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O-ring goes in the box up in the groove, you do not need to add any sealant if the surface on the block is clean and smooth.

Hard learned lesson, make sure the metal sleeves that your 13mm Bolts go through are flush with the bottom side of the box. Had a breather box that the sleeves stuck down past the plastic far enough that it prevented enough squeeze on the o-ring.

A word of warning, there is a tube that goes down into the oil pan on the left hand side of the breather box. You do not want to have that push down into the oil pan. Getting back out involves pulling the oil pan. To pull the oil pan involves a lot more work. So be gentle, line it up before pushing down. It should set down flush before you install and tighten the bolts.

Bet if you clean the old box and then look closely at the old breather box groove you find a flat black o-ring that looks like it is part of the box itself. Thought the same thing when removing my old breather box, no o-ring. It was there after all, just very hard to see until after cleaning the box.

Good Luck and hope this helps,

Paul