Cummins Onan Small Generator Impeller: Where's the Shaft? (Split-Housing Pump)
- Jul 6
- 8 min read
Some small Cummins Onan generators use a split-housing Sherwood raw water pump that throws boat owners off the first time they open it up. There is no thin front cover with the shaft sitting right behind it. The housing comes off as a section, with the impeller in one side and the shaft on the other.
In this walkthrough, Eddie Protzeller services a small Cummins Onan MDKBM generator impeller in a very tight space and shows why careful removal, correct impeller orientation, and clean reassembly matter.
What You'll Learn
Why this small Cummins Onan pump looks different from a standard face-plate impeller pump
How the Sherwood split-housing pump comes apart
Why the brass lip around the impeller is easy to damage
How the tiny keyway affects new impeller alignment
Why the correct impeller, the right O-rings, and a water test matter before you call the job done
"What you don't want to do is put a screwdriver behind the housing and pry against it. Look how thin that groove is. Pry in there and the chance of damaging that brass is pretty high." - Eddie Protzeller
Where Is the Shaft on a Small Cummins Onan Generator Pump?
On this small Cummins Onan MDKBM, the raw water pump is a split-housing Sherwood pump, so the shaft is not sitting behind a thin front cover the way it is on many pumps. The housing comes off as a section, with the impeller in one side and the shaft on the other. To reach the impeller you close the raw water valve, remove three stainless bolts, disconnect two hoses so the pump can move, and take the housing off backward. Once you know the housing splits this way, the rest of the job is straightforward.
Step-by-Step Overview
1. Identify the Pump Style
This small Cummins Onan MDKBM generator uses a Sherwood raw water pump wedged into a tight space between the Volvo Penta engine and the generator. The pump sits almost against the side of the engine, with hoses, wires, and a dipstick crowding the work area. Unlike a pump with a thin bolt-on face plate, this housing splits apart as a section, so the impeller is not sitting behind a cover with the shaft right there.
2. Turn Off the Raw Water Supply
Close the raw water valve before opening the pump. The sea strainer is tucked out of sight, but the system is isolated so water does not keep feeding the pump while the housing is off.
3. Remove the Hoses for Access
The work area is extremely tight. Eddie removes the hoses to open up enough room to reach the three stainless bolts and to rotate the pump housing during reassembly.
4. Remove the Three Housing Bolts
The housing is held on with three stainless metric bolts. With those out, the housing comes off backward and exposes the impeller inside the housing section.
5. Inspect the Old Impeller
The old impeller comes out in one piece. It is not missing vanes, but it shows wear: fine lines in the rubber and grooves where it has been riding against the cam. That is enough to retire it.
6. Avoid Prying on the Brass Lip
The groove around the impeller is very thin. A screwdriver used for prying can nick or distort that brass sealing area, and damage there can create a leak later.
7. Remove the Impeller with a Clean Grip
Eddie lifts the tiny impeller out with a steady grip on the rubber instead of prying against the brass. The point is a clean, even pull with no leverage against the delicate housing.
8. Confirm the Correct Replacement Impeller
The first impeller grabbed for the job is wrong: the bore is too large for this shaft. Small generator impellers look alike, so the correct one is selected and lubricated before it goes in.
9. Clock the Keyway Correctly
This pump uses a tiny keyway, so the new impeller has to be oriented so the keyway lines up with the shaft. Eddie points it toward the upper hole, roughly halfway between the two housing holes.
10. Install the New Impeller and O-Rings
The correct impeller is lubricated and set into the housing. The kit includes a gasket that this pump configuration does not use, plus the two O-rings that it does.
11. Reassemble the Housing
The stainless plate and housing go back on. Eddie starts all three bolts before snugging them down evenly.
12. Reconnect Hoses and Run a Water Test
With the hoses back on, the raw water valve is opened, the sea strainer is checked, and the generator is run to confirm it is moving water.
Mechanic Insight
The lesson on this pump is understanding how the housing comes apart before you force anything. On a split-housing Sherwood pump, the impeller often comes out with the housing instead of sitting behind a thin face plate. Once it is open, the brass lip around the impeller is thin and easy to scar. A screwdriver used for prying can turn a simple impeller service into a sealing problem, so the goal is a controlled grip, a clean pull, and careful reassembly.
Common Questions
Why was the first replacement impeller wrong?
The first impeller did not fit because its bore was too large for this pump's shaft. Small generator impellers look alike, so match the bore, the shaft size, and the keyway to the old part before installing. Staging the new impeller against the old one catches a mismatch before it costs you the job.
How do you clock the impeller on this Onan pump?
This pump uses a small keyway, so the new impeller has to be oriented so the keyway lines up with the shaft. Eddie points it toward the upper hole, roughly halfway between the two housing holes. If it is not clocked correctly, it will not seat properly during reassembly.
Why shouldn't you pry the impeller out with a screwdriver?
The brass lip and the groove it sits in are thin, and a screwdriver levered against that area can nick or distort it. That damage can create a sealing problem later, and the housing costs more to replace than the impeller. A steady, even pull avoids the risk.
Do you use the gasket or the O-rings from the kit?
This pump configuration uses the two O-rings, not the gasket the kit includes. Match what came apart, inspect the old seals, and fit the correct new O-rings before reinstalling the housing.
The generator is not pumping water after service. What should I check?
Confirm the raw water valve is open, the hoses are tight, and the sea strainer is sealed. If it still is not moving water, recheck the hose connections, the impeller orientation in the housing, and the pump seal. A missed valve or a mis-clocked impeller is the usual cause.
Full Transcript (Cleaned for Readability)
We're still down here in a very tight space. There is not a lot of room between the Volvo Penta engine and the generator, but tight spaces are where this tool helps.
There are hoses, wires, and other components in the way, so we'll remove what we need to get access. This pump has three stainless bolts holding the impeller housing in place.
The thing I want to point out is how close this Sherwood pump is to the side of the engine. It is almost touching it.
This is a Sherwood pump attached to a small Cummins Onan generator, an MDKBM unit. With small equipment, you often get very tight spaces.
We'll remove these three metric-headed bolts, and after we turn the water off, the housing will come off backward. What you do not want to do is put screwdrivers behind the housing and pry against it.
There is a dipstick in the way, hoses in the way, and wires in the way, but I think we'll be able to get a clean pull.
First, we need to find the raw water valve and turn it off.
The water is now closed. The sea strainer is hidden, but it runs underneath and up into the bottom of the system, so we should be good to open the pump.
This area is ridiculously tight. I can barely get the head of the tool on the bolt. I was hoping there might be enough room to avoid taking the hoses off, but for this video I'm going to remove them so we can show the job clearly.
Now that the hoses are off, we can remove the housing.
This is not terrible once it is apart. When we install the new impeller, we need to pay attention to the tiny keyway. It needs to be clocked toward the upper hole, about halfway between the two housing holes.
The question is whether it will be easier to install the impeller into the housing first and then slide the housing back onto the pump. That looks like the better way. We'll also install the new O-ring and lubricate everything.
The old impeller came out in one piece.
For removal, I pushed the teeth down across from each other so they grab the rubber, without using the threaded pulling portion. The point here is no prying. Look how thin this groove is. If you take a screwdriver and pry in there, the chance of damaging that brass area is pretty high.
With a good grip on the rubber, we'll try pulling it out one-handed.
The impeller comes out cleanly. It is not missing pieces, but it is definitely worn. You can see small lines in the rubber and grooves where it has been working against the cam.
Now let's look at the new parts. The kit includes a gasket we do not need for this pump, along with two O-rings.
The first impeller we grabbed is not the correct one. The shaft size is too big.
Now we have the correct impeller. We'll lubricate it and get some lubricant around the housing. Then we'll lower the impeller in and clock it so the keyway lines up correctly.
This part is difficult because the work area is so tight.
Once the impeller is clocked correctly, we can slide the housing back into place.
Now we pick up the stainless plate and start the bolts. I like to get all three started before tightening them down. Once two are started, we get the third one in and snug everything evenly.
With the hoses removed, it is much easier to rotate the housing, remove the old impeller, install the new impeller, and roll it into place. Without removing the hoses, you would be working in an even more confined space.
All three bolts are now snug.
Now we reconnect the hoses. One hose goes on, then the second hose. We'll tighten the clamps, run the generator, confirm that it is pumping water, and call it a night.
The sea strainer is tight, the raw water valve is open, the hoses are on, and the pump is assembled. Now we can start it and verify water flow.
Related Marine Maintenance Videos
A Note on the Tool
Eddie used the small ImpelPro puller here mainly for grip, to lift the tiny impeller out without prying against the thin brass. On a pump this delicate, a clean, even pull is what protects the housing.
Topics Covered
Cummins Onan small generator impeller replacement
Cummins Onan MDKBM impeller service
Sherwood raw water pump impeller
split-housing raw water pump
small generator impeller replacement
marine generator raw water pump service
no shaft behind impeller cover
impeller keyway alignment
brass pump housing damage
tight space generator impeller service
About the Author
Eddie Protzeller is a Seattle tugboat and yacht mechanic and the inventor of the ImpelPro impeller puller. With 16 years of hands-on experience servicing inboard engines and generators, he specializes in marine cooling systems and impeller maintenance. For more about Eddie, visit About Us. See the full lineup of ImpelPro impeller pullers at impelpro.com
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