Cam and Set Screw Gone Pink - Jabsco Raw Water Pump

[rardiH36]

Raw water circulation for my old Yanmar is done by a belt driven small Jabsco pump. Model 2760-0003 to be exact.

I replaced my original pump with a new one 2.5 years ago. Only 150 running hours since. Last week, I noticed the pump leaking. Removing it for inspection, I first checked the bronze (or maybe it is brass) set screw for the pump's cam piece. With hardly any twist at all on it, the set screw simply sheared off. Upon taking off the pump cover plate and removing the impeller, I discovered the set screw had turned pink and brittle. Fortunately the broken segment still inside the cam extracted easily when I drilled through the center with a 1/16" bit. The cam itself is still good yellow metal.

Then I decided to take a look at the cam from my old pump which I had kept around. It is very pink looking. And actually I broke piece off of it with just a light poke at it with a screwdriver.

Is this a common occurance with these pumps?

For now, I have replaced the corroded pink set screw with an SS screw.

Will I be setting up even faster electrolysis due to disimilar metals?

Thanks for any insights.

 

[Charles Erwin]

Good choice actually. The screws/cam you see failing have a high zinc content in their recipe hence the decay. The new screw is probably 304 thus the more noble. But no matter because the part of the screw exposed to seawater (just the tip) is very small compared to the pump body. Don't forget to put some sealant (like permatex # 2 ) on the screw threads . You can also paint the end of the screw with nail polish or similar if you want.

Charles

 

[rardiH36]

Charles:

Many thanks. Yes, I did use Permatex sealant on the SS screw threads and around the head. Plus a good dab on the cam itself so that the compound might be forced up into the pump body. And also maybe the sealant will provide some electrical isolation between the cam and the pump body. I while had the pump apart, I replaced the water-side lip seal and the O-ring as well. Hope I got the leak problem fixed! I will be remounting it this afternoon.

But still its worrying about the de-zincing after such a short period.

 

[Charles Erwin]

Do not despair - this is an opportunity. You have a fresh assembly and so can observe how this new combination performs. I am betting there will not be any decay but let us know what happens.

Charles

p.s. Verify no sealant can migrate into the contact surfaces - impeller to pump body.

 

[jigray3]

Jumping in and old thread, but I have almost the identical problem,. Mine is a Volvo Penta 2002 seawater pump, made by Johnson. In my case the screw is also brass, the washer for the screw is copper, and the cam and body of the pump is bronze. If it were as simple as using a stainless screw (the pump shaft is stainless), I think that's what they'd be using. I'm left to believe the brass screw and copper washer serve a purpose, though it's clear it's meant to be inspected and replaced periodically. On my pump the screw is tiny, unique, and NLA from Volvo... of course.

There are no threads for the screw in the pump body, the screw insets through the body and protrudes inside the pump where it engages the threads in the cam. So the screw clamps the cam into place. That tiny 7mm screw does a big job. If it fails and lets go the cam goes bashing around the inside of the pump body as the impeller spins. My pump started leaking from the screw, but because I I believed it was the lip seal behind the impeller that had failed, I had to remove the cam to gain access and I was fortunate to catch the actual problem before it failed. End of the screw and threads were pink and fit was very loose in the cam with just a few threads still there. Now I will keep spares and check it when I replace the impeller.

When I removed the screw from the pump, I found some sort of sealant had been used at the screw head and washer. This was an OEM replacement pump, not rebuilt or aftermarket. I want to replace the sealant, but it's an odd application. It's under pressure, in contact with 3 dissimilar active metals, salt water, potentially even oil and diesel, exposed to heat, and acting as a gasket/sealant. It's not really an adhesive or threadlocker, because I want to be able to remove and inspect the soft brass screw without damage once in a while, and because it doesn't thread into the body. I talked to both Permatex and Loctite, and each came up with some pretty far flung industrial recommendations, neither company liked any type of silicone RTV for the job.

Took a while, but I managed to source the screws, washers, and cam and want to find an appropriate sealant for the job. Probably overthinking it, but what I've learned about plumbing I've learned the hard way. So, I'm still looking.

 

[jigray3]

Fixed my pump this weekend, and I may have figured out why the screw that secures the cam is brass. The cam I removed showed wear and was noticeably thinner that the new cam I replaced it with. A brass screw will wear along with the bronze cam so the screw does not protrude above the cam surface. Stainless being harder might not do that, and a stainless screw would ultimately start to protrude and tear the impeller blades up as the cam wore down. That's guess anyway. Still don't like that the cam is held in place by a single, very small and seemingly fragile screw. I think I managed just 2 rotations of the screw in the cam.