1972 Mercury Thunderbolt 500 Help Needed

[revedup04]

I'm having a hard time figuring out why I have gear oil leaking out of a small hole located next to the vent screw on the lower unit. Can anyone tell me what this hole if for? Does this mean I have a bad seal in the lower unit? And if so a few steps on how to fix this problem. All help is appreciated.

 

[carholme]

Re: 1972 Mercury Thunderbolt 500 Help Needed

That hole is a drain hole to allow water to drain and prevent possible rupture of the case from freezing. The fact that you are seeing oil there indicates oil leakage through the shift shaft seal.

You haven't provided your engine s/n but if you are sure it is a 1972, here is the parts list:

MERC 500 - Mercury, Mercruiser, Force, etc. Parts Lookup

For the shift shaft seal, see item 40 GEAR HOUSING ASSEMBLY, COMPLETE (PAGE 1) which contains the seal, item 41.

You can verify this by dropping the lower unit and looking for evidence of oil.

It may also be the seals, items 16/17 under the base of the water pump but dropping the LU will help to narrow it down.

Gerry

 

[revedup04]

Re: 1972 Mercury Thunderbolt 500 Help Needed

Well I finally has time to get out there and tear it apart. The good news is I replaced the bushing and did a complete water pump kit with the housing. I got it all back together and ran it for like 5-10 min. No oil leaking out of that hole any more. Bad news is after I thought everything was running good for 10 min. I now have gear oil pumping out just below the power head on the outside of the motor. I'm guessing something went wrong when installing the water pump. Can anyone confirm this? Or and some input. Thanks

 

[Aqualift]

Re: 1972 Mercury Thunderbolt 500 Help Needed

That wouldn't be gear oil up there. More than likely, spent exhaust oil residue. Possible bad powerhead to exhaust extension plate gasket.

 

[revedup04]

Re: 1972 Mercury Thunderbolt 500 Help Needed

No, it's gear oil for sure. Because the lower unit keeps filling up with water. I took it apart and found the oring on the bottom of the water pump housing had split. Got a new one tried it again and the same thing happens. Any ideas?

 

[carholme]

Re: 1972 Mercury Thunderbolt 500 Help Needed

If it is gear oil, it is escaping from the gearbox through the shift shaft seal or the seals at the base of the waterpump. Did you change the seals and if so, are they in the correct way?

Gerry

 

[revedup04]

Re: 1972 Mercury Thunderbolt 500 Help Needed

I'm pretty sure it's coming from the water pump. Yes I did replace all seals and thought I put them in the correct way. Do you have a diagram? The one I have isn't very clear. Thanks

 

[carholme]

Re: 1972 Mercury Thunderbolt 500 Help Needed

I don't have a diagram handy but the upper seal is installed with the lip facing up and the lower, with the lip facing down. The upper keeps the water out of the gearbox and the lower keeps the oil in the gearbox.

Gerry