Pressure Washer Pump Parts: Replacement Guide
The parts inside a pressure washer pump, how to match a replacement to your washer, and 4 pumps with their stated figures.
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Saturday morning, driveway wash day: you pull the starter, the engine runs, and the wand spits a weak dribble instead of a fan of water. The pump has given up, and the fix is usually a new pump rather than a new washer. Pressure washer pump parts are the pieces inside that pump: the head, the pistons, the seals, the unloader valve and the fittings that join it to the engine and the hoses. This guide names those parts, shows how to match a replacement pump to your washer, and lists 4 replacement pumps with their stated figures.
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Below: the parts inside a pump, how to match a replacement to your washer, and 4 replacement pumps with their stated figures.
AAA 4400 PSI 4.0 GPM Horizontal Replacement
A horizontal triplex plunger pump kit rated 4400 PSI at 4.0 GPM with a 1-inch shaft, PowerBoost Technology and a 5 year warranty.
Pros
- 4400 PSI at 4.0 GPM with PowerBoost Technology
- Sight glass shows the oil level without a dipstick
- 5 year warranty stated
- Hardware and siphon tube included in the kit
Cons
- No safety listing stated
- Self-priming not stated
- Fits gas washers only; match engine orientation, PSI and GPM first
- No thermal relief valve stated
WOJET High-Flow Triplex Plunger Pressure Washer
A horizontal triplex plunger pump rated 4400 PSI at 4.0 GPM with a 1-inch shaft, self-priming and an integrated unloader, for 9 to 15 HP engines.
Pros
- 4400 PSI at 4.0 GPM for heavy-duty cleaning
- Self-priming: draws water from a bucket, pool or tank
- Integrated unloader valve cools the pump in bypass
- Brass head with polished ceramic pistons and forged brass manifold
Cons
- No warranty term stated
- Direct crankshaft mount only; no adapters fit
- 17.5 lb, heavier than the 16.0 lb SIMPSON
- Needs a 9 to 15 HP horizontal-shaft engine
VEVOR Pressure Washer Pump, 1'' Horizontal
A horizontal triplex plunger pump rated 4400 PSI with up to 4.4 GPM flow and 5 nozzles, with a recommended working pressure of 4000 PSI.
Pros
- 4400 PSI with up to 4.4 GPM, the highest flow here
- 5 nozzles included
- Recommended working pressure of 4000 PSI stated
- Fits Simpson MorFlex 40224 with a 1-inch shaft
Cons
- No warranty term stated
- No safety listing stated
- 21.58 lb, the heaviest pump in this set
- No inlet or outlet sizes stated
Hourleey 7/8" Shaft Vertical Pressure Washer Pump
A vertical pressure washer pump rated 3400 PSI at 2.5 GPM with a 7/8-inch shaft, pre-filled with oil and fitted with a thermal relief valve.
Pros
- 3400 PSI at 2.5 GPM for walls, fences, sidewalks and vehicles
- Pre-filled with oil and maintenance-free crankcase
- Thermal relief valve guards against overheating in bypass
- Detergent hose, bolts, shaft key, pads and washers included
Cons
- Lowest pressure and flow in this set
- No warranty term stated
- Vertical shaft only; check your engine orientation first
- 7/8-inch shaft, while the other pumps here use a 1-inch shaft
Pressure washer pump parts: what wears out first
A pump has a small set of parts that do the work, and each one wears in its own way. The pump head, or manifold, holds the water passages; the pistons, also called plungers, move the water; the seals keep water out of the oil and oil out of the water; the unloader valve opens when you release the trigger so pressure has somewhere to go; and the crankcase holds the oil that keeps the moving parts sliding. When a seal wears, water clouds the oil or oil weeps from the pump. When a piston wears, the pressure drops. When the unloader sticks, the pump overheats in bypass. The thermal relief valve is the last guard: it opens to protect the pump when the water inside gets too hot.
Match the replacement to the washer
Match 4 things before you order: the shaft, the orientation, the pressure and flow, and the fittings. The shaft must match the engine: a 7/8-inch shaft pump will not sit on a 1-inch crankshaft, and the other way round is just as wrong. A vertical pump fits a vertical-shaft engine and a horizontal pump fits a horizontal-shaft engine. Match the PSI and GPM to what your washer supports: the listings here state 3400 PSI at 2.5 GPM for the Hourleey and 4400 PSI at 4.0 or 4.4 GPM for the other 3 pumps. Check the inlet and outlet too: a 3/4-inch garden hose inlet is standard, while the high-pressure outlet may be a threaded M22 fitting or a 3/8-inch quick-connect. When in doubt, match the engine model and the old pump’s part number against the listing.
| Pump | Shaft | Orientation | Pressure and flow |
|---|---|---|---|
| Hourleey 3400 PSI Pump | 7/8 in | Vertical | 3400 PSI / 2.5 GPM |
| WOJET 4400 PSI Pump | 1 in | Horizontal | 4400 PSI / 4.0 GPM |
| VEVOR 4400 PSI Pump | 1 in | Horizontal | 4400 PSI / 4.4 GPM |
| Simpson AAA 4400 Pump | 1 in | Horizontal | 4400 PSI / 4.0 GPM |
Engine size matters too: the WOJET listing states it is made for 9 to 15 HP horizontal-shaft engines, so check your engine’s HP against the listing before you order. Wear eye protection when the washer runs, and never point the wand at people or pets.
Use rated jack stands on solid ground, wear eye protection with power and air tools, keep fuel-burning equipment out of a closed garage, and follow the manual for every tool. Read our Safety Disclaimer.
The gear: 4 replacement pumps
The pumps below cover the match above: check your shaft size and orientation, then pick the pump with the PSI and GPM your washer supports.
How to tell the pump is the problem
Low pressure with the engine running well points at the pump, and so does water in the oil or oil in the water. A pump that overheats in bypass may have a stuck unloader, and a pump that leaks at the head may have worn seals. Check the simple causes first: a clogged nozzle, a kinked hose, or a starved inlet can look like a dead pump. When the engine runs well, the water supply is full, and the pressure stays low, the pump is the likely cause, and a replacement pump costs less than a new washer.
FAQ: garage tools
Can I put a 4400 PSI pump on a 3400 PSI washer?
No. Match the pump to what your washer supports: the Hourleey pump is rated 3400 PSI at 2.5 GPM, while the other 3 pumps are rated 4400 PSI at 4.0 or 4.4 GPM. A pump rated above your washer’s figures will not make the washer stronger, and the engine, hoses and fittings are built for the washer’s own figures.
What shaft size does my pressure washer pump need?
Measure the engine crankshaft or check the old pump: the Hourleey pump uses a 7/8-inch shaft, while the WOJET, VEVOR and SIMPSON pumps use a 1-inch shaft. A 7/8-inch pump will not fit a 1-inch crankshaft, so match the shaft before you order.
Do I need to add oil to a new pressure washer pump?
The Hourleey pump comes pre-filled with oil in a maintenance-free crankcase, and the WOJET pump is pre-sealed and oil-filled. The SIMPSON pump has a sight glass so you can see the oil level, and the VEVOR listing states no oil details. Check the oil before the first start, and follow the manual for your pump.
What is the unloader valve on a pressure washer pump?
The unloader valve opens when you release the trigger so pressure has somewhere to go and water keeps moving through the pump. The WOJET pump has an integrated unloader valve that cools the pump in bypass, and the Hourleey pump has a thermal relief valve that guards against overheating in bypass. A stuck unloader lets the pump overheat, so a working unloader matters.
Can I use a vertical pump on a horizontal engine?
No. A vertical pump fits a vertical-shaft engine, and a horizontal pump fits a horizontal-shaft engine. The Hourleey pump is vertical, while the WOJET, VEVOR and SIMPSON pumps are horizontal. Match the orientation to your engine before you order.
Related: see our pressure washer pump repair guide, the pressure washer nozzle size chart, our best gas pressure washer picks and our car detailing hub.
