A factory-built 4 GPM contractor washer costs more than the parts, and when its pump dies the machine is finished. Build the same thing yourself around a rebuildable pump and you get a machine you can keep running for a decade, plus the ability to choose your own engine.

The RKV4G37-PKG is built for that: 4.0 GPM at 3,700 PSI, on a 1 in hollow shaft with an F24 gas engine flange, boxed with the parts that usually turn a pump swap into three trips to the hardware store.

What ships with the RKV4G37-PKG

• Pump: RKV4G37DF24VR35, 4.0 GPM at 3,700 PSI, 1 in hollow shaft, F24 flange
• VR35 unloader, factory matched to the pump’s flow
• TPP140 thermal relief valve for bypass protection
• AR1264750 garden hose adapter
• 3/8 in quick coupling plug

The unloader being matched is the part that matters most. An unloader sized for the wrong flow is the most common reason a home-built rig never holds its rated pressure, and it is the hardest thing to diagnose after the fact.

Sizing an engine for the RKV4G37-PKG

4.0 GPM at 3,700 PSI works out to about 10.1 EBHP. Gas engines lose output to heat, altitude and the fact that published horsepower is measured under conditions your driveway does not reproduce, so you want 13 HP or more at the crank.

That puts you on a 420cc class engine. A Predator 420 from Harbor Freight is the common budget choice; a Honda GX390 is the common long-term choice. Briggs, Kohler SH and other 13 to 15 HP horizontal-shaft engines work equally well.

The only two things that have to match are the shaft, 1 in, and the flange, F24. Measure the shaft rather than trusting the engine’s badge, because displacement does not reliably predict shaft size across years and markets.

Why 4 GPM at 3,700 PSI is the useful number

This is the combination most working contractors settle on, and there is a reason. 3,700 PSI cuts grease, oil and mildew without being so aggressive that it etches concrete or lifts paint by accident. 4 GPM is enough to drive a 20 in surface cleaner at a working pace rather than a crawl.

Drop to 3 GPM and the surface cleaner slows down noticeably. Go up to 5 GPM and you need a bigger engine, more water supply, and a hose you will notice carrying. 4 and 3,700 is where the curve flattens out.

Building a machine around the RKV4G37-PKG

1. Fit the pump. Hollow shaft onto the engine crankshaft, then bolt to the F24 flange. It should slide on without persuasion.
2. Fit the accessories. VR35 unloader, thermal relief valve, quick coupler. Keep the unloader as close to the pump as the plumbing allows.
3. Plumb the supply. Garden hose adapter on the inlet, and add an inlet filter if you are drawing from anything other than clean municipal water.
4. Mount the assembly on a skid, cart or trailer frame. Get it square and get it rigid. A pump under mounting strain leaks at the head long before it wears out.
5. Fill with pump oil, start, and set the unloader with the trigger open. Then close the trigger and confirm it drops into bypass cleanly.

RKV4G37-PKG variants worth knowing

Two related packages cover slightly different builds. RKV4037-PKG is the same 4.0 GPM at 3,700 PSI with dual packing, and RKV4G37-VRT-PKG substitutes a VRT unloader. If you want more pressure, RKV4G40-PKG runs 4.0 GPM at 4,000 PSI; if you want more flow for surface cleaner work, RKV5G32D-VRT3PKG gives 5.0 GPM at 3,200 PSI.

Making the RKV4G37-PKG last

The RKV is a triplex plunger pump with a nickel-plated brass head, stainless plungers, replaceable seals and oversized bearings. Every wear part is available as a kit, which is the entire economic case for building rather than buying.

• Change the oil at 50 hours, then every 300 hours or three months
• Never run it dry, and do not leave the trigger closed for long stretches even with thermal relief fitted
• Replace O-rings at the first weep rather than after the leak
• Run pump saver before any storage below freezing

Do that and the pump outlasts the engine, which is exactly the point.

Unsure whether your engine will pair with it? Send the engine model and shaft diameter and you will get a yes or a better-fitting package.