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Engine block de-rusting question

Dec 31, 2020
21
Catalina 30 Gig Harbor
Hi all. Newbie 1987 Catalina 30 MK II owner here, looking for some advice. The Oberdorfer raw water pump on my Universal M-25 diesel had a bad seal and was weeping salt water all down the engine. I’ve taken care of that part (with some key advice from forum folks, many thanks!), replacement pump is in place, all tight, engine purring.

But to finish the job, I still have to deal with the mess caused by all that salt water on the engine block. Basically the underside of the engine is all covered now in a bloom of rust; looks like that weep was discovered a bit late...

I’ve been researching my options here and I wondered what folks think about the approach I’ve penciled out for myself. First, wire brush and sandpaper to get as much of that stuff off as possible, especially any loose flakes. Acetone rubdown to clean up dust and any oil. Then a treatment with a phosphoric-acid-based product I found at the marine store, that reacts chemically with the rust and bonds to the metal as a different substance. Finally, liberal application (and regular reapplication) of the water-displacing rust inhibitor BoeShield.

Thoughts? Gotchas? Alternative approaches? The main challenge, of course, will be doing all this with the engine in place... very difficult to get at everything, especially underneath, with such a tight fit. If I use that phosphoric acid stuff, do I need to protect the bilge fiberglass? Any tricks/tools to help me reach underneath and behind the engine? I really want to get this right the first time... Thanks In advance for any guidance!
 
Jun 8, 2004
2,486
Catalina 320 Dana Point
Brass wire brushes on a small size cordless drill work well, used Dremel with buffing wheels in the semi inaccessible areas, the Dremel brass brushes didn't work as well as the synthetic buffers, but you burn thru a bunch of them.
Oh, and don't forget the shop vac as dust collector when possible.
 
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dmax

.
Jul 29, 2018
343
O'Day 35 Buzzards Bay
I did this to my engine, though the rust wasn't that bad. I raised the engine about 8 - 9 inches to allow access to the bottom and sides. If the leak has been going on for a while, you will most likely find that the bottom is pretty rusted. The only way to get to it is to raise the engine, especially on a Cat 30. This is what I did: dremel tool with a SS wire brush to get the rust off, acetone, rust converter and paint. You could also use primer before painting (some debate whether or not to do so). If you have some badly pitted areas, you can fill them with JB Weld epoxy and then sand that smooth. Once painted, there's no reason to use BoeShield, the paint will protect the metal. The oil pan metal is pretty thick so it takes a while to rust through but people have lost their engine to just that happening. Here's a picture of a typical rusty pan (way worse than mine):

rustyOilPan.jpg
 
Dec 31, 2020
21
Catalina 30 Gig Harbor
Whew... raising the engine - and realigning it to the shaft - is beyond my skill level. And the boatyards are all booked up into the summer. If I were going to do this, would it perhaps be a quicker and cleaner fix simply to replace the pan?
 

dmax

.
Jul 29, 2018
343
O'Day 35 Buzzards Bay
A new pan would be simpler and better, you could also try to locate the drain at the rear of the pan which would be a big improvement - they may be a little hard to find. To remove and replace, I would use a block-and-tackle and raise the engine to a convenient height for access to all those bolts. It is a big job. Try to get a good look at the bottom of your pan - a hand mirror and flashlight will give you a pretty good idea of how bad it is. The one pictured above could probably go on a long time without rusting through (assuming the salt water leak was stopped). If your's is not real bad, you could use rust converter on everything you can reach to slow it down and replace/fix the pan when it's more convenient. The pan pictured above could be restored to like new condition.
 
Jun 2, 2004
3,092
Hunter 23.5 Fort Walton Yacht Club, Florida
I'd make sure you can even remove the pan once it's unbolted. There are lot's of engine bits sticking down inside the pan. If you can't drop the pan down the same distance as the depth of the pan you are likely not going to be able to get it out just buy dropping down the motor is going to have to be unbolted and raised.

Ospho is the best stuff around for treating rust on mild steel and likely to be found cheaper at a hardware store than at a marine chandlery.

 
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Sep 20, 2014
1,252
Rob Legg RL24 Chain O'Lakes
Once done, I'd paint it with cold galvanizing paint. The zinc in the paint is a sacrificial layer that will help prevent rust in the future. I painted my keel with it, and found it held up pretty well.
 
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Jun 2, 2004
3,092
Hunter 23.5 Fort Walton Yacht Club, Florida
I had a thought that if I went to all the trouble to get it off and cleaned up I'd have the outside of the pan powder coated. First thought was chromed but that would be silly, awesome cool but silly.