Pencil Zinc location for Yanmar 2GM
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a 2GM raw water cooled motor, will have zincs in engine.
My 3GM raw water cooled motor has 2 zincs on starboard (fuel
filter/dip stick) side and 1 in back.
The 2GM raw water should have 2 zincs, 1 in the block on the dip stick side, just behind/above it . A second is on the rear side of the head. Check manual, should be available free on line.
...(1 could be hiding behind fuel filter like my 3GM)
My 3GM raw water cooled motor has 2 zincs on starboard (fuel
filter/dip stick) side and 1 in back.
The 2GM raw water should have 2 zincs, 1 in the block on the dip stick side, just behind/above it . A second is on the rear side of the head. Check manual, should be available free on line.
...(1 could be hiding behind fuel filter like my 3GM)
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http://j30.us/files/Yanmar-2GM20-Parts-List.pdf
Figure 1, part 18 is the zinc. Part 16 is the plug that holds it; it has a fairly large hex head on it and screws directly into the stb side of the block (in a standard installation).
Figure 11, part 24 is the zinc. Part 22 is the plug; it screws into a cast iron plate (part 19) that attaches to the back of the head.
Don't be surprised if you need penetrating oil, a cheater bar, and some choice language to get the plugs out. You can get the zincs at West Marine, or on-line from a number of sources.
Figure 1, part 18 is the zinc. Part 16 is the plug that holds it; it has a fairly large hex head on it and screws directly into the stb side of the block (in a standard installation).
Figure 11, part 24 is the zinc. Part 22 is the plug; it screws into a cast iron plate (part 19) that attaches to the back of the head.
Don't be surprised if you need penetrating oil, a cheater bar, and some choice language to get the plugs out. You can get the zincs at West Marine, or on-line from a number of sources.
After lots of reading in this any other places, I agree that the Yanmar 2GM20F (fresh water cooled- has antifreeze canister) indeed has NO zincs in it - due to the unique corrosion resistant alloy used in the heat exchanger (the only part of the engine to contact sea water). The 2GM20 (entirely sea(raw) water cooled) does have the pencil zincs since the sea water contacts various parts of the engine block during its cooling cycle.
I ran across this info during my research. . . There is a lot of controversy surrounding the proper use of zincs and electro galvanic corrosion in general and I encourage everyone to be aware that there is a huge difference in the high quality marine Bronze (which is copper+Tin alloy) through hulls and fittings and some BRASS (which is copper + Zinc alloy) thru hulls and fittings/ adapters being sold. Attaching a brass fitting to even an excellent bronze thru hull or valve creates a high potential for corrosion and catastrophic failure even if you electrically bond everything together. The Zinc in the alloy acts just like a zinc should !! Just sayin' -- know what is on or being put on your boat!
yes - one of the anodes is hidden behind the fuel filter - a really dumb location because you will probably have to take the fuel system apart to get proper access with a ratchet wrench with a 25mm (or very close imperial equivalent) socket - dont try anything else 
- it takes a lot of torque to get these puppies out and I see several posts where people have "rounded" the anode holder hex head (presumably using a monkey or pipe wrench) and now cant get them out.........
- it takes a lot of torque to get these puppies out and I see several posts where people have "rounded" the anode holder hex head (presumably using a monkey or pipe wrench) and now cant get them out.........
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I have been going nuts trying to locate the Zincs on my 2GM20F, your post is the first I have found that says there are none. But since the manual simply says they would be behind a cover labeled with a sticker that says "Anticorrosion Zincs" and no such sticker is on my 1988 engine, I'd say you are correct.
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Post #3 above is correct for locating the zincs in a raw water cooled 2GM.
As mentioned, they can be a mongrel to remove. The one on the side near the dipstick is a bit tricky because the socket wrench that you will need may foul the high pressure fuel line. By grinding metal away from the cheek of the socket you should be able to wriggle it in and use a short extension to a very long bar that you will probably need to get enough leverage.
I have found that running in salt water but giving the cooling system an occasional flush out with fresh water, I get a good 18 months before the zincs are down to about 50% . But what you get will depend much on your own installation, as no two boats are the same in the way they are wired up and how well metal components are isolated. I would certainly recommend that you check zincs after the first year in salt water until you know for sure. Don 't be alarmed if they are almost gone - that shows that they are doing their job.
If your engine has a closed cooling system (fresh water cooling), while the engine itself won't have zincs, it is possible that the heat exchanger will have at least one in the salt water jacket (and maybe not very obvious). Same advice - check it after a year. Also consider setting up a method to flush the heat exchanger itself.
As mentioned, they can be a mongrel to remove. The one on the side near the dipstick is a bit tricky because the socket wrench that you will need may foul the high pressure fuel line. By grinding metal away from the cheek of the socket you should be able to wriggle it in and use a short extension to a very long bar that you will probably need to get enough leverage.
I have found that running in salt water but giving the cooling system an occasional flush out with fresh water, I get a good 18 months before the zincs are down to about 50% . But what you get will depend much on your own installation, as no two boats are the same in the way they are wired up and how well metal components are isolated. I would certainly recommend that you check zincs after the first year in salt water until you know for sure. Don 't be alarmed if they are almost gone - that shows that they are doing their job.
If your engine has a closed cooling system (fresh water cooling), while the engine itself won't have zincs, it is possible that the heat exchanger will have at least one in the salt water jacket (and maybe not very obvious). Same advice - check it after a year. Also consider setting up a method to flush the heat exchanger itself.
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The problem here is the original poster simply said a 2GM. This engine has come in so many variations over the years. While Post number three indeed answered the original question for the engine specified, the conversation has drifted.
The 2GM raw water cooled (sea or fresh water pumped in from the body of water the boat is in) does have two zincs. The 2GM20F, fresh water cooled (as in antifreeze and water that is changed annually) does not have any zincs on the engine nor the heat exchanger(as confirmed by the dealer when I returned the zincs I had bought without fully identifying my engine).
If you add a Sen-Dure heat exchanger, an after market addition made for many different engines, it may or may not have a zinc since they make both copper (zinc required) and Cupronickel (no zinc required) models.
The Yanmar 2GM20F heat exchanger is made of cupronickel. According to Wikipedia, "Cupronickel or copper-nickel is an alloy of copper that contains nickel and strengthening elements, such as iron and manganese. Cupronickel is highly resistant to corrosion in seawater, because its electrode potential is adjusted to be neutral with regard to seawater. Because of this, it is used for piping, heat exchangers and condensers in seawater systems..."
Hope this helps those that were as confused as I was.
The 2GM raw water cooled (sea or fresh water pumped in from the body of water the boat is in) does have two zincs. The 2GM20F, fresh water cooled (as in antifreeze and water that is changed annually) does not have any zincs on the engine nor the heat exchanger(as confirmed by the dealer when I returned the zincs I had bought without fully identifying my engine).
If you add a Sen-Dure heat exchanger, an after market addition made for many different engines, it may or may not have a zinc since they make both copper (zinc required) and Cupronickel (no zinc required) models.
The Yanmar 2GM20F heat exchanger is made of cupronickel. According to Wikipedia, "Cupronickel or copper-nickel is an alloy of copper that contains nickel and strengthening elements, such as iron and manganese. Cupronickel is highly resistant to corrosion in seawater, because its electrode potential is adjusted to be neutral with regard to seawater. Because of this, it is used for piping, heat exchangers and condensers in seawater systems..."
Hope this helps those that were as confused as I was.
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