Furuno TS-331A - Manual (Page 54)

6. CATCH SENSOR OPERATION
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8. Use your finger to remove the O-ring from the battery cap.
O-ring
Battery cap, inside view
9. Coat the O-ring seat and the indented area below it with silicone grease.
10. Coat the new O-ring with silicone grease and set it in its seat.
11. Reverse steps 1-4 to reassemble the unit.
Note:
Do not overtighten the main housing and battery cap or the trigger bolt.