There is only one. It's on the heat exchanger. [url]www.marinedieseldirect.com[/url] has pictures of your engine showing where if you don't have one.

You don't say when you bought your boat so its unclear how long the zincs have gone without being replaced, but if its more than a year or two, I'd suggest checking the internals of the heat exchanger to make sure it hasn't corroded

Bill, I suggest that you remove the HX and carefully check it out, rather than just trying to remove the end caps. I've found that the HX rubs against the bracket (if you have hose clamps holding it on), so I've installed a rubber bicycle tube between the bracket and the HX to avoid rubbing. You won't know if you have that issue unless you take it off. You should also inspect the inlet port from the raw water pump where salt deposits can clog up that port. You'll also need to "Burp" the engine to avoid overheating when you get it all back together. [url]http://c34.org/bbs/index.php/topic,4518.msg26462.html#msg26462[/url]