Leaking pipe joints usually trace back to a hole drilled a size or two too small, because the wrong chart was open on the bench.
What Size Drill Bit Fits A 1 1/4 NPT Tap?
The thread runs at 11.5 threads per inch, and the hole is sized for the tap’s tapered body rather than for a straight bolt thread of the same diameter.
NPT threads taper at roughly 1 in 16, so the thread is narrower at the tip than at the shank and the tap removes more metal near the top of the hole.
Steps that keep the thread usable:
- Confirm the tap is 1 1/4 NPT. The shank should read 1 1/4-11.5 NPT.
- Center-punch the location, then step up through smaller bits to a 1 1/2 inch bit.
- Set the tap square to the surface, turn it in half a turn, then back it a quarter turn to break the chip.
- Keep cutting oil flowing and clear the flutes often, because a pipe tap removes plenty of metal.
Steady resistance and a fitting that starts by hand mean the bore is sized correctly. A squealing tap, chips packed in the flutes, or a fitting that won’t start all point to the same fix: back the tap out, clear the chips, and check the hole with calipers before turning further.
If the tap itself is still missing, our tested 1 1/4 inch pipe thread tap roundup covers the sizes and styles worth buying.
NPT Tap Drill Chart: Matching Pipe Sizes To Drill Bits
Every NPT size follows the same pattern, and the drill bit grows with the pipe.
| Pipe Thread Size | Threads Per Inch | Tap Drill Size |
|---|---|---|
| 1/8 NPT | 27 | 21/64 inch |
| 1/4 NPT | 18 | 7/16 inch |
| 3/8 NPT | 18 | 37/64 inch |
| 1/2 NPT | 14 | 23/32 inch |
| 3/4 NPT | 14 | 59/64 inch |
| 1 NPT | 11.5 | 1 5/32 inch |
| 1 1/4 NPT | 11.5 | 1 1/2 inch |
Drilling that wide goes easier in passes — start near 1/2 inch and work up, keeping the bit perpendicular.
Where NPT Sizes And Standards Get Confused
Two mix-ups wreck most fittings: reading 1 1/4 NPT as 1/4 NPT, and pulling a drill size from a chart built for a different standard.
- 1 1/4 NPT against 1/4 NPT. The drill sizes are 1 1/2 inch and 7/16 inch, nowhere near each other. Read the tap’s own markings, not the box.
- NPT against NPTF. NPTF is a dry-seal thread form with tighter tolerances, so check the tap maker’s chart instead of assuming an NPT number carries over.
- NPT against BSPT. BSPT borrows the same size names but uses a different thread form, and the two won’t thread together or share a drill chart.
- Tapered against straight. NPT is tapered, so a straight-thread chart for the same nominal size points at a different bit.
Take 1 1/2 inch to the drill press for a 1 1/4 NPT tap, and let the shank marking confirm the size before the bit touches metal. Every other row follows the same logic: pipe size first, drill bit second.
FAQs
Can I Use A Hole Saw Instead Of A Drill Bit?
Use a 1 1/2 inch twist drill and step up through smaller bits first.
What Happens If I Drill Slightly Undersized?
The tap shoves metal aside instead of cutting it, raising torque, tearing threads, and risking a cracked fitting or snapped tap. An undersized bore also keeps the thread from reaching full depth, so the fitting never seats.
Do Pipe Taps Need Cutting Oil?
Yes, particularly in steel and stainless. Cutting oil cools the tap, flushes chips out of the flutes, and leaves smoother thread walls. Feed about half a turn, back a quarter turn to break the chip, and brush the flutes clean before continuing.
References & Sources
- Engineers Edge. “Pipe Tap Drill Chart.” Lists the 1 1/2 inch tap drill for 1 1/4 NPT and the full range of NPT sizes.
- Slugger Tool. “BSP and NPT Pipe Tap Sizes.” Confirms tap drill sizes and thread pitches for pipe taps.
- Gewindebohrer. “Thread Type NPT.” Technical reference on NPT thread dimensions and drill sizes.
