An important part of the annual maintenance list is inspection of the water jacket anodes in your engine. Using salt water as the primary coolant for your Yanmar 2GM or 2GM20 engine simplifies the design tremendously but it does mean that corrosion is a constant threat. Your only defense against your engine corroding away from the inside are the two anodes which are located inside the water jacket of your engine. Regular checking and replacement of these parts is essential maintenance of your Yanmar GM2 series engine.
Anode Number 1
The first anode plug is located on the back of the engine on a removable plate. Do not remove the plate unless you can absolutely not shift the plug as there is a gasket beneath it. The anode is located in a plug which can be difficult to remove if it has been in place for a few seasons or more. This best tool for the job is a sturdy 24mm ring spanner. If it still resists movement, some have reported setting up a car jack beneath the ring spanner. Then, when the spanner is under pressure, tap the socket with a small hammer. When it’s out, inspect the zinc. If it’s any more than 25% corroded, replace it. You don’t want parts of this breaking off and blocking cooling channels in the engine. If you are removing the zinc (Part# 27210-200300) clean the plug and the threads with a wire brush. This should not be greased prior to replacement as full electrical connectivity is vital to the operation of the anode. If they are removed and inspected each year, you shouldn’t have any further problems with them sticking.
Anode Number 2
The second anode is on the Starboard side of the engine and located just beneath the exhaust mixing elbow. The plug is marked “ZINC” for easy identification. Even more than Anode#1, this can be an awkward location to get a good grip. Again, the ring spanner is usually your best approach.
Remember – Dry Fit
Finally, do not be tempted to apply any copper grease or such to the threads as that may insulate the anodes from the engine. These are absolutely dry fit.
TECHNICAL DETAIL: Yanmar Anode Part Number – 27210-200300