Nearly every 1 1/2 inch NPT tap drill size lookup lands on that number. It only clicks once you know the thread is tapered, the callout is nominal, and a chart listing 1-23/32 inches for the same thread is not a misprint.
What Is The Tap Drill Size For 1-1/2 NPT?
The hole is a starting point, not the finished thread: NPT is tapered, so the drilled diameter sits under the final thread’s major diameter, and the fit depends on tap depth, material hardness, and tap class. A chart’s single number is a common starting point, not a measurement of the finished joint.
| 1-1/2 NPT Detail | Charted Spec |
|---|---|
| Nominal pipe thread size | 1-1/2 NPT |
| Threads per inch | 11.5 |
| Standard tap drill size | 1-47/64 in |
| Decimal equivalent | 1.7344 in |
| Alternate chart listing | 1-23/32 in |
| Thread form | Tapered pipe thread |
| Final major diameter | Set by tap depth, not the drilled hole |
The name causes its own trouble. A 1-1/2 NPT callout is a nominal pipe size, so the external thread measures wider than 1.5 inches. NPT, NPS, NPTF, and BSPT use different drill sizes at the same nominal size, and a straight-thread chart will not give you a hole that seals.
Cross-check the figure against the tap maker’s documentation before the bit touches metal — Allied Machine’s National Pipe Threads chart lays out the drill and tap numbers side by side for each nominal size.
How To Drill And Tap A 1-1/2 NPT Hole
The job takes four moves: confirm the thread standard, drill the charted hole square, cut the taper, then test depth with the fitting that will screw into it.
- Confirm the standard. NPT is a taper. If the fitting, tap, or drawing says NPS, NPTF, or BSPT, pull that series’ drill chart instead.
- Drill 1-47/64 in. Clamp the work, add cutting fluid, and keep the bit square. A drifting hole cuts an off-center thread, and there is no cheap fix after the taper is formed.
- Cut the thread with a tapered pipe tap. Start it square, keep steady pressure, and back the tap out a half turn every turn or two to clear chips.
- Test the depth as the taper grabs. Thread the mating fitting in after each turn near the end, because the thread tightens fast close to the bottom.
Plan for the bit itself. A 1-47/64 in drill sits outside most index sets, so you will order one or use the nearest charted size, and one reference lists 1-23/32 in for this thread. Match your pick to the tap maker’s guidance for the material.
Buying tooling rather than borrowing it? The tested 1 1/2 inch NPT tap picks we rounded up cover the sizes and shank types this thread needs.
Where 1-1/2 NPT Drill Charts Disagree On Hole Size
Drill charts disagree because they answer slightly different questions. Some publish a general-purpose starting hole, and others build in a specific engagement depth, tap class, or material. The commonly charted figure is 1-47/64 in, while an NPT/NPTF table in circulation lists 1-23/32 in for the same thread.
Those two figures sit within a few hundredths of an inch, and most jobs absorb the difference. Pressure and fluid work is the exception, because NPT seals on tapered thread interference rather than a straight mechanical fit, so confirm the standard and the sealing method before you cut metal.
- Treating 1-1/2 NPT as a 1.5-inch hole. The nominal size describes the pipe, not the thread diameter you measure.
- Grabbing an NPS, NPTF, or BSPT chart. Each series carries its own drill sizes.
- Assuming one drill size fits every material. Harder stock and deeper engagement targets shift the recommendation.
- Running a straight tap into the hole. The taper is what makes the joint hold.
The whole job, in order: confirm the standard is NPT, drill 1-47/64 in (1.7344 in) unless the tap maker’s chart says otherwise, cut with a tapered pipe tap, then keep testing depth with the fitting that will live in the joint.
FAQs
Do I Need A Special Drill Bit For This Thread?
Usually, yes. Bits at 1-47/64 inches rarely sit in standard index sets, so you will order one or use the nearest charted size, which one reference lists as 1-23/32 inches. Go by the tap manufacturer’s chart for your material rather than assuming any bit near 1.7 inches will do.
Can I Run An NPT Fitting Into A Hole Drilled For A Different Series?
Not reliably. NPS is a straight thread and NPTF is a dry-seal variant, so their charted drill sizes differ from NPT’s. An NPT joint seals on tapered thread interference, and a hole cut to a straight-thread chart leaves a connection that leaks or never seats. Keep the chart, tap, and fitting in the same series.
How Deep Should The Tap Run?
Deep enough to seat the fitting, no further. Thread the mating part in by hand after each turn once the taper starts biting, because the hole tightens quickly near the bottom. Stop when the fitting reaches the depth you need and still turns in smoothly, since extra depth only makes assembly harder.
References & Sources
- Allied Machine & Engineering. “National Pipe Threads Technical Data Chart” Lists drill and tap figures side by side for National Pipe Threads, including the 1-1/2 NPT size.
- MISUMI. “NPT Drill And Tap Chart” Used to cross-check recommended drill sizes for NPT pipe taps.
- Engineers Edge. “Pipe Tap Drill Chart” Reference chart used to cross-check alternate drill listings for 1-1/2 NPT.
