Yamaha C90 Outboard Lower Unit Drain Screw Leak

[pstephens46]

Ok. Devils advocate here... Would the stainless insert create a corrosion issue next to cast aluminum? I know it's a stainless screw. Just wondering...Lower unit more susceptible? Better than a leak for sure...

[TownsendsFJR1300]

The hole is opened up slightly for a special tap for the coil.

I use red loctite (extra insurance) when installing the Heli coil.

Remember, the heli-coil and drain plug are both coated somewhat with 85-90wt LU oil and are literally inside the LU. The threads are not exposed to air/salt, etc..

The salt water is outside the repair, so its NOT like using a SS bolt with aluminum (dissimilar metals) with 02 in the air able to help corrode it.

IE, SS bearings, parts, etc, INSIDE an aluminum LU are often pressed in, thus aluminum to SS, W/O issue..
And, currently, you have a SS drain plug in an aluminum LU housing which usually lasts the life of the engine. Really no difference BUT now you'll have much stronger threads, steel on steel.

[cpostis]

A little update. On a whim yesterday I stopped in to West Marine to see if they had any drain plug gasket washers of any kind. They had a off brand for all Yamahas. I bought a box of 6. Now the OEM Yamaha - proper part # washers were not working. In fact the ID seemed too big. Normally I have to thread the gasket washer onto the plug. Those just slipped on with ease - even a little sloppy. The generic off brand I bought at West Marine had to be threaded onto to drain screw. I teflon taped the screw. I check it this AM. No leak.... Now I notice the drain screw's first 1 1/2 threads are buggered up. Threads closest to the magnetic end. Screw goes in fine and snugged up tight and even. I wonder if that was the problem to begin with and or combo of defective yamaha gasket washers. Checked again at 7PM still no leak.

I'm glad you fell confident you have the problem solved. I do not.

In your first post, you said you "replaced the old drain screw". Now the threads are damaged on that screw? Your internal threads are probably damaged and need to be repaired one way or another. (I'd probably run a tap/chaser through them first, and replace the screw.

All my drain screw gaskets fit over the drain screw plug with clearance. You say the generic gaskets you bought from WM "had to be threaded onto to drain screw". NOW YOU HAVE THE INCORRECT GASKET, AND A DRAIN SCREW WITH DAMAGED THREADS AND, PROBABLY DAMAGED INTERNAL THREADS..

All the above suggests you have not corrected the problem, but stopped the leak temporarily by using Teflon tape where it does not belong

Young man, looks like you may not have the mechanical aptitude to accomplish a reliable repair yourself. No offense.

I still think you need to ensure your threads are straight and true, your gasket sealing surfaces are also straight, level, smooth, and parallel to each other, and ensure you torque the drain screw tight properly. If not by you, by a qualified machine shop.

Good luck!!!

[whaler82]

I have not visited thread in while. Ran motor at WOT for about 25-30 miles so far. Check oil condition it remains in virgin condition.

[TownsendsFJR1300]

Kinda like driving your car with 2 of 4 lug nuts missing and the other two barely tight...

Good luck...

[zenoahphobic]

I have not visited thread in while. Ran motor at WOT for about 25-30 miles so far. Check oil condition it remains in virgin condition.

I presume you would have removed the old Teflon tape and reapplied with new tape.
Remember Teflon tape was created to fill the gap between threads that one side is tapered creating an ever decreasing space as they are tightened. The stuff has very slippery and malleable properties (as well as waterproof). But it has no elasticity; such a joint sealing is quickly lost at the slightest undoing.
Your application is not a tapered thread, so quick as nothing you will loose your seal.
If you won't replace your damaged threads, atleast use a more appropriate thread sealer, you may need to wait a little time for it to "set", but much better insurance.