Understanding Hydraulic Oil Requirements for the Bobcat 642 Skid Steer

[MikePhua]

Introduction to the Bobcat 642’s Hydraulic System
The Bobcat 642 is one of the earlier skid steer loader models produced by Melroe Bobcat in the 1970s and 1980s. Known for its mechanical simplicity and robust construction, it remains a reliable machine for many small contractors and property owners. However, due to its age and design differences from modern equipment, determining the proper hydraulic fluid to use can be confusing. This article explores the hydraulic oil requirements for the Bobcat 642, maintenance considerations, compatibility concerns, and real-world practices.
Hydraulic Oil Functions and System Overview
Hydraulic oil in the Bobcat 642 performs several critical roles:

• Power Transmission: It transfers force from the pump to the hydraulic motors and cylinders.
  
• Lubrication: It reduces friction between internal moving parts like pump gears and control valves.
  
• Cooling: It dissipates heat generated by hydraulic operation.
  
• Contamination Suspension: It holds dirt and particles in suspension until removed by the hydraulic filter.
  

The hydraulic system on the 642 includes a gear pump, steering levers linked to control valves, lift and tilt cylinders, and a chain case integrated into the rear of the machine. While straightforward, its age means that using the wrong fluid can accelerate wear or cause malfunctions.
Original Fluid Specifications and Common Substitutes
The original specification for the Bobcat 642 hydraulic system was Bobcat Hydraulic Fluid, a proprietary formulation available through dealerships. In the 1970s and early 1980s, Bobcat recommended its own branded hydraulic oil or Type A Automatic Transmission Fluid (ATF) in some machines.
Due to availability issues and price, many operators now turn to equivalent or superior substitutes. The following are viable alternatives:

• AW32 or AW46 Hydraulic Oil: These anti-wear (AW) hydraulic oils are widely available and meet ISO VG viscosity standards. AW32 is suitable for colder climates, while AW46 is preferred in moderate temperatures.
  
• Universal Tractor Fluid (UTF): Popular in agricultural applications, UTF is used by some Bobcat owners with satisfactory results.
  
• Type A or Dexron III ATF: Some older manuals reference Type A or Dexron II/III transmission fluid as acceptable in older systems with lower operating pressures.
  

Considerations When Changing Hydraulic Fluids
Switching fluid types in a Bobcat 642 is not without risk. Key considerations include:

• Seal Compatibility: Some seals, particularly older Buna-N rubber, may swell or degrade with synthetic or detergent-rich fluids.
  
• System Cleanliness: If the machine hasn’t been flushed in years, switching fluids may dislodge varnish or sludge and clog filters.
  
• Chain Case Interaction: The Bobcat 642’s chain case and hydraulic reservoir are interconnected, meaning any change in fluid affects both systems.
  
• Filter Condition: Before any fluid change, always inspect and replace the hydraulic filter to prevent restriction or cavitation.
  

A common and effective strategy is to drain the system completely, including the chain case and reservoir, then refill with a modern AW46 hydraulic oil, which offers wide temperature performance and good anti-wear protection.
Real-World Experiences and Tips
Operators of older Bobcats often share consistent themes in their maintenance stories:

• Avoid Mixing Fluids: Mixing ATF with AW hydraulic oil can cause foaming, poor lubrication, and erratic behavior in control valves.
  
• Warm Up the System: In cold weather, it's critical to let the hydraulic oil warm up before operating under load to prevent pump damage.
  
• Check for Milky Fluid: Water contamination is common in older, uncovered machines. Milky fluid should be drained and replaced immediately.
  
• Top Off With Caution: Adding fluid without knowing what’s already in the machine may introduce compatibility issues. If unsure, a complete flush is safer.
  

One operator noted that after switching to AW46 hydraulic oil, their machine ran cooler, and the lift response improved. Another warned that their seals began to leak slightly when moving from ATF to AW32—likely due to the detergent properties cleaning built-up grime that was sealing gaps.
Refill Capacity and Maintenance Intervals
The Bobcat 642 hydraulic reservoir typically holds approximately 4 to 5 gallons (15 to 19 liters). Always check the dipstick or sight glass to confirm proper level after a fluid change.
Recommended maintenance practices include:

• Hydraulic Oil Change: Every 500 hours or annually
  
• Hydraulic Filter Replacement: Every 250 hours or with every oil change
  
• Visual Inspection: Weekly or before each use
  

Final Thoughts
Maintaining the Bobcat 642's hydraulic system with the correct oil is essential for longevity and safe operation. While original fluids may be obsolete, modern substitutes like AW46 offer excellent protection. Avoid mixing types, monitor for leaks or contamination, and commit to a consistent maintenance schedule.
The story of the Bobcat 642 illustrates how even older equipment, when maintained with care and mechanical respect, can continue to serve productively. Just as mechanics in the 1980s adapted to new standards, today’s operators must balance historical recommendations with modern fluid technology—and with the right oil, the 642 keeps moving dirt like it always has.