BIZOMAT MAKEOS · Threads
NPT Tapped Port Check (Thread Length, Depth, Boss Wall)
Is this tapped NPT pipe port deep enough for a full thread engagement, and is the boss wall around it thick enough?
Check of a tapped port for an NPT tapered pipe thread: pitch diameter at the small end, effective thread length and pitch diameter at that length (as the NPT pipe thread tool), the tapped depth needed for the effective length plus the tap chamfer threads, and the wall left between the thread major diameter and the boss outside diameter, against the depth and wall you have.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Pipe outside diameter | in | Outside diameter of the pipe (e.g. 0.840 in for ½ NPT). |
| Threads per inch | TPI | Threads per inch for the size. |
| Tap chamfer threads to allow | threads | Incomplete threads at the tap end to allow beyond the effective length. |
| Tapped depth available | mm | Depth the boss allows you to tap. |
| Boss outside diameter | mm | Outside diameter of the boss or the narrowest wall across the port. |
| Least wall around the thread | mm | Least wall you accept around the thread for strength and leak tightness. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Pipe outside diameter
- What it is
- The actual outside diameter of the pipe for the NPT size, from the pipe thread table; NPT sizes are named after an old nominal bore, so the size name and this diameter differ.
- Units and format
- Enter a number in in: inches. The form accepts 0.000001 in to 30 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.84 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter inches from the pipe size table: ½ NPT is not 0.5 in. The depth and wall inputs below are in millimetres.
What this means: Threads per inch
- What it is
- The number of thread turns per inch of length for the NPT size, from the pipe thread table; the tool takes the pitch as its reciprocal.
- Units and format
- Enter a number in TPI: threads per inch. The form accepts 0.000001 TPI to 50 TPI. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 14 TPI as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter threads per inch, not pitch: the tool works out the pitch as 1 ÷ TPI.
What this means: Tap chamfer threads to allow
- What it is
- The number of incomplete threads the tap’s lead chamfer leaves at the bottom of the hole, added to the effective thread length to give the depth to tap.
- Units and format
- Enter a number in threads: threads. The form accepts 0 threads to 10 threads. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 threads as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Each extra chamfer thread adds one pitch to the tapped depth needed.
- Get it right
- Count the chamfer threads on the tap you will use; this is a number of threads, not millimetres.
What this means: Tapped depth available
- What it is
- The depth you can actually tap into the boss or body from the port face, limited by the material behind the port or by internal passages.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter millimetres; the pipe diameter and TPI are in inches, and the tool converts between them.
What this means: Boss outside diameter
- What it is
- The outside diameter of the raised boss the port is tapped into; for a port near an edge, the narrowest dimension across the port centre.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 32 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- The wall left is half of (boss minus thread major diameter), so an off-centre port needs the narrowest width, not the nominal boss size.
What this means: Least wall around the thread
- What it is
- The least radial material you accept between the thread crests and the outside of the boss, set by your design rules for strength and leak tightness.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 200 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Method and formulas
- p = 1 ÷ n; E₀ = D − (0.05D + 1.1)p; L₂ = (0.8D + 6.8)p; E₂ = E₀ + L₂ ÷ 16; thread height 0.8p (as the NPT pipe thread tool).
- Depth needed = L₂ + chamfer threads × p.
- Major diameter near the face ≈ E₂ + 0.8p; wall = (boss − major) ÷ 2.
- Example: tap at least 17.185 mm deep (L₂ = 13.556 mm); the boss leaves 5.2186 mm of wall.
Limitations
- Formula dimensions; hand-tight engagement L₁ and tolerances come from the standard’s tables.
- Wall taken at the effective-length position, which is conservative near the face.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a tapped port in a manifold or casting
- Size a weld-on boss for a pipe thread
- Set the tapping depth on a drawing
Who it is for
- Design engineers
- Machinists
- Hydraulic technicians
What you get
- Small-end pitch diameter, effective length, pitch diameter at L₂
- Tapped depth needed
- Boss wall left
Interpreting the result
- A short port lets the fitting bottom before it seals: tap deep enough for the full effective length.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1952–1975. Topic reference; no source tables or recommended settings are reproduced.
- ASME B1.20.1 Pipe Threads, General Purpose (Inch)
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Open the NPT Tapped Port Check (Thread Length, Depth, Boss Wall) calculator