What Size Drill Bit For 38 Tap Accurate Tapping Guide

Published 30 April 2026

Selecting the correct drill bit size for a 3/8" UNC tap is a critical step in achieving precise, functional threads in metalworking and fabrication. Whether working with aluminum, stainless steel, or brass, the relationship between tap size, thread pitch, and drill bit material directly impacts thread quality, tool longevity, and project success. This guide provides a structured approach to determining the optimal drill bit dimensions, material specifications, and procedural best practices to ensure flawless tapping results. From fundamental calculations to material-specific recommendations and troubleshooting common issues, every detail is essential for professionals and DIY enthusiasts alike.

The process begins with understanding the tap drill chart for a 3/8" UNC thread, which standardizes the drill bit size to 0.334" (or 13/64") to accommodate the tap’s minor diameter and thread pitch of 16 TPI. However, variations in material hardness, drill bit type (HSS, cobalt, or carbide), and lubrication can influence the final outcome. This guide also addresses practical considerations, such as avoiding drill bit walk, preventing stripped threads, and verifying depth-to-diameter ratios to maintain consistency across projects. By integrating technical specifications with hands-on techniques, readers will gain the confidence to execute precise tapping operations efficiently.

Drill Bit Size Fundamentals for 3/8" UNC Taps

The selection of the correct drill bit size for tapping a 3/8" UNC (Unified National Coarse) thread is critical to ensuring proper thread formation, avoiding strip-out, and maintaining thread integrity. The relationship between tap size, drill bit diameter, and thread pitch is governed by standardized tap drill charts, which account for material clearance and thread engagement. Incorrect drill sizing can lead to weak threads, excessive material removal, or tap breakage, particularly in harder metals like steel or stainless steel. This section provides a structured approach to determining the appropriate drill bit size, including calculations, reference tables, and verification methods.

Relationship Between Tap Size, Drill Bit Size, and Thread Pitch for 3/8" UNC

The 3/8" UNC thread has a 16 threads per inch (TPI) pitch, meaning the distance between adjacent threads is 1/16". The minor diameter (root diameter of the thread) is smaller than the tap size due to the depth of the thread engagement. Standard tap drill charts specify the recommended drill bit size to create a pilot hole that accommodates the tap’s minor diameter while allowing sufficient material for thread formation.

For a 3/8" UNC tap, the minor diameter is calculated as:

Minor Diameter = Tap Size – (1.0825 × Pitch)

Applying this to a 3/8" UNC tap (0.375" nominal size):

Minor Diameter = 0.375" – (1.0825 × 0.0625") = 0.375" – 0.067656" ≈ 0.3073"

Step-by-Step Calculation of Drill Bit Size for 3/8" UNC Taps

1. Determine the Tap Size and Thread Class
For a 3/8" UNC Class 2A tap (common for general-purpose applications), the major diameter is 0.3750", and the minor diameter is approximately 0.3073" (as calculated above).

2. Apply the Tap Drill Chart Formula
Most tap drill charts use an empirical formula to adjust for material clearance:

Drill Bit Size = Minor Diameter – (0.013 × Pitch) + Allowance

For a 3/8" UNC tap (16 TPI):

Drill Bit Size = 0.3073" – (0.013 × 0.0625") + 0.002" ≈ 0.3073" – 0.0008125" + 0.002" ≈ 0.3085"

3. Cross-Reference with Manufacturer Specifications
Always verify against published tap drill charts (e.g., from tap manufacturers like Tapcon, HSS, or ISO standards), as empirical formulas may vary slightly based on material hardness and thread class.

Comparison Table: Tap Size, Drill Bit Size, Thread Pitch, and Recommended Drill Bit Type

Verification of Drill Bit Size Using Physical Tools

1. Tap Drill Gauge
A physical tap drill gauge is a specialized tool with holes matching standard drill sizes. To verify:

2. Digital Caliper Measurement
For higher precision, use a digital caliper to measure the drill bit diameter:

3. Visual Inspection with Thread Pitch Gauge
After drilling, verify the hole size by comparing the tap’s minor diameter to the hole:

Best Practices for Verification:

Material-Specific Drill Bit Recommendations for 3/8" UNC Taps

The following guidelines provide material-specific recommendations for drill bit selection, coatings, and preparatory steps to ensure successful tapping of 3/8" holes across common engineering substrates.

Recommended Drill Bit Materials and Coatings by Substrate

Best Practices for Avoiding Drill Bit Walk and Breakage in Hard Materials

Key Principles for Hard Material Drilling:
1. Rigidity of Setup: Use a rigid drill press or CNC setup with minimal overhang. For handheld applications, employ a drill guide bushing aligned with the hole center.
2. Pilot Hole Accuracy: A precise, undersized pilot hole (as specified in the table above) ensures the tap engages uniformly, reducing lateral forces.
3. Proper Speed and Feed: Excessive speed generates heat; excessive feed causes deflection. Follow material-specific SFM and feed rates to balance cutting efficiency and tool life.
4. Chip Control: Pecking cycles (intermittent drilling) break up long chips, preventing clogging and maintaining accuracy. For deep holes, retract the drill 0.010–0.025" per pass.
5. Lubrication: Extreme-pressure (EP) lubricants (e.g.,

Step-by-Step Drilling and Tapping Procedure for 3/8" UNC Holes

Precision in drilling and tapping ensures thread integrity, tool longevity, and component functionality. The 3/8" UNC (Unified National Coarse) thread requires careful preparation to avoid stripping, binding, or premature tool failure. Below is a structured procedure covering pilot hole drilling, tapping, and critical operational parameters for consistent results.

Preparation and Marking for Drilling

Accurate hole placement begins with precise marking to ensure alignment with design specifications. Misalignment or improper depth can lead to thread distortion or incomplete engagement. Use a scribe or center punch to mark the hole location on the workpiece, ensuring the mark is centered relative to adjacent features.

Key considerations for marking:

For flat surfaces, verify perpendicularity to adjacent edges using a square or angle gauge.

On curved or irregular surfaces, employ a depth gauge or template to maintain consistent hole spacing.

Avoid marking directly on soft materials (e.g., aluminum, brass) without a center punch, as the drill bit may slip.

Selecting Drill Bit Speed (RPM) and Feed Rate

Optimal drilling parameters minimize heat buildup, tool wear, and workpiece deformation. The recommended speed and feed for a 3/8" UNC tap pilot hole (typically 0.290" diameter for a #10 tap or 0.302" for a #8 tap) vary by material hardness and drill bit type (HSS, cobalt, or carbide).

General guidelines for common materials:

Material	Drill Bit Type	RPM Range	Feed Rate (in/tooth)
Mild Steel	HSS	500–800	0.005–0.010
Stainless Steel	Cobalt HSS	300–500	0.003–0.006
Aluminum	HSS	1,200–2,000	0.008–0.015
Cast Iron	Carbide	400–600	0.004–0.008

Notes:

Adjust speeds downward for harder materials or when using smaller drill bits to prevent overheating.

High-speed steels (HSS) require slower feeds than carbide to maintain edge integrity.

Feed rates are per tooth for twist drills; multiply by the number of flutes (typically 2–4).

Pilot Hole Depth and Center Punching

The pilot hole depth influences tap engagement and thread quality. A shallow hole risks incomplete threading, while an overly deep hole may cause tap breakage or workpiece weakening. The recommended depth-to-diameter ratio for a 3/8" UNC tap pilot hole ranges between 0.75× and 1.0× the tap diameter, accounting for thread depth (approximately 0.096" for UNC threads).

Illustration of depth parameters:

Minimum depth (0.75×): 0.2175" (ensures full thread engagement but leaves a small unthreaded section).

Optimal depth (1.0×): 0.3125" (allows complete thread formation without excessive material removal).

Maximum depth (1.25×): 0.3906" (risk of tap binding; use only for through-holes or when specified).

Center punching procedure:
1. Position the workpiece securely in a vise or fixture to prevent movement.
2. Use a center punch (typically 60° or 90°) to create a shallow dimple at the marked location.

Apply firm, controlled pressure to avoid cracking brittle materials (e.g., cast iron).

3. The dimple prevents the drill bit from slipping, ensuring straight entry and minimizing burr formation.

Lubrication and Coolant Application

Lubrication reduces friction, dissipates heat, and flushes debris from the cutting zone. The choice of lubricant depends on the material and tap type (hand or machine tap).

Recommended lubricants for 3/8" UNC tapping:

Mineral oils (e.g., SAE 10–20): Suitable for steel, stainless steel, and cast iron.

Soluble oils (e.g., 5–10% emulsion): Effective for aluminum and non-ferrous metals.

Tapping fluids (e.g., sulfurized or chlorinated oils): Enhance chip evacuation in blind holes.

Dry lubricants (e.g., graphite or molybdenum disulfide): Used for high-temperature applications or when oils are incompatible.

Application method:

Apply lubricant before drilling and continuously during tapping.

For hand tapping, use a brush or drip feed; for machine tapping, employ a flood coolant system.

Avoid excess lubricant in blind holes to prevent pressure buildup, which can cause tap breakage.

Step-by-Step Drilling and Tapping Sequence

Follow this ordered procedure to achieve consistent, high-quality 3/8" UNC threads:

1. Mark and verify hole location:
   Use a scribe or laser marker to indicate the center point. For critical applications, cross-check with a dial indicator or CMM.
2. Center punch the mark:
   Strike the center with a center punch at a 15–30° angle to create a pilot for the drill bit. Ensure the dimple is deep enough to prevent slipping but shallow enough to avoid cracking.
3. Select and install the drill bit:
   Choose a twist drill sized for the tap (e.g., 0.290" for a #10 tap). Secure the bit in a drill chuck or collet, ensuring it is straight and rigid to avoid deflection.
4. Drill the pilot hole:
   1. Start the drill at low speed (50–100 RPM) to prevent bit walk or breakage.
   2. Increase speed to the material-specific RPM once the bit engages.
   3. Maintain a feed rate aligned with the material table above.
   4. Drill to the calculated depth (0.75×–1.0× tap diameter). For blind holes, use a depth stop or measure frequently.
   5. Withdraw the drill, reverse direction, and clear debris from the hole.
5. Deburr the pilot hole:
   Use a deburring tool, reamer, or countersink to remove sharp edges. A slight 30–45° chamfer eases tap entry and reduces thread distortion.
6. Install the tap and apply lubricant:
   Secure the tap in a tap wrench or tapping attachment. Apply lubricant to the tap flutes and hole walls.
7. Start tapping:
   1. Align the tap perpendicular to the workpiece surface (use a square or angle gauge for verification).
   2. Engage the tap by hand for 1–2 threads, then switch to machine feed if available.
   3. Apply even, moderate pressure—do not force the tap. Let the tap "cut" rather than "push."
   4. For blind holes, reverse the tap every 2–3 threads to clear chips and prevent binding.
   5. Complete the thread to the required depth, ensuring the last thread is fully formed.
8. Inspect the threaded hole:
   Verify thread quality using a thread gauge, GO/NO-GO plug, or optical comparator. Check for:
   • Full thread engagement (no stripped or incomplete threads).
   • Clean, uniform thread profile without burrs or tears.
   • Proper depth (measured with a depth gauge or caliper).

Critical Safety Warnings and Common Mistakes

Safety and operational warnings for 3/8" UNC tapping:

• Incorrect drill angle: Tilting the drill bit (>2°) causes oval holes and misaligned threads. Always use a square or angle gauge to ensure perpendicularity.
• Insufficient lubrication: Dry tapping generates excessive heat, leading to tool failure or workpiece damage. Use appropriate lubricants for the material and tap type.
  

  Essential Tools and Accessories for Accurate 3/8" UNC Tapping

  Precision in tapping 3/8" UNC threads requires specialized tools beyond the drill bit to ensure thread quality, depth control, and lubrication. Proper selection of tap wrenches, depth stops, and lubricants minimizes stripping, breakage, and incomplete threads. Additionally, accurate measurement and verification methods guarantee compliance with thread specifications, particularly in applications requiring mating parts like fasteners or fittings.

  The following tools and techniques support consistent, high-quality tapping for 3/8" UNC threads, addressing critical aspects such as torque application, depth control, and thread verification.
  

  Tap Wrenches and Their Applications

  Tap wrenches provide the necessary torque and control to engage taps effectively. The choice of wrench depends on the material, accessibility, and torque requirements of the application.

  - T-Handles: Ideal for manual tapping in accessible locations. They offer high torque and are commonly used for general-purpose tapping in steel, aluminum, or brass.
  

• Example: A 1/2" drive T-handle with a 3/8" square shank accommodates most 3/8" UNC taps.
• Consideration: Ensure the handle aligns with the tap’s shank to prevent wobbling, which can lead to stripped threads.

- Magnetic Tap Wrenches: Suitable for tapping in tight or hard-to-reach areas, particularly in sheet metal or thin-walled components. The magnetic base secures the tap without requiring additional support.


• Example: A 1/2" drive magnetic wrench with a 3/8" shank adapter for 3/8" taps.
• Consideration: Verify magnetic strength for ferromagnetic materials; non-magnetic materials may require alternative clamping methods.

- Ratcheting Tap Wrenches: Provide incremental torque control, reducing the risk of overtightening or breaking taps. These are essential for blind holes or when precise depth control is required.


• Example: A 1/2" drive ratcheting wrench with a 3/8" shank and adjustable depth stop.
• Consideration: Check the ratchet’s torque range to match the material’s hardness (e.g., softer aluminum requires less torque than hardened steel).

Torque Guidelines for 3/8" UNC Taps:


Steel (Soft): 40–60 in-lb

Steel (Hardened): 60–80 in-lb

Aluminum: 20–35 in-lb

Brass: 30–50 in-lb

Source: Tap and Die Manufacturer Standards (e.g., HSS, Cobalt, or Carbide taps).

Tap Stops and Depth Control Methods

Accurate tap depth ensures threads are neither too shallow (incomplete) nor too deep (weakening the base material). Depth control methods vary based on application requirements, such as through-hole or blind-hole tapping.


Standard Tap Depth for 3/8" UNC:


Through-Hole: Full engagement until the tap’s pilot point exits the opposite side.

Blind-Hole: Typically 1.25–1.5 times the nominal diameter (e.g., 0.5–0.625" depth for 3/8" taps) to avoid bottoming out.

Depth Control Tools and Techniques:
1. Adjustable Tap Stops (Depth Collars):
   These collars screw onto the tap shank and limit insertion depth. They are reusable and ideal for batch production.
   
2. Example: A 3/8" UNC tap with a threaded collar set to 0.550" (for blind holes).
3. Advantage: Eliminates manual measurement errors and ensures consistency across multiple holes.
4. Scribe Mark Method:
   A manual technique where a scribe line is marked on the tap at the desired depth (e.g., 0.500" for a blind hole). The tap is inserted until the mark aligns with the workpiece surface.
   
5. Example: For a 3/8" UNC tap in aluminum, mark the tap at 0.450" to account for material softness.
6. Consideration: Requires precise marking and visual alignment; less accurate for high-tolerance applications.
7. Depth Gauges (Vernier or Digital):
   Used to verify tap depth before or after insertion. The gauge measures from the workpiece surface to the tap’s end.
   
8. Example: A digital depth gauge set to 0.525" for a through-hole application.
9. Advantage: Provides real-time feedback and is suitable for critical applications like aerospace or medical devices.
10. Threaded Stop Plugs:
    Permanent solutions for production environments where taps are inserted repeatedly. A threaded plug is installed in the hole to limit tap depth.
    
11. Example: A 3/8" UNC threaded stop plug installed in a jig for consistent depth.
12. Consideration: Requires additional machining to install the plug and is not reusable across different hole sizes.
Comparison of Depth Measurement Methods:

Method	Accuracy (±)	Reusability	Best For
Adjustable Tap Stop	±0.005"	High (multiple uses)	Batch production, high-volume tapping
Scribe Mark	±0.010–0.020"	Low (one-time use)	Prototyping, low-precision applications
Depth Gauge	±0.002"	High (tool-based)	Critical tolerances, inspection
Threaded Stop Plug	±0.003"	Medium (hole-specific)	Automated jigs, repetitive tapping

Thread-Cutting Lubricants and Their Selection

Lubricants reduce friction, dissipate heat, and flush away metal chips, preventing tap breakage and thread damage. The choice depends on the material being tapped and environmental conditions.


Lubricant Selection Guidelines for 3/8" UNC Taps:


Soluble Oils: Universal for steel, stainless steel, and cast iron. Diluted with water for easy cleanup.

Example: Cutting oil with 5–10% dilution ratio.

Dry Lubricants: Suitable for aluminum, brass, and non-ferrous metals where water-based oils are incompatible.

Example: Lithium-based grease or molybdenum disulfide (MoS₂) paste.

Synthetic Lubricants: High-temperature applications (e.g., titanium or exotic alloys).

Example: Extreme-pressure (EP) synthetic oils for aerospace-grade materials.

Application Techniques:
1. Pre-Application: Apply lubricant to the tap, drill hole, and surrounding area before tapping. Excessive lubricant can pool in blind holes, requiring post-tapping cleanup.
2. Recirculation: For through-holes, recirculate lubricant via a flood coolant system to maintain cooling and chip evacuation.
3. Avoid Contamination: Ensure lubricants are compatible with the workpiece material to prevent corrosion or residue buildup (e.g., avoid water-based oils for aluminum if galvanic corrosion is a risk).

Verification of Thread Quality in 3/8" Tapped Holes

Post-tapping inspection ensures threads meet dimensional and functional requirements. Visual and physical verification methods include:

Visual Inspection:


• Thread Pitch Gauge: A specialized tool with teeth matching the UNC thread pitch (16 threads per inch for 3/8"). The gauge is run along the threads to confirm pitch consistency.
• Example: A 16 TPI pitch gauge applied to a 3/8" UNC tapped hole should seat fully without gaps.
• Limitation: Does not verify depth or minor pitch deviations.

Physical Verification:


• Mating Nut Test: A correctly sized nut (e.g., 3/8" UNC) is threaded into the hole by hand. The nut should:
• Seat fully without excessive force.
• Not bind or require



Common Issues and Troubleshooting for 3/8" Tap Drilling

Accurate tapping for a 3/8" UNC thread requires precise hole preparation, proper tool selection, and adherence to procedural best practices. Deviations in material hardness, drill alignment, or lubrication can lead to frequent complications, including thread stripping, tap breakage, or misaligned holes. Understanding these issues, their root causes, and systematic corrective measures ensures reliable results in both industrial and precision machining applications. Below are five prevalent problems encountered during 3/8" tap drilling, along with structured troubleshooting guidance.


Five Frequent Problems in 3/8" Tap Drilling

Properly identifying symptoms and their underlying causes is critical for preventing defects and optimizing thread quality. The following issues often arise due to mechanical, material-related, or procedural factors, each requiring distinct mitigation strategies.

• Stripped Threads: Incomplete or uneven threads result from excessive cutting pressure, improper drill size, or misaligned tapping. This is particularly common in softer materials like aluminum or brass, where threads lack structural integrity.
• Broken Taps: Tap failure occurs when excessive torque is applied, often due to inadequate lubrication, incorrect pilot hole size, or hard spots in the material. Hand taps are especially vulnerable to snap-off if not used with proper technique.
• Misaligned Holes: Off-center or angled holes compromise thread engagement, leading to poor fitment or assembly issues. This typically stems from improper drill guide alignment or excessive drill wander in deep holes.
• Incomplete Thread Engagement: Shallow or inconsistent threads result from insufficient tap penetration, often caused by incorrect tap length selection or premature withdrawal during tapping. This is critical in applications requiring full thread depth for load-bearing.
• Drill Bit Deflection or Walk-Out: Drill bit flexing or lateral movement, especially in long or thin-walled components, leads to oversized or tapered holes. This is exacerbated by high-speed drilling without proper support or rigid setup.

Structured Troubleshooting Table for 3/8" Tap Issues

A systematic approach to diagnosing and resolving tap-related problems involves cross-referencing observable symptoms with their likely causes, preventive actions, and corrective steps. The following table provides a concise reference for field or workshop application.


Symptom	Likely Cause	Preventive Measure	Corrective Action
Stripped or incomplete threads	Incorrect pilot hole size (oversized or undersized). Excessive cutting speed or feed rate. Insufficient lubrication/cutting fluid. Material hardness variations (e.g., work-hardened surfaces).	Verify pilot hole diameter using manufacturer-recommended charts (e.g., 0.335" for 3/8" UNC in steel). Use appropriate cutting speeds (e.g., 20–40 SFM for steel, 50–80 SFM for aluminum). Apply soluble oil or tap-specific lubricant. Pre-drill pilot holes in stages for deep threads.	For minor stripping: Use a helical coil insert (e.g., Heli-Coil) or thread repair kit to restore proper threading. For severe cases: Re-drill with a slightly larger bit (e.g., 0.340") and re-tap with a bottoming tap. Check material hardness with a fillet or surface test; anneal if necessary.
Broken or snapped tap	Improper tap sequence (e.g., using a plug tap for bottoming). Inadequate lubrication leading to heat buildup. Excessive torque due to hard material or incorrect alignment. Tap not fully seated before reversal.	Follow tap sequence: pilot → taper → plug → bottoming. Use flood coolant for hand tapping; apply tap-specific lubricant. Ensure rigid clamping and proper drill alignment. Use a tap wrench with torque control or a tapping attachment.	Remove tap fragment with a tap extractor set or E-Z Out tool. If fragment is inaccessible, re-drill with a larger bit (e.g., 0.350") and re-tap. Inspect material for hard spots; consider stress-relief annealing.
Misaligned or off-center holes	Improper drill guide or fixture misalignment. Drill bit deflection in thin-walled or deep components. Excessive feed rate causing wander. Inadequate support during drilling.	Use a jig or drill bushings for precise alignment. Reduce feed rate (e.g., 0.005–0.010" per revolution for 3/8" bit). Support workpieces with backing blocks or vise jaws. Use a spot-facing cutter to ensure flat surfaces before tapping.	For minor misalignment: Ream hole to correct size, then re-tap. For severe misalignment: Re-drill with a center drill or spot drill for alignment, then step-drill to final size. In CNC applications, verify toolpath for collisions or incorrect offsets.
Incomplete thread engagement	Tap not fully inserted to bottom of hole. Insufficient hole depth (shorter than tap length). Premature reversal during tapping. Incorrect tap type (e.g., using a taper tap for blind holes).	Use a bottoming tap for blind holes; measure hole depth with a depth gauge. Ensure tap length does not exceed hole depth by ≥0.125". Apply steady, even pressure without abrupt reversals. For deep holes, use a tap guide bushing to maintain alignment.	For partial threads: Extend hole depth with a step drill, then re-tap. Use a thread chasing tool to restore threads in blind holes. For critical applications, verify thread depth with a thread pitch gauge.
Drill bit deflection or walk-out	Excessive overhang of drill bit (unsupported length >3× diameter). High-speed drilling without rigid setup. Dull or improperly ground drill bit. Material hardness causing bit flex.	Minimize unsupported drill length; use extension sleeves if necessary. Reduce speed (e.g., 3 Mastering the art of tapping a 3/8" hole requires more than just selecting the right drill bit—it demands an understanding of material science, tool compatibility, and procedural precision. From calculating the correct 0.334" drill bit size for UNC threads to choosing the appropriate HSS, cobalt, or carbide bit for substrates like stainless steel or aluminum, each decision impacts thread integrity and tool performance. By following the structured guidelines—including speed and feed rates, lubrication techniques, and depth verification—users can minimize common pitfalls such as broken taps or stripped threads. Whether troubleshooting a misaligned hole or salvaging a damaged thread, the principles outlined here ensure reliability in both industrial and DIY applications. Ultimately, precision in tapping not only enhances project quality but also extends tool life and operational efficiency. FAQ What size drill bit should I use to start a hole for a 3/8-inch Tapcon anchor? For a 3/8" Tapcon anchor, use a 5/16" drill bit (approx. 0.3125"). Always pre-drill to avoid over-tightening or cracking the material. What drill bit size is needed to tap a 3/8-inch thread? For a 3/8"-16 thread, use a #9 drill bit (approx. 0.1405"). For 3/8"-24 thread, use a #8 drill bit (approx. 0.1285"). What size drill bit is required for a 3/8-inch Tapcon screw? For a 3/8" Tapcon screw, use a 5/16" drill bit (same as the anchor). Pre-drilling ensures proper alignment and prevents stripping. What drill bit size should I use for a 3/8-inch Tapcon concrete anchor? For a 3/8" Tapcon in concrete, use a 5/16" drill bit (0.3125"). Check the manufacturer’s specs, as some recommend a 1/4" bit for softer materials. What drill bit size is correct for a 3/8-inch Tapcon bolt installation? For a 3/8" Tapcon bolt, use a 5/16" drill bit (0.3125"). This matches the anchor’s pilot hole size for secure fastening. What size drill bit is needed to tap a 3/8-inch NPT thread? For 3/8"-18 NPT threads, use a #7 drill bit (approx. 0.161"). For 3/8"-14 NPT, use a #6 drill bit (approx. 0.138"). Always tap slowly to avoid cross-threading. drill-bit-size-guides material-specific-drilling metalworking-tools tapping-techniques thread-pitch-calculations Leave a Comment Name Comment Post Comment Comments are moderated before appearing. The data you submit is processed according to the Privacy Policy of Velirium.