Choosing The Right Pump and Gearbox For Your Application

Choosing The Right Pump and Gearbox For Your Application

A common problem people have when building their own gear driven system, is not knowing what will actually work for the build.  At we have built hundreds of gear driven machines.  With this being said, we have definitely discovered some hurdles.  I want to make this easy for you to understand.  I am going to cover the engines we have built gear driven machines with.  The SAE J609a is what you will find on most gearboxes.  This flange is commonly seen on the General Pump ZK, Comet and AR gear boxes.  We only carry the gear boxes with an SAE J609a flange moving forward.  

You can find many of the models here - WashMart Gearboxes

The SAE J609b is the flange found on the “PA” gear box is more versatile and will fit on pretty much any engine used in the pressure washer industry.  They are rebranded by other companies with many different names, but essentially the bulky silver one?  Yeah, that is the PA gear box.  We no longer carry this model, but can be found at many different stores.

So, here is the part that will make your life easier, which ones will and won’t work with what engines. Here is what we have found.  

SAE J609A will fit on the following engines (from our experience).

Honda GX390, 630, 690, IGX700 and IGX800

Predator 420cc and 670cc

Kohler CH440

Vanguard 14, 18 and 23 HP

We have found that they WILL NOT fit on the Kohler EFI 26.5, CH730, CH 750, CH980 and CH1000.  This is important to know for you Kohler fans.  There are other models they may or may not fit.  But we have found that they will NOT fit on these engines.  Here is a link to the Kohler Command series we are referring to - Kohler Command Series

If you look at page 5, you can see the bolt pattern we are referring to.

The SAE J609b will fit on any engine we have used so far, which includes all of the above.


Now that we have covered engines, let's cover pumps!

There are some unusual things we have discovered with the different gearboxes in the market.

What is unusual is that we have the majority of gear boxes running at 1450 RPM which causes the gear boxes to run at a 2.1(ish)% ratio.  What does that mean?  I can make this simple, instead of turning this into a pissing contest of who’s math works.

Kohler CH440 14 HP with an AR RKA55G3N pump (3000 PSI 5.5 at 1750 RPM) with a 1450 RPM gear box (like the PA or Comet) will run at 3000 PSI at 5.5 GPM (barely).  With the AR1690 gear box that is 1750 RPM we can get 3000 PSI at 6.5 GPM (barely). 

You will ask yourself, why in the hell does this happen?  In Europe engines run 50 cycles instead of 60 cycles (3000 RPM instead of 3600 RPM).  So the gear boxes are essentially made by Italians for Italians.  But AR, having such a huge footprint in the USA, got smart and realized that there was a market here and created a 1750 RPM gear box series.  

Now what is frustrating with the AR gear boxes is that they have a very small oil bath and are temperamental to heavy vibration.  So when they begin leaking oil, you have to address it immediately or bam, your gearbox is toast.

We have been using the Comet series gearboxes more often because of their rigidity, liability and large oil bath.  We have had very few warranty issues with this gearbox model.  But they only make it as a 1450 RPM option.  This has been frustrating for us and we have repeatedly asked Comet to have a 1750 RPM version of this gear box made so that we can get the same performance that AR has been able to give us.

I point this out because you have a choice here.  Do you want a more reliable system?  Or do you want to gain 1 free gallon per minute?

OH, BEFORE I FORGET!  There is a hack I’d like to share!  We found the best unloader for the 6.5 build is the VRT3.  We have tried several unloaders and would only get 3000 PSI at 5.5 GPM.  When we used the VRT3 we somehow managed to climb to 6.5 GPM.  Another special note is on the Kohler CH440s, we would get 6.5 GPM 19 out of 20 builds.  Why ?  I have NO IDEA.  But we have built approximately 200 of this particular model and originally sold it as a 6.5 GPM.  After 6 months we ran into one machine at the test station and it barely hit 3000 PSI at 5.5 GPM.  Once we saw there were some engines that just “didn’t do it”, we changed the title to the machine being a 5.5 GPM power washer.  However, they were absolutely delivering 6.5 GPM at 3000 PSI 19 out of 20 of the builds.  

Let me reiterate in English.  Kohler CH440, AR RKA55G30, VRT3, AR1690 = 6.5 GPM at 3000 PSI 95% of the time.  This logic will also apply to the Vanguard 14 and the Predator 420 (they are all 3 a 429cc engine).

We manufacture a baseplate, designed for this model, if you haven’t already purchased one (or if you are lazy and don’t want to fab one yourself), order here - R Washer Baseplate

On a Honda GX390 we have been able to hit 3000 PSI at 6 GPM with the VRT3, RKA55G30, VRT3 and AR1690.

It gets even crazier.  We have found that using a PA gear box or Comet gearbox(1450 RPM) with a Honda IGX800 with the AR XWAM8G35N will only deliver 3600 PSI at 8 GPM.  With the AR XWAM8G35N and the AR1670 (1750 RPM), we have been able to acquire 4200 PSI at 8.5 GPM which has absolutely blown our minds!  We did one time experiment and found we can actually get 4400 PSI at 8.5 GPM but it blew seals within 150 hours.  We found that the 3500 PSI pumps limit really is 4200.

 I have created a little cheat sheet for you.

  • AR North America AR1690- 1" Drive Shaft (Honda gx390, Predator 420, Duromax, Kohler CH440
  • AR1690 will fit on the RKA55G30N, but will not fit on the 8 GPM XW series pumps. 
  • AR1670- 1 1/8" Drive Shaft (Honda GX690, IGX700, IGX800) This model will fit on the AR XW 8 and 10 GPM pumps no problem.
  • AR 1697- 1" Drive Shaft (Vanguard 18 and Predator 670cc) This model will fit on the AR XW 8 and 10 GPM pumps no problem.
  • Comet 5005.0207.00- 1" Drive Shaft will fit on all engines listed above and will fit with most Comet, General Pump and AR Pumps.  This is the one we recommend with most pumps.  Just keep in mind they will not work with a Kohler V-Twin model.

A special note is that the Vanguard 23 and Honda GX630 work with these gear boxes but they vary and sometimes I have seen them with a 1” shaft and sometimes they have a 1 ⅛” shaft.  This has to be verified before you just purchase pumps and gearboxes.

There are many engines I haven’t listed, because we simply haven’t worked with them.

You may wonder why I am sharing this information as an OEM.  It is because we are no longer building gear driven, gas powered machines for mass production and have strictly moved to 3 phase and diesel powered pressure washer equipment.

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