Mercury 50hp Thunderbolt Lower unit lube problems/questions

[Wasax3]

Early 80's Mercury 50hp Thunderbolt. Can't find a serial. 2cycle, 4 cyl.
After taking my boat out of the water I noticed white grease around the inside the prop. I have owned the motor for a year and have not done any maintenance to it so I figured I'd change the lower gear oil... but where is it? Is there any? I can't find a place to put it in/drain it. I can't seem, to find anything on the internet about it.

 

[fisherguy123]

Re: Mercury 50hp Thunderbolt Lower unit lube problems/questions

My motor is a 1982 merc 50hp.....there is a lower gear oil drain screw near the front of the gear housing just below the round part and a vent above it on the shaft housing. They are both just large common slotted screws. Be sure to relpace the gaskets for a good seal....also the lower screw has a magnetic tip...check for metal filing...a very small amount on it is normal...or so I`m told. It takes about 1/2 litre to fill it. Get a pump attachement for your gear oil bottle and pump up from the bottom hole until it runs out the top one. The motor must be verticle. There are several good videos on U TUBE on changing ouboard gear oil.....a pic is attached of the lower end showing the drain screw at the left lower part.....if the old oil is milky or water runs out you may have bigger problems and have to do more than replace the oil......Good Luck

 

[fisherguy123]

Re: Mercury 50hp Thunderbolt Lower unit lube problems/questions

I just found a better diagram for you.......my screws are on the opposite side but same place.

 

[Sunfish12]

Re: Mercury 50hp Thunderbolt Lower unit lube problems/questions

Change the fluid like fisherguy says then watch that prop area closely for oil expulsion after you run it. Sounds like you have a seal leak in there.

 

[Wasax3]

Re: Mercury 50hp Thunderbolt Lower unit lube problems/questions

I was just blind. Couldn't find the drain plug because it was in the back. I drained the oil that was in there, about 6 ounces of black black oil, and refilled it with about 12-15 ounces of fresh new oil.

 

[fisherguy123]

Re: Mercury 50hp Thunderbolt Lower unit lube problems/questions

If you live in a cold area like I do be sure to do this again before any freezing weather and storing your boat and motor in case any water is in there or it can freeze and expand and break your gearcase.....also you will be set for next season !!!

 

[joed]

Re: Mercury 50hp Thunderbolt Lower unit lube problems/questions

If the oil was black then you didn't have a leak. Water in the oil would have made it look milky.

The stuff you saw from the inside of the prop was likely unburnt oil from the gas oil fuel mix.