Why does lower have to be filled from bottom?

Just curious as to why you can't fill the lower unit on my Mariner 25 from the upper hole. That's the way a car differential would be filled. Looks like it would be the same difference.

By all means you can. However if you do not take your time you will end up with air pockets in the lower unit, when this occurs and the air bubbles rise to the surface, overtime you'll end up with a low oil condition. If you do it an ounce at a time, you should be okay. Let it sit for up to 15 minutes counts. Filling from the bottom fills all orifices and pushes out all air creating a 100% filled lower unit

or just go buy a six dollar lower unit oil pump and be done with it. It takes about two minutes with this pump.

I used one of the squeeze tubes of quicksilver oil, and one wasn't enough, and is why I asked.

I see. Then I would say grab another tube and fill from the top. Just take it slow, and if possible allow the motor to sit overnight with the top plug out. Go back the following day and check oil level. If good, close her up and rip. If it's low due to air burping out, fill to top hole and then let her rip.

I found that if you take a 6" piece of fish aquarium air hose,
or, drip irrigation water hose, heat it a little, it will slide
right onto the pointed bottle cap. Put the hose on first so you
will know how much to cut off the tip. Then, you can easily introduce
the oil into the top hole. A small diameter hose allows air
to escape without bubbling the oil out.
And, you can cut the bottle in half and use it like a funnel.
and like said above, let it sit for awhile, rotate the prop, tap it with a
wood stick, rubber mallet, etc. and check the level again.
It took a real engineer years to perfect this dumb bottom fill idea
then even longer to convince every outboard builder to accept it.

or you can fill it from the bottom like your supposed to. :LOL2:

I'm afraid that when I remove the plug to put in the second tube, it will drain out.

LOL not to worry.
an ENGINEER designed the two plug system so that when changing the oil,
it all falls into the golden plan of marine motor maintenance.
get your new tube of lube at the ready, loosen the top plug but don't remove it just yet.
then, with one hand, remove the bottom plug and drop it on a clean cloth or newspaper.
then quickly insert the tube of lube. then, with your other hand, remove
the top plug. with two hands, squeeze the tube of lube and try to fill it until
it starts to run out of the top plug, then, with your spare third hand, quickly
put in the top plug that creates a vacuum so the oil will not run out the bottom hole.
then, remove the tube of lube and with another hand, quickly put in the bottom plug.
tighten everything up and you are good to go.

go to YOUTUBE and you will see several methods of how to change your lower unit oil.
pick one or two that might suit your skill set.

but on the serious side, the lube is so thick that very little will be wasted.
once you do it a couple of times, you loose less than a teaspoon full.
Since you are having a little problem with this - - - remove the top plug and practice
putting it in quickly and turning with your fingers to get a quick grab on the threads.
I think this where most folks get frustrated. Just can't get it threaded quickly enough.
Once you understand that part - the rest should fall into place rather cleanly.
good luck.

safetyfast said:

I'm afraid that when I remove the plug to put in the second tube, it will drain out.

Click to expand...

Just have both tubes ready to go, when switching to 2nd tube put your finger over the top hole. This will create a vacuum and not much if any oil will drain out. Fill it from the bottom to make sure no air is left in.

I just use one of these:



No more aggravation with those POS tubes, and the spillage is minimized.

Thanks all. I've worked on cars all my life, but outboards are a new adventure.

See this trick here for LU oil changes ...

https://forum.tinboats.net/viewtopic.php?f=4&t=38284

I am so confused. Why is this an issue? I've changed my LU oil twice...

Just use the $6 pump from walmart.

Fill from the bottom until it comes out the top. Reinstall the top cap.

Remove pump from bottom. Vacuum keeps oil in. Reinstall bottom cap.

What is so hard about this?

LOL LOL LOL this falls into the same category of
25 posts on how to put the plug in the drain hole.

Bowhunter1661 said:

...or just go buy a six dollar lower unit oil pump and be done with it. It takes about two minutes with this pump.

Click to expand...

^^^The task is designed around having the proper tooling, in this case a cheapo pump, otherwise you get into the needing 6 hands scenario someone described earlier.

The gear oil in a tube is how much per unit of oil vs. the oil in a bottle? Figure how many times you'll change the oil, how much are you really saving from not buying a cheapo little tool and how much aggravation and risk of under filling it is there by doing it wrong to save a measly $6???

With the pump you fill until it runs out the top and then your hands are free to plug the top and remove the pump...

Got the pump. One pump was enough to fill it on top of the tube I had already put in. Peace of mind worth it. Now I'm ready for the next service as well.

Finally made it to two pages.... can it make a third????