What You Need to Know about Taps and Dies
From time to time as you work through your restoration, you will find the need to "dress-up" the threads on bolts, nuts, or holes or add threads to new holes. For this you will need a tap and die set. A tap and die set should have both National Fine and National Coarse threads in various sizes, such as 1/4, 3/8, etc. If you have, or anticipate having, a car that uses Metric fasteners, you will need a tap/die set in those sizes as well. The good news is that these sets are widely available and very inexpensive.
Taps are used for cutting internal threads and they come in four styles. The taper tap is used to cut threads that go completely through a hole. The long gradual taper makes it easy to start. The plug tap is used to tap threads that only go part way through the hole. A bottoming tap is used when it is required that threads go completely to the bottom of a hole. The bottoming tap doesn't start easily, so it's best to use a plug tap to get things going. And lastly, a machine screw tap is used for small diameter, fine thread holes.
How to Use a Tap
After you determine the diameter, and number of threads per inch of the screw or stud that will go into the hole, you use a tap drill size chart (see below) to find out what size hole to drill.
Let's say you want a threaded hole for a 3/8 stud with coarse (National Coarse) threads. You look at the chart under Coarse Standard Thread and see that 3/8 will have 16 threads per inch. Continuing across the chart you can see that the tap drill size is listed as 5/16. This means that you must drill a 5/16 hole for a 3/8 NC stud with coarse threads. If you don't have a large selection of drill bits or get a little flustered at trying to read bit sizes, use the "rule of thumb" system: that is, hold the tap you intend to use against a bright light, then bring up the shaft of a likely drill bit in front. When you hold a drill bit of sufficient diameter in front of the tap and you can "just see" the threads of the tap and not any of its shaft, the drill bit is close enough to proper size.
If the hole is to be tapped part way through, use a 3/8 x 16 NC plug tap. Put the tap in the handle and carefully start the tap in the hole. Be sure to keep the tap square with the hole. Add a little tap lubricant*. After about two turns, back the tap out of the hole about a quarter to half-turn. This breaks the metal chips from the hole and keeps the tap from binding. From time to time it may be necessary to remove the chips from the hole to prevent clogging. Dressing up existing threads is done by running a tap through them with a little oil.
* Tap lubricant is known as "cutting oil," a heavy oil mixed with chlorinated parafins, available at some hardware stores and home centers. If none is available, use 30-weight engine oil or gear oil.
Dies are used to cut external threads. A die of the correct size is inserted into the handle and turned. Keep the die lubricated, and back up every turn or two to break metal chips loose.
Taps and dies are precision tools and should be kept lightly-oiled and stored in a protective case.
Coarse threads are easier and faster to use. They provide an easier "start" of the fastener, with less possiblility of cross-threading. Nicks and burrs from handling are less likely to affect assembly, they are less likely to seize in temperature applications and in joints where corrosion is likely. Coarse threads are less likely to "strip" and are more easily tapped into brittle materials.
Fine threads provide superior fastening (typically 10% stronger holding power than coarse) in hard materials. They can be adjusted more precisely due to their shallower helix angle. They are better in situations where length of engagement (depth) is limited and where wall thickness is limited, again because of their smaller thread cross-section (coarse threads are cut deeper into the shaft of the fastener than fine.)
The links below for tap drill sizes charts are to PDF files, so that you can easily print them out and tack them up on your garage wall for easy reference.