Mercury - Replacement O-Ring V12 Verado / SeaPro Series Rubber Seal 8M0177478. Overview. Any boater who's spent real time on the water knows that the smallest parts often carry the biggest responsibilities. An O-ring doesn't look impressive, but when it's sealing a critical system on a Mercury V12, you feel its importance every time you push the throttles forward. That quiet confidence - the kind that lets you run offshore without second-guessing what's happening under the cowling - starts with