Fixing Rear Main Seal Leaks on the Komatsu PC78MR-6

[MikePhua]

The PC78MR-6 and Its Engineering Background
The Komatsu PC78MR-6 is a compact radius hydraulic excavator introduced in the early 2000s, designed for urban construction, utility work, and tight job sites. With an operating weight around 17,000 lbs and powered by the Komatsu S4D95LE-3 engine—a four-cylinder diesel developed in collaboration with Yanmar—the PC78MR-6 balances power, fuel efficiency, and maneuverability. Komatsu, founded in 1921 in Japan, has long been a global leader in construction machinery, and the PC78MR series has been widely adopted across North America, Europe, and Asia.
The machine’s mid-size footprint and advanced hydraulic system make it a favorite for trenching, grading, and demolition. However, like many compact excavators with wet flywheel housings, it can develop oil leaks at the rear main seal, especially after 5,000+ hours of operation.
Identifying the Rear Main Seal Leak
A common symptom of rear main seal failure on the PC78MR-6 is engine oil dripping from the starter motor area or bellhousing drain plug. In some cases, previous owners may attempt temporary fixes using silicone sealant or installing drain fittings to relieve pressure—methods that rarely address the root cause.
Key indicators include:

• Oil dripping from the bellhousing seam or starter cavity
  
• Residual oil inside the bellhousing when the inspection plug is removed
  
• Evidence of sealant or makeshift drain fittings added to the housing
  
• Engine oil loss without visible external leaks
  

The PC78MR-6 uses a wet flywheel housing, meaning the flywheel and damper operate in an oil-filled cavity. This design improves lubrication and dampening but requires precise sealing at the rear main and input shaft seals. If either fails, oil can accumulate and leak externally.
Inspection and Confirmation Procedures
To confirm the leak source, technicians should:

• Locate the drain plug near the bottom of the damper case—typically a pipe plug about 12 mm from the base
  
• Remove the plug and observe oil flow; minimal seepage is normal, but a steady stream indicates internal leakage
  
• Inspect the breather canister on top of the bellhousing; if clogged, it can cause pressure buildup and force oil past seals
  
• Check for oil contamination around the starter motor and flywheel teeth
  

In one case, a contractor discovered that the breather tube was blocked with debris, causing pressure to build inside the housing. After cleaning the breather and replacing the rear main seal, the leak stopped entirely.
Seal Replacement and Component Considerations
Replacing the rear main seal on the PC78MR-6 requires removing the engine or separating it from the transmission housing. While labor-intensive, it’s critical to address both the rear main seal and the input shaft seal on the hydraulic pump simultaneously, as both can contribute to oil intrusion.
Recommended steps:

• Drain engine oil and remove the starter motor
  
• Disconnect hydraulic lines and electrical harnesses
  
• Separate the engine from the bellhousing
  
• Inspect the flywheel and damper for oil saturation
  
• Replace the rear main seal using OEM or high-quality aftermarket parts
  
• Replace the input shaft seal on the hydraulic pump
  
• Clean and inspect the breather assembly
  
• Reassemble with new gaskets and torque fasteners to spec
  

If the damper or Lovejoy coupler has been exposed to oil for extended periods, it may need replacement due to rubber degradation or slippage. A contaminated coupler can cause vibration and premature wear on the crankshaft flange.
Preventive Measures and Long-Term Reliability
To prevent future seal failures, operators should:

• Monitor oil levels and inspect the bellhousing drain plug during routine service
  
• Replace breather filters every 1,000 hours or annually
  
• Use high-quality engine oil with proper viscosity and detergent levels
  
• Avoid overfilling the crankcase, which can increase internal pressure
  
• Inspect the starter cavity for oil residue during electrical service
  

Adding a magnetic drain plug to the bellhousing can help detect early signs of seal wear by capturing metallic particles from the damper or flywheel.
Field Experience and Practical Advice
A technician in Iowa reported receiving a PC78MR-6 with 5,360 hours and no service history. After noticing oil dripping from the starter, he removed the bellhousing plug and found significant oil accumulation. The previous owner had applied red silicone to the housing seam and installed an air coupler as a drain—neither of which solved the issue. After replacing both seals and cleaning the breather, the machine returned to full service without further leaks.
In another case, a rental fleet operator replaced the rear main seal on three PC78MR units after noticing consistent oil loss. He began inspecting the bellhousing plugs during every 500-hour service interval, catching future leaks early and reducing downtime.
Conclusion
Rear main seal leaks on the Komatsu PC78MR-6 are a known issue, especially in machines with over 5,000 hours or poor maintenance history. By understanding the wet flywheel housing design, inspecting breather systems, and replacing seals proactively, operators can restore reliability and extend the life of their excavators. With proper care, the PC78MR-6 remains a powerful and efficient tool for compact earthmoving tasks—proving that even complex repairs can be tackled with methodical precision and a bit of field wisdom.