Note
- Coolant Overflow Tube: A relief pathway that allows excess coolant or pressure to escape from the radiator cap.
- Head Gasket: A seal between the engine block and cylinder head, critical for maintaining compression and preventing fluid mixing.
- Radiator Cap Pressure Rating: The threshold at which the cap allows coolant to escape, typically measured in psi or kPa.
- Thermal Expansion: The increase in coolant volume as temperature rises, which can trigger overflow if not properly regulated.
Operators have reported a peculiar leak pattern: coolant only escapes when the machine is climbing uphill under load. On flat ground or downhill, the leak disappears. This suggests a pressure-related issue rather than a continuous mechanical failure. The most likely culprit is the radiator cap relief tube, which may be venting excess pressure due to thermal expansion or cap fatigue.
When ascending a slope, the engine works harder, generating more heat. If the radiator cap is old or improperly rated, it may open prematurely, allowing coolant to escape. The fan orientation also plays a role—if set to blow rather than pull, it may redirect escaping coolant, making the leak harder to trace.
Engine Temperature and Cap Pressure Considerations
The Detroit 4-53 engine typically operates between 85°C and 95°C under load. In the reported case, the temperature climbs into the high 90s Celsius when pulling uphill, which is within acceptable limits but close to the threshold for pressure relief. If the radiator cap lacks markings or has degraded over time, it may vent at a lower pressure than intended.
Recommendations include:
- Replacing the radiator cap with a properly rated unit (typically 7–13 psi depending on system design)
- Verifying the fan direction—pulling air through the radiator is generally more effective for cooling
- Installing a coolant recovery tank to capture overflow and prevent loss
While head gasket failure is a common concern in older machines, the absence of performance loss, white smoke, or coolant in the oil suggests the gasket is intact. However, if the leak worsens or coolant loss becomes chronic, a compression test or chemical block test may be warranted.
In forestry environments, head gaskets are often stressed by rapid temperature changes and heavy loads. Preventive measures include:
- Monitoring coolant levels daily
- Using high-quality antifreeze with corrosion inhibitors
- Avoiding sudden shutdowns after hard pulls to prevent thermal shock
In 2018, a logging crew in British Columbia noticed their Clark 666C leaking coolant only during uphill skids. After replacing the radiator cap and reversing the fan direction, the issue disappeared. The operator had previously assumed a cracked head, but the fix cost under $40 and restored full functionality. The machine went on to complete a 1,200-hour season without further cooling issues.
Maintenance and Inspection Strategy
To manage coolant leaks effectively:
- Inspect the radiator cap for wear, corrosion, or missing seals
- Check hoses and clamps for signs of bulging or seepage
- Clean the radiator fins monthly to ensure airflow
- Use infrared thermometers to monitor engine and radiator surface temperatures
- Keep a log of leak occurrences, terrain, and load conditions
Clark Equipment Company, founded in 1916, was a pioneer in material handling and forestry machinery. The 666C was one of its most successful skidders, often paired with Detroit Diesel engines due to their simplicity and field serviceability. The 4-53 engine, introduced in the 1950s, remains in use today in marine, industrial, and military applications.
Despite its reputation for oil consumption and noise, the 4-53 is valued for its durability and ease of rebuild. Many operators continue to run these engines well past 10,000 hours with proper maintenance.
Conclusion
Coolant leaks in the Clark 666C are often terrain-dependent and pressure-related rather than catastrophic. By understanding the interplay between engine load, radiator cap function, and fan orientation, operators can resolve these issues without major repairs. The 666C remains a workhorse in the woods, and with a few targeted adjustments, it can continue pulling timber with confidence and minimal downtime.
