Bobcat 975 Engine Replacement Challenges and Casting Number Identification

[MikePhua]

The Bobcat 975 and Its Powertrain History
The Bobcat 975 skid steer loader was introduced in the late 1970s and remained in production through the early 1980s. Designed for heavy-duty applications, it featured a robust frame, high lift capacity, and a rear-mounted diesel engine. Unlike smaller Bobcat models that used gasoline or compact Kubota diesels, the 975 was equipped with a John Deere industrial engine—specifically the 4276DF, a naturally aspirated four-cylinder diesel known for its torque and reliability.
John Deere’s 4276 series was widely used across OEM platforms, including air compressors, generators, and forklifts. However, by the mid-1990s, the 4276 was phased out and replaced by the 4045DF and 4045TF series, which offered improved emissions compliance and optional turbocharging. This transition created a parts gap for legacy machines like the Bobcat 975, especially when engine blocks were damaged beyond repair.
Failure Mode and Block Damage
In one documented case, the 4276 engine suffered catastrophic failure due to a worn balance shaft bushing. The bushing dropped into the oil pan, was picked up by the oil pump, and shattered the pump shaft. This chain reaction led to severe internal damage, including ovaled-out balance shaft bores in the engine block. Attempts to repair the block were abandoned when machinists could not guarantee alignment tolerances for the shaft journals.
Terminology annotation:

• Balance Shaft: A rotating shaft designed to counteract engine vibration, especially in inline four-cylinder configurations.
  
• Ovaled Bore: A cylindrical hole that has become distorted into an oval shape due to wear or mechanical stress.
  
• Casting Number: A unique identifier molded into the engine block during manufacturing, used to match parts and verify compatibility.
  
• Short Block: An engine assembly consisting of the block, crankshaft, pistons, and connecting rods, but excluding the cylinder head and accessories.
  

Casting Number Identification and Serial Plate Loss
The engine in question had been sandblasted and repainted prior to purchase, erasing the original serial plate and obscuring key identifiers. Without a serial number, sourcing a replacement block became nearly impossible through standard dealer channels. The only remaining clues were casting numbers found on the damaged block:

• R59790
  
• R54680
  
• R57078
  
• 63647T
  

These numbers correspond to internal John Deere casting references, but without a serial number, most dealers were unable to match them to a complete engine assembly. Some casting numbers may refer to specific block revisions, oil gallery configurations, or mounting flange patterns.
Recommendations:

• Use casting numbers to cross-reference with salvage yards and rebuilders familiar with Deere industrial engines
  
• Contact legacy parts suppliers such as Cascade Diesel or Engine Rebuilders in North Dakota
  
• Search for compatible engines from Lull forklifts, Ingersoll Rand compressors, or other OEMs that used the 4276 block
  
• Consider a repower using the 4045DF or 4045TF series, noting that the turbo variant may require exhaust modifications
  

In Alaska, one operator replaced his 4276DF with a 4045TF001 crate engine. The swap required cutting and welding a new 4-inch exhaust elbow, but the engine bolted directly to the mounts and delivered an additional 35 horsepower.
Dealer Limitations and Aftermarket Solutions
Bobcat no longer supports the 975 with factory engines or short blocks. The last service bulletin referencing the 4276 was issued in 1976, and replacement engines—if available—can exceed $8,000. John Deere also discontinued the 4276 line, and most dealers now focus on Tier 3 and Tier 4 compliant engines.
Aftermarket options:

• Rebuilt 4276 blocks from salvage yards
  
• Crate 4045 engines with adapter kits
  
• Custom rebuilds using matched casting numbers and donor parts
  
• Engine swaps with Cummins 4BT or Perkins 1004 series, requiring fabrication
  

In British Columbia, a technician used a flexible borescope to locate casting numbers on a friend’s 975 engine without removing it. This allowed them to match the block to a donor unit 500 miles away and arrange shipping.
Repower Considerations and Hydraulic Compatibility
When repowering a Bobcat 975, it’s critical to consider hydraulic pump compatibility. The original Sunstrand pumps were designed for the torque curve of the 4276. Installing a higher-output engine may overstress aging hydraulic components.
Precautions:

• Verify pump input shaft dimensions and RPM limits
  
• Install pressure gauges to monitor system behavior post-swap
  
• Upgrade cooling system if turbocharged engine is used
  
• Reinforce engine mounts and frame brackets if weight increases
  

In Minnesota, a contractor installed a turbocharged 4045TF into a 975 and noticed increased hydraulic heat under load. Adding an auxiliary cooler resolved the issue and extended pump life.
Conclusion
Replacing the engine in a Bobcat 975 without a serial number is a complex challenge that blends mechanical detective work with parts sourcing ingenuity. Casting numbers offer a lifeline, but success depends on persistence, networking, and a willingness to adapt. Whether rebuilding a 4276DF or repowering with a modern alternative, the goal remains the same—keep the machine working, keep the hydraulics flowing, and keep the legacy alive.