Swapping Fuel Tanks Between Caterpillar 311 and 312 Excavators

[MikePhua]

The 311 and 312 Series in Caterpillar’s Compact Excavator Line
Caterpillar’s 311 and 312 hydraulic excavators are part of the 300 series, designed for mid-size earthmoving tasks with a focus on maneuverability, fuel efficiency, and serviceability. The 311 was introduced as a compact, short-tail swing model ideal for urban and utility work, while the 312 offered slightly more reach and capacity, making it a favorite for general construction and light quarrying.
Both machines share a similar design philosophy and many components, including undercarriage layout, hydraulic architecture, and cab ergonomics. However, differences in frame geometry, engine placement, and auxiliary routing can complicate part swaps—especially when it comes to fuel tanks.
Terminology Notes

• Fuel Tank Assembly: The complete unit including the tank shell, mounting brackets, filler neck, and internal pickup.
  
• Mounting Points: Welded or bolted locations on the frame where the tank is secured.
  
• Return Line: A hose that routes unused fuel back into the tank from the injection system.
  
• Vent Line: A tube that allows air to escape or enter the tank to prevent pressure buildup.
  

Feasibility of Swapping a 311 Tank into a 312
While the 311 and 312 tanks may appear similar at first glance, there are critical differences that affect compatibility:

• Mounting Geometry
  • The 311 tank is designed for a shorter tail swing frame, with mounting brackets positioned differently than on the 312
    
  • Solution: Custom fabrication of brackets or adapter plates may be required
    
• Capacity and Shape
  • The 312 typically has a larger tank, shaped to fit its longer rear frame
    
  • Using a 311 tank may reduce fuel capacity and affect balance
    
• Line Routing and Fittings
  • Return and feed lines may be located differently, requiring rerouting or extension
    
  • Vent lines and filler necks may interfere with body panels or hydraulic lines
    
• Sensor Compatibility
  

• Fuel level sensors may use different resistance ranges or connectors
  
• Solution: Swap sensors or adapt wiring to match the 312’s gauge cluster
  

A technician in Alberta successfully installed a 311 tank into a 312B by fabricating new mounts and extending the return line. The machine ran without issue, though the fuel gauge required recalibration due to the smaller tank volume.
Best Practices for Tank Swaps
To ensure a successful swap:

• Measure both tanks precisely before removal
  
• Photograph line routing and bracket positions for reference
  
• Use high-quality fuel-rated hose and clamps for any extensions
  
• Pressure test the tank for leaks before installation
  
• Clean the tank interior thoroughly to prevent injector damage
  

If modifying the tank, avoid welding near sealed seams or sensor ports. A contractor in Texas once cracked a seam while repositioning a bracket, leading to a slow leak that went unnoticed until the machine stalled mid-shift.
Alternative Solutions and Recommendations
If a direct swap proves too complex:

• Source a used 312 tank from salvage yards or equipment recyclers
  
• Consider repairing the original tank with epoxy or patch kits if damage is minor
  
• Retrofit a universal tank with custom mounts and external fuel pump
  

Some operators install auxiliary tanks for extended runtime. A crew in Argentina added a 20-gallon saddle tank to their 312D, connected via a gravity feed and manual shutoff valve. This allowed longer shifts without refueling and reduced downtime.
Operator Anecdotes and Field Wisdom
A retired operator in Montana recalled swapping tanks between two CAT machines during a remote job. With no welder on site, he used ratchet straps and rubber bushings to secure the tank temporarily. It held for three weeks until proper mounts were fabricated.
In British Columbia, a forestry crew used a 311 tank on a 312C after their original was punctured by a falling branch. The swap worked, but they had to shorten the filler neck to clear the counterweight and reroute the vent line to avoid hydraulic interference.
Conclusion
Swapping a fuel tank from a Caterpillar 311 to a 312 is possible but requires careful planning, fabrication, and attention to detail. While the machines share many design elements, subtle differences in frame geometry and plumbing can complicate the process. With the right tools and a methodical approach, operators and technicians can restore functionality and keep the job moving—even when parts aren’t a perfect match. In the field, adaptability is often the best tool in the box.