Roma Tappitt 3/8" for stud holes.
Using the tap:
With a wire brush or stud hole cleaner, remove dirt from the stud hole. Should you be struggling to get a stone or piece of grit out, try applying a drop of baby oil and leave it for a minute or two, as this should loosen it.
When you have cleared the holes, use the tap to gently clean the thread. Turn the tap slowly in a clockwise direction. Take care not to turn it in at a skew angle, as this will cross the thread. If the tap gets tight, simply turn it out a bit and then continue.