Why push the gear oil up?

[Carolina Stevie Ray]

I'm doing some routine maintenance on my Tohatsu 70 and see the procedure in the owners manual for changing gear oil. Why is it filled from the bottom up? Why not top down? I have the right tool, but just curious. Thanks

 

[Rocky_Road]

Re: Why push the gear oil up?

Avoids having an air pocket...air escapes out the vent hole, as the lube is introduced from the bottom!

 

[Carolina Stevie Ray]

Re: Why push the gear oil up?

thanks for the explanation - that makes good sense

 

[marquette]

Re: Why push the gear oil up?

i have always wondered the same thing. i remember my dad changing LU lube 50 yrs ago and it was the same system we use today on a outboard. it seems that in years of outboard engineering we could have come up with a more convient method. i don't understand the logic of filling from the bottom to prevent an air pocket. the gears and shaft are running in an oil bath for lubrication. the top of the gear case is empty. much like the rear end in a car. we don't fill them from the bottom. we don't fill a 4x4 transfer case from the bottom. we drain them from the bottom and fill from the top. i can't beleive on a assembly line that LU's are filled from the bottom. i imagine they use a fill tube 2/3 the diameter of the fill hole to let air escape that automatically dispenses a preset amount of gear lube.

 

[gss036]

Re: Why push the gear oil up?

Have ever tried to fill a lower unit from the top? Try it sometimes and you wil see why one should do it from the bottom.

 

[Mas]

Re: Why push the gear oil up?

If you fill it from the top, you'll just create pressure inside the LU.

MAS

 

[marquette]

Re: Why push the gear oil up?

why would you create pressure if the fill tube is smaller than the fill opening?

 

[boat1010]

Re: Why push the gear oil up?

The gear oil is so thick it creates air pockets and will not let the air escape. You CAN fill from from the top. BUT ya better have a good day to sit there and let it run in. By filling it from the bottom it is forced in and insures that there are no air pockets. If it is filled from the top and you "think" it is full the first time it is ran the pocket goes away and your lower unit is only half full.

 

[Mas]

Re: Why push the gear oil up?

Have ever tried to fill a lower unit from the top? Try it sometimes and you wil see why one should do it from the bottom.

No need to debate, as GS said, "try it!"

MAS