New impeller won't prime.

[Ajax]

Riddle me this fellas:

Oberdorfer 202M7 pump.
Engine has sat for at least 5 years, properly winterized on the hard.

I remove old impeller. It's not cracked, but is a bit deformed.
I install a new, seemingly identical impeller. The package even said "Oberdorfer" on it, and referenced the pump models it's compatible with.

With the new impeller, the pump simply cannot pull water from the seacock.
I put the old impeller back in, and "bam" pulls water and pumps like a champ.
Obviously, running with a 5 year old impeller is not acceptable. I tried lubing the new impeller, but it didn't help. The old impeller is not lubricated and still pumps fine. No, the metal hub is not torn out center of the new impeller. The old and new impellers seem identical in every measurement.

Also- What is the difference between the "N" designation and the older, 202M7 model?

Lastly, here's something that makes NO sense to me whatsoever:
When I replace an impeller on my Atomic-4, it slips easily off the shaft when I remove the cir-clip. When I needed to remove and install the impellers from this other engine, I needed to use a hammer and drift to tap the shaft through the impeller hub. That's not normal, is it?

What say the quorum?

[hanleyclifford]

Did you measure the thickness of each impeller?

[Ajax]

Well, I stacked them on top of each other and placed them side by side. They seemed identical. I guess I'll check again.

I'll also post a photo of the pump to ensure that its the model I actually think it is.

[hanleyclifford]

Look at them carefully side by side. Even a slight variance can leave the thinner unable to prime. This issue has come up here before.

[roadnsky]

Here's a post from Don back in 2010 about this very issue...


We receive a handful of reports each year in which an older Oberdorfer impeller has swelled to a size considerably larger than the dimensions of a new impeller. If an impeller is kept in service until it enlarges significantly beyond its original size (particularly in width), it will cause accelerated wear on the inside of the impeller chamber. In some few cases, assuming the impeller itself survives, wear within the impeller chamber will so closely match the aging/swelling impeller that the pump will continue to function longer than expected. Unfortunately, these pumps will usually not accept a new (properly sized) impeller.

New Oberdorfer impellers measure 2” in diameter, and 7/8” across the width. If your pump will not work with a new impeller with these nominal dimensions, you have several options:

1) Reinstall the old impeller and continue to use the pump (not really recommended except as an emergency measure to get to a safe port).
2) If you have an older Oberdorfer M3 series pump (usually with big raised letters on the back plate), you can try replacing its cam shoe with a new M7 shoe (product number - CSOB_08_69 in our online catalog at moyermarine.com). The M7 cam shoe is twice as thick as the M3 (1/8” versus 1/16”). The increased performance of the M7 shoe might allow the pump to continue working, at least as an emergency spare.
3) If wear within the impeller chamber shows up primarily as excessive depth, you might be able to work enough metal off the rear face of the housing (using sand paper on a flat surface) until seeing a small amount of “crush” on the new impeller when installing the back plate. In these cases, you will probably also have to reface the back plate using the sand paper on a flat surface to get rid of any grooves.
4) Replace the pump and begin changing the impeller after every second season or after 200 hours whichever comes first (after every third season or 300 hours for freshwater cooled engines)

[ndutton]

I have personally resurrected two worn (excessive cavity depth) Oberdorfer pumps following the chamber facing protocol described in Roadnsky's quote. It takes a delicate touch to keep the surface flat and future gaskets will likely need a light coating of Permatex #2.

I would not recommend an O ring style faceplate after such facing.

[JOHN COOKSON]

Try Priming The New Impeller

I went through this identical scenario many years ago- a new impeller would't prime. When primed the new impeller by pouring water into it instead of pulling water into it, it worked fine. Now whenever I open the pump up I prime it before I start the engine. It always works great. I sure my pump is slightly worn as mentioned but I run at 150 degrees+\- in my RWC engine so I'm getting plenty of water through the engine. I've left it alone over the years.
My best guess in your case is that the pump is worn a bit and even though the old and new impellers appear to be the same the old one is swollen in some aspect just enough to pull a prime.
If the new impeller won't hold a prime you will have to get a new pump or work on the old one.

TRUE GRIT

[Ajax]

Thanks guys. This certainly seems possible. I will try to measure the old and new impeller and the cavity this week.

Question for Neil: when attempting to re-face the pump, what grit of sandpaper did you use? Did you wrap it around a block and just go to town on the face? How did you know when you went far enough?

The only thing that causes me any doubt about this diagnosis is that I seemed to have some "crush" on the new impeller when installing the back plate. Still, it's worth investigating. I want to be cautious about just pulling the trigger on a new pump because when it works, this pump moves water very well.

[ndutton]

I used a stationary sander with a worn 60 grit 6" x 48" belt. Patience is a virtue with this project. Use a very light touch and an orbital motion , stop frequently and measure the cavity depth to compare to the new impeller** (you certainly don't want to overdo it). Use the depth gauge of dial calipers out the long end. One other thing to watch for is the exterior of the casting at the in/out ports. I took mine down to where the sander just kissed the casting exterior.

The key is holding the pump body plumb, sanded face perpendicular to the shaft axis and even pressure all around. The silver lining is there's no real risk. If your pump is at a point where this repair attempt is indicated it's not useful as it is anyway so if you mess this up entirely you're no worse off than when you started. You've got nothing to lose.

The power sander I used may be a little aggressive for some.


**Oddly, the Oberdorfer spec sheet does not provide a measurement for cavity depth so you'll have to come up with your own and in doing so you'll get an indication if this procedure is warranted in the first place. Measure the depth of the new impeller and compare to the depth of the cavity. You ultimately want the cavity only one or two thousandths less than the impeller.

[Ajax]

Ok, it's time to come clean-

This Oberdorfer is mounted on a Universal M30 diesel. The pump is mounted on the front of the engine, out in plain sight. Based on what you're saying, a light touch with a sanding block that touches the entire face at once, would be safe and maybe even one of those old "buzzing" palm sanders (not an orbital sander).

I wouldn't even have to remove the pump. The engine can hold it plumb for me.

[sastanley]

Ok, it's time to come clean-

Ok, you've only come half clean..

But, I'll provide my $0.02 while I am here...It will be easy to round off the edges of the pump housing's face with a 'buzzy' sander while the engine is holding it, so be careful. You are sanding metal, but not grinding down steel or your lawnmower blade...this metal is pretty soft in comparison.

[Clucas]

I had a priming issue several years back after changing out an impeller -- I tried manually priming with no success. The issue was rectified when I changed out the shoe inside the pump which had worn thin over time. The new shoe made all the difference in the world.

[Ajax]

Ok, you've only come half clean..

But, I'll provide my $0.02 while I am here...It will be easy to round off the edges of the pump housing's face with a 'buzzy' sander while the engine is holding it, so be careful. You are sanding metal, but not grinding down steel or your lawnmower blade...this metal is pretty soft in comparison.

Don't hate me. I'll still be here buying parts.

Ok, I'll try sanding by hand, with a block first, *after* I take more measurements and make sure I want to go down this path.

[ndutton]

Ajax, I won't tell you not to do it the way you want, it's up to you

BUT

the advantage of using a sander like I described is the belt rides on a flat metal surface, not a soft pad like every hand sander I've ever seen. I believe you'll have a much better chance at a flat result when there's no 'give' to the abrasive. I think that's pretty much what Shawn said. Please re-read Don's directive in post #5, para 3, add 'hard' in front of 'flat surface'.

[tenders]

Personally I'd try another impeller before I abraded a pump known to be working. And maybe even a replacement shoe.