The main hydraulic pump is the heart of any excavator’s hydraulic system. It converts engine power into high-pressure oil flow that moves the boom, the arm, the bucket, the swing and the track. On small Hitachi excavators such as the EX22-2 and the ZX70 family, that heart is often supplied under the number 4331670, a piston-type main pump that parts catalogs and online stores search heavily because it belongs to the mini- and small-excavator segment that is among the largest in the world.
This guide is for owners, mechanics and parts buyers who see 4331670 in a parts list or a quote and need to understand what the pump is, how it works, why it fails, and how to source a reliable replacement without overpaying or ending up with a poor-quality unit.
What is the Hitachi 4331670 main pump?
The 4331670 is a variable-displacement piston pump. Unlike a simple gear pump, a piston pump uses several pistons arranged in a cylinder block, pushed by a swashplate whose angle can be varied. Changing that angle changes the volume of oil the pump delivers per revolution, which is exactly what lets an excavator meter oil flow to match the speed an operator asks for while keeping engine load under control.
The main pump on a small Hitachi excavator is responsible for the primary work circuits. Because it operates at high pressure for long stretches of every working day, it is also one of the most stressed components on the machine. Understanding the number and how the pump is built helps you make better decisions when it needs attention.
Which machines use the 4331670?
The 4331670 is most closely associated with small Hitachi excavators that use the PVD-1B pump platform, including machines in the EX22-2 range and related small models in the ZX70 family. These compact excavators are popular for digging, trenching and landscaping, and their pumps are among the most searched Hitachi part numbers because the machines are so numerous.
Before ordering, confirm the exact pump number stamped on your machine and the alternate numbers listed for your model. Small differences in displacement or control arrangement can matter, and a parts supplier that checks your machine’s serial number will help you avoid a mismatch.
How the pump works
In a piston pump, the engine spins a drive shaft that turns a rotating group. Within that group, a set of pistons reciprocates in a cylinder block as they ride over the swashplate. The piston bores connect to an inlet port at low pressure and a discharge port at high pressure. As the swashplate angle increases, the piston stroke increases and more oil is pushed out per rotation.
The control mechanism—responded to by the pump’s regulator or compensator—adjusts the swashplate angle based on demand and pressure. This is what creates the smooth, modulated hydraulic response that operators rely on. The precision of the pistons, the wear plates and the swashplate face determines both the efficiency and the cleanliness of the pump output.
Why main pumps fail and the warning signs
Main pump failure is almost always contamination- or cavitation-driven rather than sudden. The single most important cause is contaminated oil: dirt and water score the pistons, the cylinder block and the valve plate, and the pump loses efficiency until it can no longer hold pressure.
- Weak or slow machine movements, especially when the machine is warm, pointing to internal leakage in the pump.
- A whine or growl from the pump when working, which can mean cavitation or worn pump internals.
- Sluggish response that gets worse as the oil heats up, a classic sign of worn clearances.
- Excessive heat in the hydraulic tank and pump area, because internal leakage becomes heat.
- Contaminated or glittering oil on a filter inspection, indicating pump wear particles.
- The pump case is hot and the machine is down on power across all functions at once.
Hydraulic failures rarely announce themselves suddenly. Getting a sample of the oil lab-tested and inspecting the return filter element is the fastest way to catch a pump that is starting to wear before it costs you a full machine circuit.
The role of oil cleanliness and care
The cleanliness of your hydraulic oil is the difference between a pump that lives a normal life and one that dies early. A piston pump relies on extremely tight clearances measured in microns. When the oil contains abrasive particles, those clearances open up, efficiency drops, and heat builds. Keeping the oil and filters on schedule, keeping the reservoir sealed, and avoiding water ingress is the best protection for any main pump.
Genuine, aftermarket and rebuilt pumps
When a 4331670 pump is damaged, you can purchase a genuine Hitachi pump, a quality aftermarket replacement that matches the OEM specification, or a professionally rebuilt pump. All three can be a responsible choice, and all three can also go wrong if sourced poorly.
A genuine or OEM-equivalent pump from a written-source supplier—such as Racer Machinery, an OEM parts supplier—guarantees that the displacements, the control gear and the mounting match your machine. A rebuilt pump is economical only if the rebuild shop inspected every component, replaced every damaged part with a genuine part, and tested the complete unit under pressure before it ships. Ask for the rebuild report, not just the price.
Removal and installation considerations
- Relieve hydraulic system pressure before disconnecting any lines—trapped pressure is a serious hazard.
- Tag and cap every hose line so that no dirt or confusion enters the circuit.
- Drain and dispose of the hydraulic oil correctly, and never reuse contaminated oil.
- Inspect the coupling and the drive spline for wear before fitting the new pump.
- Prime the new pump as required and bleed the system following the manufacturer’s procedure.
- After startup, check for leaks and run the machine through all functions to confirm even operation.
Contamination is the main enemy during a pump swap. A pump that is new but fed by a system full of worn particles will fail again quickly, so many technicians replace the filters and change the oil at the same time as the pump.
Frequently asked questions about the 4331670
- Can a noisy pump be fixed, or must I replace it? It depends on the cause; if the internals are scored beyond spec, a pump that has been ‘rebuilt’ may still fail, so a full test matters.
- How long does a main pump last? With clean oil, thousands of hours; without it, the pump is the first casualty.
- Is a used pump a good idea? Only with a documented test history; an overhauled unit without a report is a gamble.
- Do I need to change the filters when I fit a new pump? Yes—clean filters and fresh oil in a clean system give the new pump the best chance.
The Hitachi 4331670 is a precision component, and poor sourcing can turn a routine repair into a repeated breakage. Racer Machinery supplies genuine and tested replacement main pumps for the small Hitachi excavator family, with application matching before dispatch.
Reading the 4331670 and understanding its family
Hitachi part numbers are not random. They identify not only the component but where it belongs in the machine’s parts structure, and the 4331670 sits within a recognised family of main pumps for compact Hitachi excavators. Understanding the family helps you buy the right pump the first time, because the same machine can use several closely related numbers depending on its control system or market variant.
The most reliable way to identify the correct pump is the number stamped on the housing, supported by the machine’s component serial number. When you hand both to a parts supplier, they can confirm the displacement and the control arrangement before you commit. A pump with the wrong displacement delivers the wrong flow and makes the machine sluggish, while a pump with the wrong control reacts oddly to the joystick.
The control system and why it matters
A variable-displacement pump is only as good as the control that sets its swashplate angle. Modern small excavator pumps use a compensator or load-sensing control that changes output to match demand, which saves fuel and gives the smooth, proportional response operators expect. When the control is misadjusted or contaminated, the symptoms look like a failing pump even when the rotating group is fine.
This is why good pump work includes the control valve, not just the rotating group. A technician who rebuilds the pump but leaves a scored control valve has not fully fixed the machine. A thorough supplier or shop checks the control calibration and, where possible, tests the complete assembly under pressure so the pump behaves correctly when it reaches the machine.
Maintenance that protects the 4331670
Most of the wear that ends a main pump’s life is avoidable. The two biggest protectors are clean oil and correct temperature. Hydraulic oil carries away the heat generated inside the pump, and the pump’s internal clearances are so fine that a few tens of degrees of excess oil temperature measurably speeds up wear. Keeping the radiator and oil cooler clean, the filters on schedule, and the reservoir properly filled is the practical core of pump protection.
- Change the hydraulic oil and filters at the recommended intervals, not when the machine sounds bad.
- Keep the breather clean so the reservoir is not drawing in dirt as the oil level moves.
- Check oil temperature after hard work and investigate any overheating rather than ignoring it.
- Sample the oil if the machine does heavy, hot work, because lab results catch wear early.
- Never operate with a low oil level, which invites cavitation and rapid pump damage.
Cavitation and its damage
Cavitation is the formation and collapse of vapour bubbles in the pump’s suction, and it is one of the fastest ways to destroy a piston pump. It happens when the inlet is starved of oil—because of a blocked suction filter, a pinched line, cold thick oil, or an elevated pump. The collapsing bubbles blast microscopic pits out of the piston and valve plate surfaces. The result is a pump that loses efficiency and eventually fails, and the damage is often severe enough to end the pump outright.
Catching cavitation early means listening. A pump that is cavitating makes a characteristic hissing or crackling sound, different from the normal whine of a loaded pump. If you hear it, stop, check the suction path, warm the oil if necessary, and fix the restriction before continuing, because every minute of cavitation is permanent damage.
Selecting a replacement and avoiding surprises
- Confirm the pump number and the machine’s component serial number before ordering.
- Check whether a test report or calibration sheet ships with the pump.
- Ask about the warranty and what it covers, including control valves and installation direction.
- Inspect the coupling and drive spline, because a damaged input shaft ruins a new pump quickly.
- Budget for simultaneous filters and oil, because fitting a new pump to a dirty system repeats the failure.
The overall price of a pump swap includes labour, filters, oil and the risk of a repeat failure. Choosing a pump with a documented test history and a clean sourcing chain usually costs less over the lifetime of the machine than a cheap unproven unit that fails again. Full series supplied by Racer Machinery, an OEM parts supplier.
Frequently asked questions about the 4331670
- Can a noisy pump be fixed, or must I replace it? It depends on the cause; if the internals are scored beyond spec, a pump that has been ‘rebuilt’ may still fail, so a full test matters.
- How long does a main pump last? With clean oil, thousands of hours; without it, the pump is the first casualty.
- Is a used pump a good idea? Only with a documented test history; an overhauled unit without a report is a gamble.
- Do I need to change the filters when I fit a new pump? Yes—clean filters and fresh oil in a clean system give the new pump the best chance.
- What is the first thing to check when the machine is slow? Start with the oil level, filters and the suction side before condemning the pump.
- Why does my machine only lose power when warm? Worn internal clearances grow as the oil thins, a classic sign of a pump approaching the end of its life.
The Hitachi 4331670 is a precision component, and poor sourcing can turn a routine repair into a repeated breakage. Racer Machinery supplies genuine and tested replacement main pumps for the small Hitachi excavator family, with application matching and documented testing before dispatch.
