1991 OMC Cobra 4.3l Lower unit issue

[kylep123]

I recently had hit a few rocks on my lower unit and broke some of the blades off of my aluminum prop, with no other damage to the lower unit but a few scrapes on the skeg. On the way back to the landing you could feel a vibration in the boat so we went slow, after arriving it was clear the it was vibrating due to the broken prop. Upon replacing the prop I noticed there is some oil in the exhaust/prop area. I checked the oil and it all seemed ok so I took the boat for a 10 minute ride just to make sure everything was working ok and it seemed to actually preform better with the new prop. Being concerned about the oil I took the bottom drain plug out and sure enough there is water in the oil. There was little to no shavings on the magnetic plug so I drained the oil out and took the stern drive off. I did take the lower unit off of the upper gear case and am going to order the seal kit (only $50) to replace and inspect everything in there. Just looking at the shaft it seems okay and have read if you have an aluminum prop you usually don't do damage to the shaft...Would the vibration on the way to the landing cause the seals to go out? and is there anything else I should be looking for? any help would be very much appreciated.

 

[jbetzelb]

I have hit rocks twice in the last 20 years with my OMC Cobra and both times the prop shaft seal started leaking. Once on an aluminum prop and once on a steel prop. It didn't bend the shaft either time.

 

[kylep123]

That sounds reassuring, I am going to just get the seals and replace those, I am guessing there is no internal crack that would cause this since there wasn't much damage done other than the prop. When putting the lower unit back on to the upper gear case is there a marine sealant I should use? if so what kind should I buy?

 

[bruceb58]

Before I did anything, and that mainly means taking it apart and splitting the drive, i would do a pressure test and try to confirm that the prop shat seal is leaking.

 

[Idlespeedonly]

There are o rings on the propshaft carrier, and o rings on the 4 bolts that hold the carrier in place. It could be leaking in either of these places also. But I would pressure test it first so you KNOW where the problem areas are. IMO, hitting rocks will not cause the seal to start leaking, unless the shaft is bent. The seal is rubber, and will give alot more than the needle bearing behind it. I would bet it has been leaking for a while. You just noticed it when you removed the prop.
Also, If it is only the propshaft seals leaking, they can be replaced without any disassembly.

 

[kylep123]

Ok that does sound like the way to go but I do have the unit already split apart and do not have access to a pressure/vacuum tester. It obviously is leaking somewhere, would it be wise to just order the whole OMC seal kit for $50 and just replace it all? I know it would be a little work but would possibly be good for it anyway since it is a 23 year old boat. If I do this is there a trick to pulling the bearing carrier and re installing it?