Simpson ALH4240 4200 PSI 4.0 GPM 60688 Pressure Washer

Three things tell a contractor everything they need to know about a pressure washer before they even touch it: the engine, the pump, and the frame. The Simpson ALH4240 checks all three without compromise. A Honda GX390 commercial engine that starts on the first pull and runs all day, a CAT Pumps triplex plunger pump rated for 4200 PSI at 4.0 GPM, and an aircraft-grade extruded aluminum frame that simply does not rust. You already know what those names mean. That's the point.

Put those specs together and you get 16,800 cleaning units, enough to handle deck restoration, fleet washing, graffiti removal, and heavy-duty surface prep without ever switching machines. Whether you're running one job a week or putting this thing through five days of commercial work, the ALH4240 is sized and built to keep pace. Rated 4.8 stars across hundreds of verified buyers, it's a machine that consistently earns its keep in the field.

Keep reading for the full breakdown, or scroll down to the Downloads section for the spec sheet, manual, and parts manual. And if you have questions before buying, our team is here to help you make the right call.

• Honda GX390 commercial engine: 389 cc, 13 HP gross output, recoil pull-start, built-in low-oil shutdown alert. The Honda GX390 pressure washer engine is the benchmark in this class for a reason: it starts reliably, runs smoothly under load, and carries a 3-year commercial warranty.

• CAT Pumps triplex plunger pump (Model 67DX): 4200 PSI at 4.0 GPM, direct drive. The CAT pump pressure washer pairing means you're working with a commercial-grade pump that carries its own 5-year commercial warranty. External easy-start unloader and thermal relief valve included.

• 16,800 cleaning units: 4200 PSI times 4.0 GPM. That number is what actually determines how fast and thoroughly a machine cleans. Most jobs in this class don't require you to dial anything back.

• Aircraft-grade aluminum frame: brushed extruded tubing, corrosion resistant, built to outlast the pump and the engine. Where steel frames rust from the inside out, the aluminum frame pressure washer structure just keeps going. Simpson backs it with a 10-year commercial warranty.

• 50-foot steel-braided Monster Hose: 3/8 in. diameter, polyurethane outer jacket, kink and abrasion resistant, quick-connect fittings on both ends. Serious reach without the mess of a hose that fights you every time you coil it.

• 5 quick-connect nozzles: 0°, 15°, 25°, 40°, and soap (black). Swap tips in seconds without touching a wrench. The soap nozzle activates the downstream chemical injection for low-pressure detergent application.

• Ergonomic rear-load spray gun: safety trigger lock included, quick-connect fitting, designed for extended use without hand fatigue.

• 31-inch quick-connect spray lance: keeps your hands well back from the surface and pairs with all five included nozzles.

• 13-inch premium pneumatic tires: roll over job-site terrain without the headache of flat tires or hard plastic wheels that won't move over anything but smooth concrete.

• 1.6-gallon fuel tank: adequate runtime between fills for extended sessions. Runs on regular unleaded, minimum 87 octane, up to 10% ethanol (E10).

• Cold water only: not rated for hot water input. Water supply minimum: 5 GPM at 20 PSI.

• 49-state compliant: not for sale in California.

• Dimensions: 38 in. L × 23.5 in. W × 29 in. H (96.5 cm × 59.7 cm × 73.7 cm). Weight: 133 lbs (60.3 kg).

The frame is where most pressure washers eventually lose. Steel frames corrode from road salt, fertilizer runoff, and plain old moisture, and once the corrosion starts, it doesn't stop. Simpson built the ALH4240 around an aircraft-grade aluminum frame, specifically extruded tubing that's been brushed for finish and sized for rigidity. That 10-year commercial frame warranty isn't marketing language: it reflects a design choice that takes the rust problem off the table entirely. If you're comparing aluminum frame pressure washers in this class, this is the one built to still be standing when the pump eventually needs attention.

The hose and spray kit match the machine's professional intent. The 50-foot steel-braided Monster Hose with its polyurethane jacket gives you serious reach and resists the kinking and abrasion that kills cheaper hoses fast. The ALH4240 manual covers full maintenance intervals, and the Simpson ALH4240 parts manual is available in the Downloads section below if you're stocking up on Simpson 60688 parts ahead of heavy use. Quick-connect fittings throughout mean you're not threading anything by hand in the field.