If you're standing in the garage wondering what size drill bit for a 3 8 pipe tap you need to grab, the short and sweet answer is 9/16 inch. That's the standard for NPT (National Pipe Taper) threads, which is what you're likely dealing with if you're working on plumbing, air lines, or automotive fittings.
It can feel a bit counterintuitive at first because, logically, you'd think a 3/8" tap would use something closer to 3/8". But pipe sizing is its own weird beast, and if you try to shove a 3/8" drill bit in there and then force a tap through, you're going to have a very bad afternoon. Let's break down why that 9/16" bit is the magic number and a few tricks to make sure your threads actually hold pressure.
Why 9/16 is the magic number
The most confusing thing for anyone starting out with pipe threads is that the "size" of the pipe doesn't actually refer to its outside diameter. When we talk about a 3/8" pipe tap, we're talking about the nominal size of the pipe it's meant to fit. Because pipe walls have to be thick enough to hold pressure and have threads cut into them, the actual outside diameter of a 3/8" pipe is usually around 0.675 inches.
Since the tap has to cut into a hole to create those threads, the hole needs to be slightly smaller than that 0.675-inch mark but significantly larger than 3/8 of an inch. That's where the 9/16" drill bit comes in. At roughly 0.5625 inches, it leaves just enough "meat" for the tap to cut clean, deep threads without putting so much stress on the tool that it snaps off in your workpiece.
Tapered vs. straight threads
Before you start drilling, it's worth double-checking that you're actually using a standard NPT tap. Most of the time in North America, a pipe tap is tapered. This means the further you screw the pipe in, the tighter it gets. This taper is what actually creates the seal, usually with the help of some Teflon tape or pipe dope.
If you happen to be using an NPS (National Pipe Straight) tap, the hole size is technically the same, but the way you use the tool changes. Straight threads don't seal on the threads themselves; they usually rely on a gasket or an O-ring. However, for 99% of DIY and shop projects involving a "3/8 pipe tap," you're looking for that 9/16" hole for a tapered fit.
Choosing the bit based on your material
While 9/16" is the textbook answer for what size drill bit for a 3 8 pipe tap, the material you're working with might make you want to nudge that size just a tiny bit.
If you're tapping into soft aluminum or plastic, 9/16" is perfect. The metal is forgiving, and the tap will slide right through. However, if you are working with something notoriously difficult like stainless steel or a hardened alloy, you might find that a 9/16" hole feels incredibly tight. In those cases, some machinists will jump up to a 37/64" bit. It's only a sixty-fourth of an inch larger, but that tiny bit of extra clearance can be the difference between a successful thread and a snapped tap that ruins your entire project.
On the flip side, if you go too big—say, jumping to a 19/32" bit—your threads will be "shallow." They might look okay, but the second you put a wrench on a fitting or crank up the air pressure, those threads could strip right out. Stick to 9/16" unless you have a very specific reason not to.
Tips for a clean tap
Knowing the drill size is only half the battle. Pipe taps are beefy, and they require a lot of torque. If you just chuck the tap into a T-handle and start cranking away, you might end up with crooked threads or a jagged finish.
Start with a center punch Don't just eyeball the drill location. Use a center punch to create a divot so your 9/16" bit doesn't wander across the metal. Since 9/16" is a fairly large bit, it's often a good idea to drill a smaller pilot hole first—maybe 1/4" or 5/16"—to give the larger bit a path to follow.
Use plenty of lubrication Never tap dry. If you're working with steel, use a dedicated cutting oil. If you're in a pinch, even some motor oil is better than nothing. For aluminum, WD-40 or kerosene works wonders to keep the metal from "galling" (which is basically when the metal gets hot and starts smearing into the teeth of your tap).
The "half-turn back" rule When you start turning the tap, you'll feel it biting into the metal. After every full turn (or even every half turn if the material is hard), back the tap off about a quarter turn. You'll hear or feel a little "click." That's the chip of metal breaking off. If you don't break the chips, they'll jam up in the flutes of the tap and eventually lock the whole thing up.
Dealing with the taper depth
Because an NPT tap is tapered, the deeper you run the tap into the hole, the larger the diameter of the opening becomes. This is a common place where people mess up. If you only run the tap in a few threads, your pipe fitting won't even start. If you run the tap all the way in until it bottoms out, your fitting will be loose and will likely leak, no matter how much tape you wrap around it.
A good rule of thumb is to tap until about half to two-thirds of the tap's threaded length is in the hole. Then, back it out, clean the shavings away, and "test fit" your actual pipe or fitting. You want it to go in a few turns by hand before it gets tight. If it only goes in one turn, run the tap a little deeper and check again.
Essential tools for the job
Aside from the drill bit and the tap itself, you're going to want a few other things on your bench: * A sturdy vise: You cannot hold a piece by hand while trying to run a 3/8" pipe tap through it. The torque will rip it right out of your grip. * A large tap handle: Don't try to use an adjustable wrench. You need the two-sided leverage of a real tap handle to keep the pressure even and the tap straight. * Deburring tool or a larger drill bit: After you drill your 9/16" hole, the edges will be sharp. Use a deburring tool or just a much larger drill bit to lightly "countersink" the top of the hole. This helps the tap align itself much easier.
Wrapping it up
It's funny how a simple question like "what size drill bit do I need" can lead down such a rabbit hole of tool geometry and material science. But at the end of the day, as long as you have that 9/16" bit and a bit of cutting oil, you're pretty much golden.
Take your time, keep the tap straight, and remember to break those chips. Pipe threading isn't exactly high-speed work, but there's a real satisfaction to it when you finally thread that brass fitting in and it cinches up perfectly. Just don't forget the Teflon tape when you're doing the final assembly, or all that hard work might still end up with a slow drip!