Originally posted by TJW:
PS - your new tap and die set should come (hopefully) with a chart of some sort which will tell you exactly what size to drill the hole for each respective tap.
You can also buy taps or dies seperately at
your local hardware or auto parts stores. Most
all taps have the recomended drill size right on their shanks.
To go to a larger size you will drill out the hole
with the recomened bit. Then being sure to keep
the tap square start to thread it in, only go a
turn or so then back it out a half turn. Cast is
easy to tap but still use a good cutting fluid
or WD-40 at the least. when you reach the bottom
of the hole back the taper tap out. If you want/need
to have threads all the way to the bottom of the
hole you will need a plug tap to follow up with.
It will cut the last 3 or 4 threads at the bottom
of the hole.
Without looking I imagine 3/8-16 NC is what you will
need to go to. Like I mentioned before you can use
a socket head (allen wrench) screw or if you have
some small head header bolts they should work also.