1990 Johnson Evinrude "ES" 60 thru 70 Service Manual, P/N 507873, Page 36Get this manual
AnodesTesting and Replacement Erosion or disintegration in salt or brackish water indicates the anodes are performing their functionThe anodes should be inspected and or replaced at intervals or corrosion of the engine will increaseINotellf any anode has been reduced to two-thirds its original size (one-third eroded), it must be replaced! To increase the duration of the anodic protection, the starboard stern bracket (on some models) is drilled to accept an additional anode
171 f81 Test for proper installation of the anode