{
 "version": "2026-10-20",
 "license": "Dimension sheets are written by Xem Studio from the cited sources and are free to use (CC BY 4.0, attribute 'Xem Studio'). Linked files belong to their publishers under the license shown; Xem Studio does not host them. 'license_verified: false' means the license shown is from memory or the publisher's page as last read and should be confirmed at the link before commercial use.",
 "slug": "npt-thread-envelopes",
 "name": "NPT male thread envelopes 1/8 to 1 in (ASME B1.20.1)",
 "kind": "hardware",
 "category": "Hardware",
 "group": "Stirling & thermoacoustic",
 "year": null,
 "question": "What is the outside diameter of a 1/4 NPT thread and the drill size for it?",
 "tags": [
  "npt",
  "pipe thread",
  "tap drill",
  "thread",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "1/4 npt dimensions",
  "npt tap drill size",
  "1/8 npt od",
  "npt thread chart mm",
  "npt major diameter"
 ],
 "overall_mm": {
  "length": null,
  "width": null,
  "height": null
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": true,
 "free_files": 6,
 "url": "https://xemstudio.com/objects/npt-thread-envelopes/",
 "sheet_txt": "https://xemstudio.com/objects/npt-thread-envelopes/sheet.txt",
 "sheet": {
  "basis": "ANSI/ASME B1.20.1 NPT major diameters, TPI, tap drills and effective thread length L2",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "Each file is a plain cylinder at the NPT major OD, the length of the effective thread (L2), with a 0.8 mm chamfer on the end; the 1:16 taper and the thread form are not modelled.",
   "Major OD: 1/8 10.29, 1/4 13.72, 3/8 17.15, 1/2 21.34, 3/4 26.67, 1 in 33.40 mm (the pipe ODs). L2: 6.7, 10.2, 10.4, 13.6, 13.9, 17.3 mm.",
   "Tap drills: 1/8 8.6 (R), 1/4 11.1 (7/16), 3/8 14.7 (37/64), 1/2 18.3 (23/32), 3/4 23.4 (59/64), 1 in 29.4 mm (1-5/32).",
   "Hand-tight engagement is about 4 mm on 1/8, 6 mm on 1/4 and 3/8, 8 mm on 1/2 and 3/4; NPT is a 60° thread and does not interchange with BSP.",
   "On my engine heads 1/8 and 1/4 NPT are the pressure-tap and thermocouple ports."
  ],
  "table": {
   "columns": [
    "size",
    "od_mm",
    "tpi",
    "tap_drill_mm",
    "l2_mm"
   ],
   "rows": [
    [
     "1/8",
     10.29,
     27,
     8.6,
     6.7
    ],
    [
     "1/4",
     13.72,
     18,
     11.1,
     10.2
    ],
    [
     "3/8",
     17.15,
     18,
     14.7,
     10.4
    ],
    [
     "1/2",
     21.34,
     14,
     18.3,
     13.6
    ],
    [
     "3/4",
     26.67,
     14,
     23.4,
     13.9
    ],
    [
     "1",
     33.4,
     11.5,
     29.4,
     17.3
    ]
   ]
  }
 },
 "sheet_text": "NPT male thread envelopes 1/8 to 1 in (ASME B1.20.1) — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/npt-thread-envelopes/  (CC BY 4.0)\nBasis: ANSI/ASME B1.20.1 NPT major diameters, TPI, tap drills and effective thread length L2\nCatalog version: 2026-10-20\n\nSizes: see table below\n- Each file is a plain cylinder at the NPT major OD, the length of the effective thread (L2), with a 0.8 mm chamfer on the end; the 1:16 taper and the thread form are not modelled.\n- Major OD: 1/8 10.29, 1/4 13.72, 3/8 17.15, 1/2 21.34, 3/4 26.67, 1 in 33.40 mm (the pipe ODs). L2: 6.7, 10.2, 10.4, 13.6, 13.9, 17.3 mm.\n- Tap drills: 1/8 8.6 (R), 1/4 11.1 (7/16), 3/8 14.7 (37/64), 1/2 18.3 (23/32), 3/4 23.4 (59/64), 1 in 29.4 mm (1-5/32).\n- Hand-tight engagement is about 4 mm on 1/8, 6 mm on 1/4 and 3/8, 8 mm on 1/2 and 3/4; NPT is a 60° thread and does not interchange with BSP.\n- On my engine heads 1/8 and 1/4 NPT are the pressure-tap and thermocouple ports.\n\nsize  od_mm  tpi  tap_drill_mm  l2_mm\n1/8  10.29  27  8.6  6.7\n1/4  13.72  18  11.1  10.2\n3/8  17.15  18  14.7  10.4\n1/2  21.34  14  18.3  13.6\n3/4  26.67  14  23.4  13.9\n1  33.4  11.5  29.4  17.3\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- Wikipedia: National pipe thread — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/National_pipe_thread\n- Wikipedia: British Standard Pipe — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/British_Standard_Pipe\n- Long Earth Society: Engine builds that use these parts (videos and write-ups) — web, video — LES site — https://longearthsociety.com/engineering/\n\nFree files (CC BY 4.0, Xem Studio; STEP + STL, 6 parts): https://xemstudio.com/files/hardware/npt-thread-envelopes/index.json\nAPI: https://xemstudio.com/api/geometry?object=npt-thread-envelopes\n",
 "sources": [
  {
   "publisher": "Wikipedia",
   "title": "National pipe thread",
   "url": "https://en.wikipedia.org/wiki/National_pipe_thread",
   "formats": [
    "web"
   ],
   "license": "CC BY-SA 4.0 (text)",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Wikipedia",
   "title": "British Standard Pipe",
   "url": "https://en.wikipedia.org/wiki/British_Standard_Pipe",
   "formats": [
    "web"
   ],
   "license": "CC BY-SA 4.0 (text)",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Long Earth Society",
   "title": "Engine builds that use these parts (videos and write-ups)",
   "url": "https://longearthsociety.com/engineering/",
   "formats": [
    "web",
    "video"
   ],
   "license": "LES site",
   "license_verified": true,
   "hosted": false
  }
 ],
 "xem": {
  "status": "free-files",
  "sheet_free": true,
  "mesh_price_usd": null,
  "step_price_usd": null,
  "tolerance_mm": null
 },
 "related": [
  "bsp-thread-envelopes",
  "schedule-40-pipe",
  "tri-clamp-end-cap",
  "o-rings-metric"
 ],
 "fits": [
  {
   "role": "Goes in or on",
   "items": [
    {
     "slug": "tri-clamp-end-cap",
     "why": "1/8 or 1/4 NPT port in an engine head"
    }
   ]
  },
  {
   "role": "Connects to",
   "items": [
    {
     "slug": "schedule-40-pipe",
     "why": "NPT major OD = the pipe OD"
    }
   ]
  },
  {
   "role": "Alternatives",
   "items": [
    {
     "slug": "bsp-thread-envelopes",
     "why": "BSP is 55° with different ODs; not interchangeable"
    }
   ]
  }
 ],
 "dimensions": "exact (nominal, no printing tolerance)",
 "dimension_rule": "All geometry on Xem Studio is exact (nominal, true size) with no printing tolerance. Where a part meets another part, the object lists its mating features (feature, exact size, what it mates with, fit class: press, slide, run, socket, clamp or designed). Apply your process's allowance to those features only; 'designed' gaps are part of the design and stay as drawn.",
 "mating": [],
 "faq": [
  {
   "q": "What is the OD of 1/4 NPT?",
   "a": "13.72 mm (0.540 in) at the large end, 18 TPI; tap drill 11.1 mm (7/16 in)."
  },
  {
   "q": "What is the tap drill for 1/8 NPT?",
   "a": "8.6 mm (letter R); the thread is 10.29 mm OD, 27 TPI."
  },
  {
   "q": "How deep does an NPT thread engage?",
   "a": "Hand-tight about 4 mm for 1/8, 6 mm for 1/4 and 3/8, 8 mm for 1/2 and 3/4."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "npt-1-8-male",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-1-8-male.step",
   "stl": "https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-1-8-male.stl",
   "note": "1/8 NPT male, major OD 10.29 mm, 27 TPI, 6.7 mm effective thread length (L2); plain cylinder at OD (taper not modeled)"
  },
  {
   "file": "npt-1-4-male",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-1-4-male.step",
   "stl": "https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-1-4-male.stl",
   "note": "1/4 NPT male, major OD 13.72 mm, 18 TPI, 10.2 mm effective thread length (L2); plain cylinder at OD (taper not modeled)"
  },
  {
   "file": "npt-3-8-male",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-3-8-male.step",
   "stl": "https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-3-8-male.stl",
   "note": "3/8 NPT male, major OD 17.15 mm, 18 TPI, 10.4 mm effective thread length (L2); plain cylinder at OD (taper not modeled)"
  },
  {
   "file": "npt-1-2-male",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-1-2-male.step",
   "stl": "https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-1-2-male.stl",
   "note": "1/2 NPT male, major OD 21.34 mm, 14 TPI, 13.6 mm effective thread length (L2); plain cylinder at OD (taper not modeled)"
  },
  {
   "file": "npt-3-4-male",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-3-4-male.step",
   "stl": "https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-3-4-male.stl",
   "note": "3/4 NPT male, major OD 26.67 mm, 14 TPI, 13.9 mm effective thread length (L2); plain cylinder at OD (taper not modeled)"
  },
  {
   "file": "npt-1-male",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-1-male.step",
   "stl": "https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-1-male.stl",
   "note": "1 NPT male, major OD 33.4 mm, 11.5 TPI, 17.3 mm effective thread length (L2); plain cylinder at OD (taper not modeled)"
  }
 ],
 "markdown": "https://xemstudio.com/objects/npt-thread-envelopes/index.md"
}