David Scheidt wrote:
wrote:
: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
osts 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?

The best way to do this is to get a selection of left-handed drill
bits. These have the twist going the opposite way from a normal twist
drill. You have to run your drill in reverse to drill a hole with one
of these. That means, in addition to puting a hole in the bolt, you
apply some anti-clockwise force to the bolt, which may back it out.
Start with a small bit; drill exactly in the center, exactly
perpendicular to the hole. Then move up to a bigger bit, and repeat.
Increase the size of the bit until you're using a bit the size of the
minor diamter of the bolt (the diameter of the round part of the
bolt, minus the threads.). Then you can use a pick to pick the
threads from the bolt out of the threads in the hole. You could also
drill a hole, and try an EZ-out.

There's a better description he
http://www.dimebank.com/tech/BrokenBolt.html

:Related questions:

: The bolts are for securing the water pump housing for the
:impeller. The hole for the bolt is at the edge of the water pump base
:-- meaning that one side of the hole is kind of thin -- something like
:3/32" thick. The bolt itself is like 1/4" or slightly larger. Because
:I have over-torqued the bolts, I found that the holes pretty much losed
:most of the thread. If I drill out the bolts, I probably will find
:that the holes will become even larger. If I re-thread the hole after
:drilling out the bolts, I am afraid that the holes will be even larger.
: I am afraid that there will not be enough metal left. Is my
:estimation correct? I am wondering whether I may be better off somehow
ouring in new metal into the holes and drill new holes instead of
:rethreading/enlarging the existing holes?

There are thread inserts, called heli-coils (that's the brand name of
the original, and best, there are others), that may solve this problem
for you. The hole needs to be drilled oversize, roughly to the major
diamter of the hold hole, and then it's tapped for the insert. The
insert is a coild of wire that allows the use of the original sized
fastener.

David

As mentioned in my other reply, I have removed the broken bolts by
simply removing the water pump base. And I also attempted to drill out
the broken bolt; but I could not get the drill bit to center on the
bolt because the bolt was sunk inside the hole and I could not see the
surface of the bolt that well. This ended up messing up the thread of
the hole. Hopefully, the heli-coils will fix this problem. Thanks.

Anyway, I will bring the lower unit to a marine mechanic later in this
afternoon, and I will see how he will fix the thread in the hole.

Jay Chan