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Posts: 241
| OK - my 40 hp Mercury has two ports on the top.
One is stamped "VENT", the other has no label.
Soooo....what's it for? I fill the lower unit from the bottom drain port, until it starts to overflow from the "VENT".
What is the purpose of that port?
Backdraft
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| You just used it exactly for what it's there for. If there was no vent port, you wouldn't be able to fill the lower unit with lube, because there'd be nowhere for the air inside to go. Fill until lube starts coming out the vent port, then plug the two ports and you're golden.
Edited by Chemi 4/12/2015 6:30 PM
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Posts: 5874
| Fill from the bottom. When it comes out the top, plug that one, and then remove the bottle, and plug the bottom one. |
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Posts: 73
| There's 2 at the top and 1at the bottom.....I'm picking up what your laying down, but don't have a positive answer. Extra venting? A backup in case one strips? Not sure. |
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| Hmmm... some of the pictures in my service manual show it (most don't). No idea!
Edited by Chemi 4/13/2015 10:12 PM
Attachments
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 Bottomend.jpg (49KB - 395 downloads)
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Location: Altoona IA | I've only seen it on a few models. One reason is it's in case you can not get the motor in a perfectly vertical position... Re Merc- With the outboard in the vertical position, add lubricant until it appears in the front vent hole and install the plug with sealing washer. Continue adding lubricant until it appears in the rear vent hole. Stop adding lube and install the rear vent plug and sealing washer before removing the fill tube. Remove tube and replace the fill plug and sealing washer. I have not had a 40 gearcase apart but I have seen where if you fill certain LU's too fast, oil will come out of the vent before actually reaching proper level due to casting cavities.
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