Help Stripped Lower Unit Thread

[99yam40]

I thought I would help with the pic so Rod can see it better.

[TownsendsFJR1300]

Most of the case in that area appears already broken away...

A good machine shop could probably weld some material back in there, tap, after dis-assembly, but they'd also have to machine out a smooth surface for the drain plug gasket...

[Capt No Slack]

Couple more photos (really hard to show exactly) little more obvious in person.

Looking in the drain hole the extra aluminum located between 3 O'clock and 6 O'clock

When I push on it with small screwdriver it does not move, appears to me to be part of the lower unit casing itself.

NO it does not go all the way around the whole.

did they drill and tap at the factory in the wrong place? Keep in mind this Is a small outboard motor just 30hp. Or has someone been in the gear case before and done some welding?

[fairdeal]

ugh. Now I'm starting to think I understand what I'm seeing-

more than just "stripped threads" -

I'm imagining little chucks of the casting having broken away...

hopefully I'm wrong.

And I can still not make any sense of that first photo.

[zenoahphobic]

yam40 more magic on the additional 2 pictures please.

Methinks what I'm seeing is not a stripped thread in the casing cast - smooth bore thru that. The threaded smashed piece looks like at was added inside, just like a nut. Perhaps held there by some aluminium welding tack, and that it was dislodged by some vigorous removal/replacement, and the rotating internals have made mince meat out of it.
Question is would Yamaha produce this this way? I would not have thought so but the picture definitely suggests by the plate appearance of the bit that it may have been how it was made. Or could this be an aftermarket machining and repair sold as renewed replacement?

Or it was made like a sparkplug tang poking into the casing with only half a thread as if to stop inside from falling out like a deflector or screen; and someone has put a longer bolt in pushing it in breaking it away?????????

[99yam40]

hope this helps, but I cannot figure out what that is inside the holes myself

[zenoahphobic]

Methinks again that last photo shows breakaway.
That is the thread used to be cut through a bump in the inside of the casting.
Pretty sure something inside has smashed this.
Means bad news because something has failed inside.
Curious did motor run?
Did it have oil in it?
How did not the plug wobble, fall out, leak?
Can you turn the shaft, change gear or turn the engine over ?

You are now committed to pulling it apart!

[TownsendsFJR1300]

The drain plug wouldn't have stayed with the condition of the LU in its current state.

It's VERY LIKELY the rest of the casting is INSIDE, that DOES NEED TO COME OUT.

Once the carrier is out, excess metal removed, material has to be added / threaded to function properly.


A much cheaper, backyardian fix would be;

(1) Open carrier, remove excess metal/chunks.
(2) Weld closed that drain hole compleatly (keeping the NEW, material as thick as before) .
(3) Drill/ tap a new hole in that spot.
(4) As machining a flat surface for the gasket would be best (and a little bit more expensive)
using a bolt with sealer (NOT RECESSED ANYMORE).

IF, (probably NOT), the welder could get to the inside and weld inside would be best but
do to being so deep in the casting would be extremely difficult.

[rodbolt17]

now would be a good time to stop, disassemble and carefully inspect it.
or find a shop with a decent tech.

[Capt No Slack]

Thanks again for all who have help me with this thread, I genuinely appreciate it.

I looked in the gear case drain hole again this afternoon with a good flash light and put on my reading glasses. To me is appears the "extra stuff" up in the hole is defiantly part of the gear case inner structure.


It is almost impossible to take a picture through a small hole inside a dark gear case to show what I am seeing in person with the naked eye! The dark shadows give the illusion of a loose or broken off piece of the case, but that is not it. In person with a good flash light it is fairly easy to see that this is part of the gear case.

Inside the hollow bullet shaped gear case is some kind of vertical support column, towards the "Nose Cone" section of the gear case.

My best guess is, when the factory drilled/taped the drain hole that they were a little off or out of alignment and caught the edge of that Column inside the Gear Case, thus cutting away part of it.

I am the second owner of this outboard, so I do not know if this is the original lower unit or an after market. I also do not know if this unit was damaged prior to my ownership and someone has already been in this gear case.

But the piece in the photo part of that inner forward support column, it is solid in place and does not seem to be a concern for causing problems.

I will put the Heli-Coil in the stripped hole this weekend tanks for all the tips and recommendations

take care

Capt No Slack