I totally agree with everything that Ray said above but can add some additional tips/suggestions:
Be very careful when you reinstall the rubber end cap on the raw water Hx. As Ray said, the divider plate needs to be perfectly aligned with the groove in the rubber cap. In my opinion, it's too difficult to do with the Hx installed on the engine so I only do it with the Hx removed for cleaning. This may be my incompetence, but be very careful if you try. If you don't get it right, the engine will overheat.
I have have 2 Hxs that I swap about every 4 years. One is the older version with the rubber end cap. The second is a newer version with a gasketed, easily removed/reinstalled end cap - a much better design. However, a standard length zinc is too long and needs to be shortened by about 1/2" or you'll damage the divider plate for the newer version.
The raw water Hx zinc has a very short life expectancy. I need to replace mine twice a season.
You shouldn't use any sealant when installing the zincs. If tightened sufficiently, they won't leak and the sealant could hinder the electrical contact.
I think the raw water Hx zinc is 3/8" but I just measured the OD of its pipe threads with a caliper. It's 1/2". The lube oil cooler is the next larger size.
I access the lube oil cooler zinc from the stbd side. Use a little caution here. It seems, on my boat, that the zinc swells (i.e., gets pasty) sufficiently that at times, if I'm not careful, it gets stuck and breaks off when I try to remove it. If that happens, I remove the HX and try to remove it while out of the boat. Typically, the remaining part will fall into the HX, which makes it very hard to remove....but you need to get it out!!!
Hope this is helpful.
_________________ Jim Voelxen Odyssey #191 Home Port: Osterville, MA
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