Zampa, Fred wrote:
>
> The idea is that it takes more heat to cut cast iron than
> steel, so you should be able to burn out the steel without
> hurting the cast.
A little late, but I had luck a couple of years ago, removing
and replacing some broken off, rusted in studs on a cast iron
exhaust manifold.
I was attempting to drill out the old studs, planning to retap
the holes to a larger size if needed. I drilled a series of
progressively larger holes, attempting (not exactly
successfully) to keep the holes centered. It didn't seem
to matter - by the time I used the drill bit that was the
proper size to tap the original threads, the drill
"automagically" centered itself. The drill bit wandered
into the softer steel, and didn't damage the cast iron part
of the threads. I then tapped the holes, and the tap cut
out the remaining portion of the steel threads without any
difficulty - I've had more trouble tapping aluminum - and left
a nicely tapped hole.
I'm sure that the threads are missing a bit of the metal at the
"points" of the threads, and in fact the studs were a bit loose
when screwing them in. It might not be strong enough to hold
head bolts into a block, but it's strong enough for the exhaust
clamps, and have been working well for a couple of years now.
Tim Mullen
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