Yamaha C90 Outboard Lower Unit Drain Screw Leak

[whaler82]

This summer after changing my lower unit gear oil I noticed a small drip of oil at the bottom drain screw. I drained the lower unit and pumped new gear lube. I installed new Yamaha gasket PN 90430-08020. All was fixed...So I thought. I drained it but this time replaced the the old drain screw and another new washer as well. I made sure I rolled gasket in the oil before installing. Again made sure no old part of gasket was attached to lower unit drain screw hole. Again today it has small leak at the bottom screw. Motor is 1997 C90TLRV Yamaha Outboard. Any thoughts?

[robert graham]

Mine is a 1999 C90 and thus far have not had a leak similar to yours. Can you very carefully inspect the drain hole area for hair line cracks or any casting or machining flaws? Maybe try using double washer/gaskets just to see if it helps?
Other than that, I don't have much idea what may cause your leak....let us know what you come up with on this....

[pstephens46]

Is it a leak or residual oil from the replacement? Is it puddling on ground?

[whaler82]

Is it a leak or residual oil from the replacement? Is it puddling on ground?

No its was wiped clean 24 hours ago.

[zenoahphobic]

Could you be overfilling, and is the engine stored where heat (sun/heater) can warm the lower leg up?
I.e. if the oil is put in cool could expansion pressure be forcing some of the oil out?
My outboard does this (more from the top plug) when we have heatwaves. Only a nuisance, not significant loss of oil.

[pstephens46]

No its was wiped clean 24 hours ago.

Is it dripping on the ground?

[whaler82]

Could you be overfilling, and is the engine stored where heat (sun/heater) can warm the lower leg up?
I.e. if the oil is put in cool could expansion pressure be forcing some of the oil out?
My outboard does this (more from the top plug) when we have heatwaves. Only a nuisance, not significant loss of oil.

Its in the shade. Not over filling -leaking from drain plug.

[cpostis]

You have a fiber gasket Pt # 90430-08020-00, correct?

As others have said check the casting and also check for any pitted or raised area on the housing where the fiber washer gasket sits. Another thing is that this gasket is fiber, it should not be over torqued. My same drain plug calls for only 7 NM or 5.1 FT LB torque. Overtightening the fiber gasket will crush it and cause it to fail.

[whaler82]

Is it dripping on the ground?

Yes it its weeping from drain plug traveling down skegg onto the paper towel.

[whaler82]

You have a fiber gasket Pt # 90430-08020-00, correct?

As others have said check the casting and also check for any pitted or raised area on the housing where the fiber washer gasket sits. Another thing is that this gasket is fiber, it should not be over torqued. My same drain plug calls for only 7 NM or 5.1 FT LB torque. Over-tightening the fiber gasket will crush it and cause it to fail.

Yes its the right p/n . I have another gasket I could try. I do not know how much torque is in the screw. Its defiantly tight.

[cpostis]

Yes its the right p/n . I have another gasket I could try. I do not know how much torque is in the screw. Its defiantly tight.

Maybe overtight? The torque values I gave you are not very much. Once you make contact, you want it snug, you shouldn't have to "grunt" it down. Like I said, too much tight will crush the gasket. Now this is my opinion, the gasket must remain pliable for changes in temperature. If you have access to a torque wrench and a straight blade "screwdriver socket, would be best. If not to be precise, at least get a "feel" for how much you need to tighten it.

Good luck!

[pstephens46]

Try the new gasket. Cheap $2.00 fix. I always double check it for snugness after the first run in water. After finding mine loose one time.....

[whaler82]

Try the new gasket. Cheap $2.00 fix. I always double check it for snugness after the first run in water. After finding mine loose one time.....

As I mentioned in previous posts I have replaced the gasket 2X

[pstephens46]

But maybe too tight?

[zenoahphobic]

But maybe too tight?

There is also the possibility the thread has stripped or corrosion of the aluminium side of the thread (stainless steel/ aluminium electrolysis). The solution to that could be you might Helicoil a new (and better sealing and permanent stronger) thread.