08-26-2025, 07:49 PM
Hitachi’s EX Series and the Evolution of Mid-Size Excavators
Hitachi’s EX series excavators, launched in the late 1980s and refined through the 1990s, became a benchmark in hydraulic excavator design. The EX100 and EX120 models were part of this lineage, built for general construction, trenching, and light earthmoving. The EX100 was rated for approximately 20,000 lbs operating weight, while the EX120 scaled up to around 27,000 lbs, offering greater reach, bucket capacity, and lifting power.
Both machines shared similar design language, but their powerplants differed. The EX100 typically used the Isuzu 4BG1 engine, producing around 64–68 horsepower. The EX120, on the other hand, was equipped with the more powerful Isuzu 4BD1 or 4JJ1 engines, delivering 80–85 horsepower depending on the variant. This difference in output directly influenced hydraulic pump performance, swing torque, and multi-function operation.
Engine Swaps and Rebuilt Machines in the Global Market
In many regions, especially Southeast Asia and parts of Africa, rebuilt or “recon” excavators are common. These machines are often assembled from salvaged parts, combining frames, booms, and engines from different models. While this practice extends the life of aging fleets, it can introduce mismatches in power-to-weight ratios and hydraulic compatibility.
A case involving an EX120 body fitted with an EX100 engine illustrates this challenge. The operator reported that the machine stalled or lost power when attempting simultaneous arm and boom movements—classic symptoms of hydraulic starvation or insufficient engine torque. Despite servicing the master pump, central joint, and even overhauling the engine, the issue persisted.
Hydraulic Load Matching and Engine Output
Excavators rely on a balance between engine power and hydraulic demand. When multiple functions are activated—such as boom lift and arm extension—the hydraulic pump requires sufficient torque from the engine to maintain pressure and flow. If the engine is underpowered, the pump cannot deliver adequate oil volume, leading to sluggish response or stalling.
In this case, the EX100 engine, designed for a smaller pump and lighter frame, struggles to meet the hydraulic demands of the EX120’s larger cylinders and heavier boom. Even if the pump is in good condition, the engine’s torque curve may not support peak flow rates under load.
Key parameters to consider:
Solutions and Workarounds
While replacing the engine with a correct EX120 unit is ideal, it may not be feasible due to cost or availability. Alternative strategies include:
Physically, the EX100 and EX120 engines differ in block size, intake routing, and accessory layout. The EX120 engine typically has a larger turbocharger, higher-capacity fuel injectors, and a more robust cooling system. The oil filter and starter motor positions also vary, which can affect service access and mounting compatibility.
Operators can identify the engine by checking:
Field Anecdotes and Lessons Learned
In Sarawak, a contractor purchased a rebuilt EX120 for slope terracing. The machine performed well on flat ground but stalled during uphill digging. After months of troubleshooting, they discovered the engine was from an EX100. Rather than replacing it, they installed a smaller bucket and trained the operator to avoid simultaneous boom and arm use. The machine continued to work, albeit with reduced productivity.
In contrast, a fleet manager in Thailand retrofitted an EX120 with a turbocharged 4BD1T engine from a truck. With minor modifications to the mounts and cooling system, the machine regained full function and operated reliably for years.
Conclusion
Using an EX100 engine in an EX120 excavator body is technically possible but operationally compromised. The mismatch in power and hydraulic demand leads to performance issues that can frustrate operators and reduce efficiency. While creative solutions exist, they often involve trade-offs. For long-term reliability and full functionality, matching the engine to the machine’s original specifications remains the best path forward. In regions where rebuilt machines are common, understanding these dynamics helps buyers make informed decisions and avoid costly surprises.
Hitachi’s EX series excavators, launched in the late 1980s and refined through the 1990s, became a benchmark in hydraulic excavator design. The EX100 and EX120 models were part of this lineage, built for general construction, trenching, and light earthmoving. The EX100 was rated for approximately 20,000 lbs operating weight, while the EX120 scaled up to around 27,000 lbs, offering greater reach, bucket capacity, and lifting power.
Both machines shared similar design language, but their powerplants differed. The EX100 typically used the Isuzu 4BG1 engine, producing around 64–68 horsepower. The EX120, on the other hand, was equipped with the more powerful Isuzu 4BD1 or 4JJ1 engines, delivering 80–85 horsepower depending on the variant. This difference in output directly influenced hydraulic pump performance, swing torque, and multi-function operation.
Engine Swaps and Rebuilt Machines in the Global Market
In many regions, especially Southeast Asia and parts of Africa, rebuilt or “recon” excavators are common. These machines are often assembled from salvaged parts, combining frames, booms, and engines from different models. While this practice extends the life of aging fleets, it can introduce mismatches in power-to-weight ratios and hydraulic compatibility.
A case involving an EX120 body fitted with an EX100 engine illustrates this challenge. The operator reported that the machine stalled or lost power when attempting simultaneous arm and boom movements—classic symptoms of hydraulic starvation or insufficient engine torque. Despite servicing the master pump, central joint, and even overhauling the engine, the issue persisted.
Hydraulic Load Matching and Engine Output
Excavators rely on a balance between engine power and hydraulic demand. When multiple functions are activated—such as boom lift and arm extension—the hydraulic pump requires sufficient torque from the engine to maintain pressure and flow. If the engine is underpowered, the pump cannot deliver adequate oil volume, leading to sluggish response or stalling.
In this case, the EX100 engine, designed for a smaller pump and lighter frame, struggles to meet the hydraulic demands of the EX120’s larger cylinders and heavier boom. Even if the pump is in good condition, the engine’s torque curve may not support peak flow rates under load.
Key parameters to consider:
- EX100 engine output: ~68 hp @ 2,200 RPM
- EX120 hydraulic pump flow: ~40–45 GPM
- Required engine torque for full hydraulic load: ~180–200 Nm
- Actual torque from EX100 engine: ~150–160 Nm
Solutions and Workarounds
While replacing the engine with a correct EX120 unit is ideal, it may not be feasible due to cost or availability. Alternative strategies include:
- Hydraulic Flow Restriction
Install flow control valves to limit simultaneous function demand. This reduces peak load on the engine but sacrifices speed.
- Idle Speed Adjustment
Slightly increasing engine idle RPM can improve pump response, though it risks overheating or premature wear.
- Auxiliary Hydraulic Accumulator
Adding a nitrogen-charged accumulator can buffer pressure during peak demand, smoothing operation.
- Pump Reconfiguration
If the pump is dual-stage, reconfigure it to prioritize boom and arm functions over swing or travel.
- Operator Training
Teach operators to stagger movements—lifting before extending, swinging after retracting—to avoid overloading the system.
Physically, the EX100 and EX120 engines differ in block size, intake routing, and accessory layout. The EX120 engine typically has a larger turbocharger, higher-capacity fuel injectors, and a more robust cooling system. The oil filter and starter motor positions also vary, which can affect service access and mounting compatibility.
Operators can identify the engine by checking:
- Engine plate (usually stamped near the injection pump)
- Intake manifold shape and turbo orientation
- Radiator size and fan shroud design
- Fuel line routing and filter housing
Field Anecdotes and Lessons Learned
In Sarawak, a contractor purchased a rebuilt EX120 for slope terracing. The machine performed well on flat ground but stalled during uphill digging. After months of troubleshooting, they discovered the engine was from an EX100. Rather than replacing it, they installed a smaller bucket and trained the operator to avoid simultaneous boom and arm use. The machine continued to work, albeit with reduced productivity.
In contrast, a fleet manager in Thailand retrofitted an EX120 with a turbocharged 4BD1T engine from a truck. With minor modifications to the mounts and cooling system, the machine regained full function and operated reliably for years.
Conclusion
Using an EX100 engine in an EX120 excavator body is technically possible but operationally compromised. The mismatch in power and hydraulic demand leads to performance issues that can frustrate operators and reduce efficiency. While creative solutions exist, they often involve trade-offs. For long-term reliability and full functionality, matching the engine to the machine’s original specifications remains the best path forward. In regions where rebuilt machines are common, understanding these dynamics helps buyers make informed decisions and avoid costly surprises.
