The Pressure Washer Pump

To most the pressure washer pump is just another component in your pressure washer system. Truth be known the high pressure pump is a highly reliable robust engineering marvel.

Read along to get a better understanding presented in non-technical terms.

pressure washer pump

Engineers define a pump as high pressure when it exceeds 150PSI (pounds per square inch pressure).

Pumps are designed to create a maximum amount of pressure at a specified rate of flow (gallons per minute).

Pressure is created by pushing this flow through the nozzle which creates a restriction. The nozzle has an inlet orifice which is sized to create the desired pressure at the desired flow.

Don't confuse the nozzle orifice with the outlet slot in the nozzle, the outlet slot is what creates the various fan patterns, it is the nozzle's inlet orifice that creates the restriction which in turn creates the pressure to get the job done.

Don't blame the pressure washer pump for a loss in pressure without at least checking for a worn nozzle first.

The most successful pressure washer pump found in the industry today is the "Positive Displacement Reciprocating Plunger / Piston type pump". This is an engineering term used to describe the charachteristics of this type of pump. Let's dissect this terminology.

Positive Displacement
Positive displacement is a pumps ability to move water or compress gases. It works on the what goes in must come out principle.

Reciprocating means the pump has the ability to convert rotary motion (the circular motion of a crankshaft) to linear motion (the back and forth motion of a plunger or piston in a cylinder).

Plunger Type
Uses a static seal (packings)

Piston Type
Uses dynamic seals that move back and forth with the piston

Engineers create different GPM flow rates in a pressure washer pump by varying three factors:
1. The speed of the pump (RPM)
2. The bore of the cylinder ( the diameter of the cylinder)
3. Stroke (the distance a plunger or piston travels in the cylinder)

By varying these three factors engineers create different flows for different pressure washer pump applications.

The pump is basically a system of valves and cylinders. The piston or plungers back and forth motion draws water through the inlet valve while the opposite stroke pushes the water out the outlet valve. Other valves direct the water through the pump by preventing backflow.

Pump Construction


Pressure washer pumps have two major sections that are bolted together these are:

1. Crankcase (power-end)this is coupled to the power source and contains the oil which in effect creates an oil bath for the crankshaft and connecting rods. This works on the same general principle as the automobile engine.

2. Manifold (head or wet-end)this is the section that water flows through.


This is connected to the power source directly in the case of direct drive setups or via a pulley and belt system in a belt drive pressure washer or by means of a gear reducer in gear drive systems.

The crankshaft turns in a bearing with an oil seal where it enters the crankcase. Cams on the crankshaft move connecting rods back and forth this is the reciprocating process which converts rotary motion to linear motion.

Plunger guides are attached to the connecting rods. These plunger guides extend through the crankcase into the cylinders of the manifold(wet-end). Oil seals prevent oil from entering the wet-end and creates an oil bath for the moving parts in the crankcase.

Plungers fit snugly into the cylinders of the manifold and are connected to the plunger guides via retaining bolts.



There are two inlet ports on the lower side of the manifold and two outlet ports on the upper side, all have female threads and one of each of these will be sealed with plugs. These plugs are interchangeable which can allow you to setup the pump left handed or right handed.

On a triplex pump there are 6 check valve caps. 3 inlets on the lower side and 3 outlets on the upper side. Removal of these caps reveal the check valves which are sealed with an o-ring. Check valve assemblies have a guide two seats and a spring. the guide holds the assembly together and in proper alignment. The spring holds the seats together holding the valve in closed position. When the water flow overcomes the pressure of the springs the seats will seperate opening the valve allowing water to pass. These check valves are press fitted into the manifold. Inlet and outlet check valves are all the same so they are interchangeable.

When you seperate the crankcase from the manifold the cylinders in the manifold become exposed. In a triplex pump the manifold has three cylinders. Each cylinder has a packing assembly held into proper position by a retainer. If you study a parts breakdown this should all become more clear.

View a breakdown of the AR- XMV series Pump

In Conclusion

This is not intended to have you doing your own pump rebuilds. A pump rebuild that is not done correctly will dramatically shorten the useful life of your pump. However, after further in depth study you can develop the ability to properly rebuild your pump and those of others.

Fortunately the major pump manufacturers provide great materials in the form of printed service guides and even how-to videos.

We hope we've helped you gain a further understanding and appreciation of this engineering marvel called the high pressure pump.

Get a great pressure washer pump at Sun Brite Supply

About Basic Pump Theory

Learn About Pump Cavitation

Learn About The Unloader Valve

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