Which Pilot Hole Should You Drill for a 3/8-18 NPT Thread?

Quick answer

Ray Kowalski August 19, 2026 · 13 min read

Quick answer: 37/64 inch is the commonly listed size

Quick answer

Thread: 3/8-18 NPT

Common tap drill: 37/64 inch

Decimal diameter: 0.578125 inch

Approximate metric conversion: 14.68 mm

Do not drill a 3/8-inch pilot simply because the thread is called 3/8 NPT.

For a general lookup, 37/64 inch is the size listed by several supplied commercial references for tapping a 3/8-18 NPT internal thread. MISUMI’s table identifies 3/8 NPT as an 18-threads-per-inch thread and pairs it with a 37/64-inch drill, rounded there to 0.578 inch. See MISUMI’s NPT drill-and-tap chart.

That value is common, but it is not universal. Allied Machine lists 9/16 inch for 3/8 NPT/NPTF and states that its NPT/NPTF drill sizes apply without a taper reamer. For controlled work, follow the data for the exact tap, the part drawing, and the approved shop process rather than treating either value as universally correct. Compare Allied Machine’s national pipe-thread chart.

The practical decision is:

Use 37/64 inch as the common general-reference answer when no higher-level specification applies.
Use 9/16 inch when the exact tap documentation, drawing, or approved process specifies it.
Do not substitute one for the other casually on drawing-controlled or inspected work.
Verify the completed tapered thread by the required gauge or engagement criterion; the pilot diameter alone does not define the finished fit.

The “3/8” in 3/8 NPT is a nominal pipe size, not the required pilot-hole diameter. A 3/8-inch hole would be much smaller than either documented tap-drill value.

What 3/8-18 NPT means—and why the hole is larger than 3/8 inch

The designation 3/8-18 NPT contains three important elements:

3/8 is the nominal pipe size.
18 is the pitch in threads per inch.
NPT identifies a tapered pipe thread.

The nominal 3/8 label should not be treated like the measured diameter of a conventional 3/8-inch bolt. A commercial tool reference identifies the required tool as a 3/8-18 NPT pipe tap, lists the pitch as 18 TPI, and gives 37/64 inch as the common pilot drill. Review the 3/8-18 NPT drill-and-tap specification.

A 3/8-18 NPT tap is not interchangeable with:

a 3/8-16 UNC tap;
a 3/8-24 UNF tap;
a 3/8 NPS or NPSF tap; or
a tap selected using a generic straight-thread formula.

The thread letters matter as much as the nominal size. Allied Machine’s pipe-thread chart, for example, lists 9/16 inch for 3/8 NPT/NPTF but 19/32 inch for 3/8 NPS/NPSF, even though both entries show 18 TPI. See the NPT and NPS values in Allied Machine’s chart.

Thread callout	Pitch or identification	Pilot-drill information supported here	Key distinction
3/8-18 NPT	18 TPI	Common reference: 37/64 in; alternative chart: 9/16 in	Tapered pipe thread; requires a 3/8-18 NPT tap
3/8 NPS/NPSF	18 TPI in the cited chart	19/32 in in the cited chart	Different pipe-thread system
3/8-16 UNC	16 TPI	Not provided here	Straight machine-thread system
3/8-24 UNF	24 TPI	Not provided here	Straight machine-thread system

Matching only “3/8” or even “3/8” plus the pitch can therefore produce the wrong tool or pilot hole. Confirm the complete callout before drilling.

The taper also explains why the starting-hole diameter does not settle the finished fit. As an NPT tap advances, it cuts the tapered thread farther into the workpiece. A fitting may then enter farther before reaching a comparable engagement condition. Two ports drilled to the same diameter can therefore have different fitting positions if one is tapped more deeply.

The drill-size lookup selects a starting hole. It does not, by itself, establish the acceptable finished thread.

37/64 inch versus 9/16 inch: why the charts do not agree

The supplied references contain a genuine conflict:

Several commercial charts and matched drill-and-tap kits list 37/64 inch for 3/8-18 NPT.
Allied Machine lists 9/16 inch for 3/8 NPT/NPTF and marks that column as applying without a taper reamer.

The dimensions are close but distinct:

37/64 inch = 0.578125 inch
9/16 inch = 0.5625 inch
Difference = 0.015625 inch, or 1/64 inch

Because 9/16 inch is smaller, it leaves more material for the tap to remove. That does not establish a specific change in torque, tool life, or thread quality; the supplied technical references do not quantify those effects.

Users in one Pirate 4x4 discussion reported that a 3/8 NPT tap may be harder to start or tighten sooner in a 9/16-inch hole. Those statements are unverified shop anecdotes, not engineering limits or a substitute for the tap manufacturer’s instructions. Read the forum discussion of the two starting sizes.

A separate Grassroots Motorsports forum discussion associates 9/16 inch with a starting hole later finished using a tapered pipe reamer. That may describe one shop practice, but it does not establish a universal rule—and it does not reconcile Allied Machine’s explicit “without taper reamer” note for its 9/16-inch entry.

The available evidence therefore does not support either of these blanket statements:

“9/16 inch is always the reamer size.”
“37/64 inch is always the direct-tapping size.”

Nor does it provide enough information to declare either chart wrong. No governing standard excerpt or engineering explanation from the makers of the particular taps is available here to reconcile the recommendations. Factors such as tooling, process, engagement requirements, and inspection may be relevant, but the evidence does not prove which factor produced either chart entry.

Use this decision hierarchy:

Exact tap manufacturer’s data: Follow instructions covering the particular tap whenever available.
Part drawing or shop specification: Confirm the complete thread callout, process notes, and acceptance requirements.
Approved process: Determine whether the operation is direct tapping or includes a controlled reaming step.
Inspection requirement: Use the specified gauge or engagement criterion for acceptance.
General reference: If none of those controls exists, 37/64 inch is the value represented by several supplied commercial references.

Do not enlarge an existing hole merely to make it match a different online chart. A 1/64-inch change alters the material left for tapping; whether that change is permitted depends on the governing process, not on whether the two fractions appear close.

For pressure-containing, leak-critical, drawing-controlled, or inspected work, resolve the discrepancy before drilling. This article provides a size comparison, not an engineering approval for a component or connection.

Decimal and metric conversions—and the limits of substitution

The common and alternative drill values convert as follows. The 37/64-inch reference is also published as approximately 14.68 mm by a commercial tool supplier. See the published decimal and metric values.

Fractional size	Exact decimal inches	Approximate metric value	How it appears in the evidence
37/64 in	0.578125 in	14.68 mm	Listed by several commercial references
9/16 in	0.5625 in	14.29 mm	Alternative value in Allied Machine’s chart
Difference	0.015625 in	about 0.40 mm	9/16 inch is 1/64 inch smaller

These metric figures are conversions, not independent drill specifications. A converted diameter may not match a drill in a metric index exactly, and rounding changes the resulting hole size.

Accordingly, 37/64 inch and 9/16 inch are not interchangeable labels. Their diameters differ by 0.015625 inch, or approximately 0.40 mm. That difference changes how much material remains for the tap.

The evidence also does not establish 14.5 mm, 14.7 mm, or another nearby metric drill as an approved substitute. Numerical closeness alone does not show that the resulting hole meets the requirements of the tap, drawing, or inspection process.

If only metric tooling is available:

Identify the pilot diameter required for the exact tap and operation.
Ask the tap manufacturer or process owner whether substitution is permitted.
Compare the actual metric drill diameter with the specified fractional value.
Confirm how the finished thread will be accepted.
Do not round 14.68 mm automatically to the nearest available drill.

For production or inspected work, document an approved substitution rather than improvising it at the machine.

Choose the pilot size from the tap and process—not from a generic formula

Before selecting a drill, read the tap markings and confirm that the tool is marked 3/8-18 NPT. Do not proceed merely because the tap appears to be about the right diameter.

Check that you do not have:

3/8 NPS or NPSF;
3/8 NPTF where the documented process distinguishes it from NPT;
3/8-16 UNC;
3/8-24 UNF; or
a tap with unreadable or unverified markings.

A generic straight-thread tap-drill formula is not an adequate substitute for pipe-thread data. The “3/8” in the NPT designation is a nominal pipe size, so it does not supply the conventional measured thread diameter assumed by that shortcut.

Use the following order of control before relying on a general web chart:

Tap manufacturer’s data: Find the chart or instructions for the exact tap.
Part drawing: Confirm the complete callout and any depth, engagement, or inspection notes.
Approved shop process: Check whether the operation involves direct tapping, taper reaming, or another controlled step.
Inspection requirement: Identify the required gauge or engagement result.
General chart: Use only when no higher-level control applies.

Commercial kits are useful evidence of common pairing, not universal suitability. MaxTow, for example, sells a set containing a 37/64-inch drill and a 3/8-18 NPT tap. That shows that the two tools are commercially paired; it does not prove suitability for every material, part, or acceptance requirement. See the components listed for the MaxTow set.

Do not infer a reaming operation solely from the pilot diameter. A 9/16-inch chart entry does not prove that taper reaming is required, and a 37/64-inch entry does not prove that it is prohibited. The documented process must establish the required sequence.

Other variables may include:

workpiece material;
required fitting engagement;
inspection method;
available wall and hole depth;
repair versus repeatable production work; and
restrictions on fitting position or intrusion into an internal passage.

Those factors are reasons to consult the controlling documentation—not permission to invent a material-specific drill size.

Hole depth also requires job-specific information. A through-hole and a blind hole have different space constraints, but the supplied evidence does not establish one universal pilot depth or a fixed allowance beneath the usable thread. For a blind hole, follow the drawing or tap manufacturer’s data for the actual tooling and required thread.

A practical pre-drilling checklist is:

[ ] Complete callout confirmed as 3/8-18 NPT
[ ] Pilot size confirmed from the controlling source
[ ] Tap type and markings confirmed
[ ] Direct-tapping or reaming process confirmed
[ ] Through-hole or blind-hole depth checked
[ ] Internal clearance checked
[ ] Intended fitting or specified gauge available
[ ] Inspection criteria identified
[ ] Any drawing, chart, or tooling conflict resolved
[ ] Application-specific design and safety requirements identified

If those items cannot be confirmed, a drill-size lookup alone has not supplied enough information to make the hole.

Tap depth and inspection determine the finished NPT fit

There is no universal number of tap turns or fixed tapping depth that can be specified for every 3/8-18 NPT port.

Because the thread is tapered, advancing the tap changes the developed thread and the eventual fitting position. Tapping farther can allow the fitting to enter farther into the port. The required stopping point must therefore come from the applicable engagement or inspection criterion rather than a remembered turn count.

For controlled or production work, a commercial NPT reference recommends checking the thread with the appropriate NPT gauge and following the governing process instructions. It also identifies a mating fitting as a possible shop check where that is the accepted method.

A cautious fitting check can help monitor engagement when formal gauging is not required:

Advance the tap gradually.
Withdraw it as needed under the applicable tapping process.
Clean the thread sufficiently for checking.
Try the intended fitting without forcing it.
Continue only if additional depth is required by the accepted engagement condition.

It should not be presented as proof of dimensional conformance.

Tap depth may also affect internal clearance. In the Grassroots Motorsports discussion, forum users noted that deeper fitting entry could allow the fitting to protrude into and partially obstruct a fluid passage. This is a practical anecdote rather than a quantified design rule. See the discussion of fitting entry and passage clearance.

A bounded workflow is:

Drill the pilot diameter specified by the controlling source.
Maintain alignment between the tap and the hole.
Advance the 3/8-18 NPT tap gradually.
Clear chips as required by the tool, material, and approved process.
Inspect progressively.
Stop at the specified gauge result or permitted engagement condition.
Check internal clearance where fitting protrusion could interfere with a passage.
Complete any required final inspection or testing.

Do not stop solely because a remembered number of turns has been reached. Tap geometry, the starting hole, the workpiece, and the required fitting position can differ between jobs.

Thread fit must also be distinguished from sealing or pressure performance.

In short, the pilot diameter starts the operation; controlled tap depth and the required acceptance check determine whether the tapered thread is finished correctly.

Mistakes to avoid and when this lookup is not enough

The most consequential sizing error is using a 3/8-inch drill merely because the tap is labeled 3/8 NPT. The common 37/64-inch reference equals 0.578125 inch, demonstrating that nominal pipe size is not the required starting-hole diameter. MISUMI lists 37/64 inch for 3/8 NPT.

Other errors to avoid include:

using an NPS or NPSF pilot value for NPT;
selecting a 3/8-16 UNC or 3/8-24 UNF tap;
applying a generic straight-thread formula;
calling 37/64 inch universally mandated;
declaring 9/16 inch universally wrong;
assuming that sizes 1/64 inch apart are automatically interchangeable;
inferring a taper-reaming step from drill size alone;
running the tap to a remembered number of turns;
treating a fitting check as equivalent to gauge inspection; and
assuming that pilot diameter guarantees fitting position, sealing, or pressure performance.

Forum discussions can reveal practical questions, such as whether a smaller pilot makes a tap harder to start or whether deeper fitting entry affects clearance. They do not replace manufacturer data, a drawing, an approved process, inspection, or qualified engineering judgment.

This lookup also does not validate the design of a pressure-containing component. It cannot establish whether a base material, wall thickness, weld, tapped port, bung, coupling, or completed assembly is suitable for pressure service. Where those matters have not already been validated, use a connection design selected and approved under the application’s governing design, fabrication, inspection, and testing requirements.

A forum thread about a proposed homemade compressed-air tank illustrates why the boundary matters: participants debated tapping plate versus using weld-in ports, but the discussion did not provide a validated pressure rating, weld design, inspection procedure, or completed test result. It is not an engineering specification.

Stop and check: If the part is pressure-containing, safety-critical, drawing-controlled, or subject to inspection, resolve both the pilot size and the finished-thread acceptance criteria before drilling.

For an ordinary general-reference lookup, 37/64 inch is the commonly listed drill for a 3/8-18 NPT tap. It equals 0.578125 inch, or approximately 14.68 mm. Because Allied Machine also lists 9/16 inch, follow the exact tap manufacturer, drawing, approved process, and inspection requirement whenever those controls exist. Confirm that the thread is NPT, control tap depth, and verify the completed tapered thread instead of assuming that the pilot diameter guarantees the result.

Can I use a 9/16-inch drill instead of 37/64 inch for a 3/8 NPT tap?

Possibly, but it is not an automatic substitution. Allied Machine lists 9/16 inch for 3/8 NPT/NPTF, while several other supplied commercial references list 37/64 inch. The 9/16-inch hole is 1/64 inch smaller, so it leaves more material for the tap.

Use 9/16 inch when the exact tap documentation, drawing, or approved process specifies it. Otherwise, resolve the discrepancy before drilling controlled or inspected work.

What is the metric equivalent of a 37/64-inch drill bit?

37/64 inch equals exactly 0.578125 inch and approximately 14.68 mm.

The metric figure is a conversion, not approval to substitute a nearby metric drill. Check the tap manufacturer or process specification before rounding it.

Is 3/8-18 NPT the same as 3/8-16 or 3/8-24?

No. 3/8-18 NPT is a tapered pipe thread with 18 threads per inch. 3/8-16 UNC and 3/8-24 UNF are different straight machine-thread systems.

Use a tap marked 3/8-18 NPT and pilot-drill information specifically covering that pipe thread.

How do I check whether a 3/8-18 NPT thread has been tapped deeply enough?

Use the inspection method required by the drawing or process. For controlled work, that may be a specified NPT plug gauge. Where formal gauging is not required, cautious checking with the intended fitting may be an accepted practical method, but it is less rigorous.

There is no universal number of tap turns for every port. Advance gradually and stop at the required gauge or engagement result.

Does a 37/64-inch pilot hole guarantee that the NPT fitting will seal?

No. A 37/64-inch pilot is a commonly listed starting dimension, not a guarantee of the completed connection.

Finished fit depends on the developed thread and tap depth, while sealing or pressure suitability must be established through the applicable component, assembly, inspection, and testing requirements.