Some Honda GX390 variants carry an internal 2:1 gear reduction, which turns a 3,400 RPM engine into a 1,700 RPM output shaft. That is good for pump life and bad for compatibility, because a standard direct-drive pump expects engine speed and a standard flange.

The RKA4G40HJ-F22 exists for that specific pairing: 4.0 GPM at 4,000 PSI at 1,750 RPM, on a 25 mm shaft with an F22 flange.

RKA4G40HJ-F22 specifications

• Flow: 4.0 GPM (15.1 L/min)
• Pressure: 4,000 PSI (275 bar)
• Speed: 1,750 RPM
• Shaft: 25 mm solid, F22 flange, for 2:1 gear reduction engines
• EBHP: 11.0
• Head: nickel-plated brass
• Connecting rods: bronze
• Oil capacity: 15 oz
• Temperature: not published for this model. Confirm before hot-water use

Why the gear reduction matters

A direct-drive pump on a GX390 runs at 3,400 RPM. This one runs at 1,750, which roughly halves the wear cycles per hour on the plungers, seats and seals.

That is a real difference in service life, and it is the reason to specify a reduction engine in the first place. The pump runs cooler, pulsation is gentler, and the machine is quieter.

The tradeoff is that this pump only fits a reduction engine. Fitting it to a standard GX390 gives you 2.0 GPM, not 4, because the pump would be turning at half its rated speed.

Confirm the engine first

This is the whole selection question. You need a GX390 with the internal 2:1 reduction (the K1 and LX2 variants among others), a 25 mm output shaft, and an F22 flange pattern.

Measure the shaft and check the flange before ordering. Engines with the same displacement and badge come in several shaft and flange configurations, and the reduction versions are the minority.

If your engine is a standard GX390 with a 1 in hollow shaft, you want a direct-drive pump instead: RKV4G40-PKG delivers the same 4.0 GPM at 4,000 PSI on that setup.

Sizing an engine for the RKA4G40HJ-F22

4.0 GPM at 4,000 PSI is 11.0 EBHP. A GX390 at around 13 HP covers it with reasonable margin, which is why this pairing is the one AR built the pump around.

Where the RKA4G40HJ-F22 is used

• Contractor rigs: concrete, fleet and equipment cleaning at 4,000 PSI
• OEM builds on GX390-powered machines
• Surface preparation: paint stripping, graffiti removal, concrete washdown
• Agricultural washdown: equipment, barns, livestock facilities
• Portable and trailer-mounted industrial rigs

RKA4G40HJ-F22 options and service

• Viton water seals in Ø18 for chemical and hot water service
• High temperature kit available. Ask us for the rating it gives this pump
• Single cylinder piston kit for quick field repairs, which is useful on a mobile rig where one cylinder fails and the others are fine
• Full repair kits for valves, pistons, seals, support rings and oil seals

The nickel-plated head and bronze rods are the durability story: plating resists hard water and detergent, bronze handles sustained load and heat better than aluminium.

Not sure whether your engine has the reduction? Send the engine model and shaft diameter and you will get a yes or the right alternative.