JoeSpareBedroom wrote:
wrote in message
ups.com...
I over-torqued four bolts in the lower unit of an outboard, and the
heads of three bolts broke off. I managed to remove one of the broken
bolt using a Dremel tool and a diamond grind bit. But I cannot remove
the other two broken bolts using the Dremel tool. Based on the past
posts in this newsgroup, I learn that the best option in this situation
is to drill out the broken bolts. I would like to learn the details of
the way to drill out the broken bolts. My question is:

Which size of drill bit do I need? Should it be as large as the hole
where the broken bolt is?


Call around to hardware stores and ask if they sell spiral screw extractors.
Before you jump in the car, measure the diameter of the bolt shafts you need
to remove. And, if you know the socket size that fits the heads, write that
down too.

I tried that already. Either it didn't work or I went too easy on the
hammer. Anyway, it could not grab the bolt and could not spin the bolt
out. I attempted to drill one of the broken bolt away. But this
simply messed up the thread in one of the hole.

Finally, a copy of the Mercury Service Manual arrived last night. I
read it and realized that the water pump base could be removed just by
removing 6 bolts, and each broken bolt went through a threaded hole in
the water pump base into another threaded hole in the lower unit. Seem
like the threaded holes in the water pump base had some chemical
reaction with the bolts, and the threads in the hole in the water pump
base had completely corroded away (the thread on the bolt mating with
the threaded hole in the water pump base was also corroded away
significantly and that was part of the reason why the bolt head broken
off easily before the thread got damaged by being over-torqued). In
other words, the water pump base was not holding on the broken bolt,
and only thing that was holding on with the broken bolt is the threaded
hole in the lower unit, and I can remove the water pump base easily.
After I had removed the water pump base, I could see a large section of
the broken bolt sticking out in the open, and I easily removed them
using a channellock plier.

Is this a happy ending? Not really. In the attempt of drilling out
one of the broken bolt, I ended up messing up the thread in one of the
hole in the lower unit (the other three are fine). I don't know what
is the best way to deal with this. I will bring the lower unit to a
marine mechanic in this afternoon to see if he can help.

The other thing is that I can simply replace the water pump base and
the broken bolts, and this should take care of the corroded threaded
holes in the water pump base. But I am wondering why the holes and the
bolts can get so corroded. The bolts must be stainless steel, right?
I didn't know that stainless steel bolts can react to casted alumimum
lower unit. How should I prevent this problem from happening again?
Any idea? Thanks.

Jay Chan