Lower Unit Help For A 74 Merc 850 Outboard

Hey guys and gals,


I'm a first time poster, but long time Merc fan. I have a 1974 Merc 850 with thunderbolt ignition. I am the 2nd owner of the motor. Its on a 1980 Venture bass boat. During the last outing of the season last year, upon tying down the transom straps on the boat, I noticed there was milky-white lower unit oil
seeping out of the midsection ( looks like a weep-hole ) of the engine. So I drained out the fluid and figured I'd wait till spring to deal with it. Well, its now spring and I'm dealing, so I pulled off the lower unit. Pulled off the water pump upper housing, impeller, and lower impeller housing. Looks like that weep-hole that I saw fluid coming out of, was exactly that. A weep-hole. I'm pretty sure that the lower housing also has a shaft seal on the bottom end that obviously keeps water from getting into the lower unit bearings. I believe that's where it was leaking was that lower housing seal. the rubber looks questionable as well as the gasket. My question is this. Is there any other seals or special things I should be aware of when I go to replace this housing?? Looks like pretty much a cut and dry repair: Scrape the surface good, clean off the area, ETC. Install New lower and upper housing and slap it back in. Do I need to replace any other seals?? Is there somebody on the forums that's done this? Am I missing anything??

Thank you for reading. My apologies for such a long post.

Mike S
Schererville, IN

Re: Lower Unit Help For A 74 Merc 850 Outboard

Ttt

Re: Lower Unit Help For A 74 Merc 850 Outboard

I would pressure test the LU to see where the leak is actually coming from.

Many times it is the lack of gaskets in the fill and drain vents, (be sure to replace those when ya change the gear oil), they can be purchased here on iboats.

FYI: you should always drain the unit and refill with fresh marine geal oil before the cold weather as the water inside has been known to crack the LU case, (unless ya keep it inside; or it doesn't freeze where you live).

BTW I have a real nice 1978 800 LU 20" shaft, (with the speedo pick up in the unit), if ya hapen ta need one.

PM me if ya have an interest. JR

Re: Lower Unit Help For A 74 Merc 850 Outboard

THANK YOU SIR. MUCH APPRECIATED!! I had planned on replacing both vent screw gaskets. Their still in good shape, but wanted to eliminate doubt when putting it back into service. What about seals on the prop-shaft?? is there any that should be replaced while i've got the prop off??

Re: Lower Unit Help For A 74 Merc 850 Outboard

Mike, I would replace the waterpump base unit. The base has a seal for the driveshaft, which can easily be the problem you are having. Clean up the driveshaft, where the seal rides, so the seal will last. You will want to install a new pump base gasket and oring, as well as gaskets on the stainless steel wearplate, and a new impeller.

The seal around the propshaft is much more difficult to replace. the good news is that the last a long time, unless fishing line gets behind the prop. Make sure the motor and MerControl are in neutral when you reassemble. also make sure the shift rod guide (black plastic piece) is in the midsection when you reassemble. It takes a bit of effort to line up the driveshaft and shift shaft splines. Grease 'em before reassembly.

Re: Lower Unit Help For A 74 Merc 850 Outboard

i cant get my lower unit back on ?!!!!!!!!!! its a 1975 merc 850 any tips?
goes in until about an inch from the engine


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