Draining and refilling Lower Unit???

Hi fellas,I just bought a merc 200 ('73) and it has a slight leak at the prop shaft seal and I don't know if I can get parts for it anylonger????Anyway what is the best method for adding lube to this lower unit? It has a top and bottom plug.I was told to put the gear oil in the bottom hole until it runs out the to hole?HOW could this be????? if you remove the fill fitting when topped off,then what is to keep the lube from running back out! while you are fumbling and trying to get the plug back in.......Seems to me you drain with the bottom plug out and fill thru the top plug until it runs over?????????any info would be greatly appreciated! and what is the best lube to use in this gearcase and where to get it?????? thanks in advance!

Re: Draining and refilling Lower Unit???

Buy a small pump at walmart or a marine store and pump the gear oil in from the bottom until it runs out the vent hole. This eliminates any air pockets. Replace the bolt in the vent hole, making sure to put on a new seal/washer. With the vent hole sealed, a vacuum is formed in the lower unit and you can remove the pump fitting from the bottom hole and (quickly) install the bolt in the bottom. The vacuum will hold the oil in and you probably will only lose a few drops. Use standard hypoid gear oil, 90wt available at walmart or marine store. The hypoid oil has additives in it to emulsify any water that gets in.

Re: Draining and refilling Lower Unit???

If you have any water in the unit, it will be on top of the oil and will be discharged if you follow the above instructions. You need a little bit of room for expansion.
If you don't know how to change the oil in the lower unit, you definitely do not want to attempt to change the seal yourself.

Re: Draining and refilling Lower Unit???

Water is heavier than oil, water will eventually separate to the bottom not float to the top. always fill from the bottom port, in fact you couldn't fill from the top if you tried because there would be nowhere for air to vent and allow the oil to go in.

Re: Draining and refilling Lower Unit???

gss036 said:

If you have any water in the unit, it will be on top of the oil and will be discharged if you follow the above instructions. You need a little bit of room for expansion.
If you don't know how to change the oil in the lower unit, you definitely do not want to attempt to change the seal yourself.

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what????