Oil Loss at the Filter Bolt in the CAT 955L and How to Fix It

[MikePhua]

The CAT 955L and Its Mechanical Heritage
The Caterpillar 955L track loader was introduced in the 1970s as part of Caterpillar’s evolution from cable-operated machines to fully hydraulic loaders. With an operating weight of around 32,000 lbs and powered by a turbocharged six-cylinder diesel engine, the 955L was designed for rugged earthmoving, demolition, and quarry work. Caterpillar, founded in 1925, had by then become the global benchmark for tracked equipment, and the 955L was a staple in fleets across North America, Europe, and the Middle East.
Despite its robust design, the 955L—like many older machines—can suffer from age-related issues, especially in the engine’s lubrication system. One recurring problem involves oil leaking from the central bolt that secures the spin-on oil filter.
Understanding the Filter Bolt Assembly
The oil filter on the 955L is mounted using a central hollow bolt, which serves two purposes:

• It secures the filter to the engine block
  
• It channels pressurized oil through the filter media
  

This bolt is typically sealed using an O-ring or copper crush washer, depending on the design. Over time, these seals degrade due to heat cycling, vibration, and oil acidity. If the seal fails, oil can leak around the bolt threads or base, often mistaken for a faulty filter.
Identifying the Source of the Leak
To diagnose the leak:

• Clean the area thoroughly and run the engine for a few minutes
  
• Observe whether oil seeps from the bolt center, the filter gasket, or the mounting base
  
• Use a UV dye and inspection light if the leak is slow or intermittent
  
• Check for cracks in the filter housing or stripped threads in the bolt
  

In one case, an operator in Greece noticed oil dripping from the bolt after replacing the filter. The new filter was installed correctly, but the leak persisted. Upon inspection, the O-ring inside the bolt recess had hardened and cracked, allowing oil to bypass the seal.
Replacement and Repair Procedure
To fix the issue:

• Drain the engine oil and remove the filter
  
• Unscrew the central bolt and inspect the sealing surface
  
• Replace the O-ring with a high-temperature Viton equivalent or a copper washer if applicable
  
• Clean the threads and apply a small amount of non-hardening thread sealant
  
• Reinstall the bolt and torque to manufacturer specifications
  
• Install a new filter and refill oil
  

Always verify that the replacement seal matches the original dimensions. Using an undersized O-ring can cause blowout under pressure, while an oversized one may prevent proper seating.
Preventive Maintenance and Best Practices
To avoid future leaks:

• Replace the O-ring or washer every time the filter is changed
  
• Use OEM or high-quality aftermarket filters with proper gasket profiles
  
• Avoid over-tightening the filter, which can distort the sealing surface
  
• Inspect the bolt threads for wear or galling
  
• Keep a record of oil changes and seal replacements
  

In high-hour machines, consider replacing the entire filter mount assembly if corrosion or pitting is present. Some operators retrofit spin-on adapters with external lines for easier access and improved sealing.
Field Anecdotes and Operator Wisdom
One technician recalled a similar issue on a CAT 977L, where oil loss was traced to a missing copper washer on the filter bolt. The washer had been discarded during a rushed service, and the leak went unnoticed until oil pressure dropped. After replacing the washer and topping off the oil, the machine returned to full operation.
Another operator shared that he carries a small kit of O-rings and crush washers in his toolbox, allowing him to address leaks on-site without waiting for parts. This habit, developed over years of field work, has saved countless hours of downtime.
Conclusion
Oil loss from the filter bolt on a CAT 955L is a common but easily fixable issue. By understanding the role of the central bolt, inspecting the sealing components, and applying proper torque and sealing techniques, operators can restore the integrity of the lubrication system. In older machines, small leaks can lead to big problems—so attention to detail and preventive care remain the best defense against downtime and engine damage.