{
 "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-bsp-thread-envelopes",
 "name": "Pipe thread envelopes: NPT (ANSI B1.20.1) and BSP (ISO 228 / 7), 1/8 to 1 inch",
 "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": [
  "thread",
  "npt",
  "bsp",
  "pipe fitting",
  "tap drill",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "1/4 npt dimensions",
  "npt tap drill size",
  "1/8 npt od",
  "bsp vs npt",
  "g1/2 thread od",
  "pipe thread sizes mm"
 ],
 "overall_mm": {
  "length": null,
  "width": null,
  "height": null
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": true,
 "free_files": 12,
 "url": "https://xemstudio.com/objects/npt-bsp-thread-envelopes/",
 "sheet_txt": "https://xemstudio.com/objects/npt-bsp-thread-envelopes/sheet.txt",
 "sheet": {
  "basis": "ANSI/ASME B1.20.1 NPT major diameters and tap drills; ISO 228-1 (G, parallel) and ISO 7-1 (R/Rc, taper) BSP major diameters",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "NPT 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 Schedule pipe ODs). Taper 1:16; hand-tight engagement 1/8: 4 mm, 1/4 and 3/8: 6 mm, 1/2 and 3/4: 8 mm.",
   "NPT tap drills: 1/8 → 8.6 mm (11/32), 1/4 → 11.6 (7/16), 3/8 → 15.0 (37/64), 1/2 → 18.0 (23/32)... use the listed drill then the taper tap to 2–3 turns past the first full thread.",
   "BSP (G) major OD: 1/8 9.73, 1/4 13.16, 3/8 16.66, 1/2 20.96, 3/4 26.44, 1 in 33.25 mm, 55° thread, parallel (G) seals on a washer or O-ring, taper (R) on the thread. NPT and BSP do not interchange: 1/2 NPT is 14 TPI 60°, 1/2 BSP is 14 TPI 55° with a different OD.",
   "In a print: model a plain hole at the tap-drill size and tap it, or use a brass insert; printed pipe threads leak above about 2 bar.",
   "Why it's here: 1/8 and 1/4 NPT are the pressure-tap and thermocouple ports on the LES engine heads; 1/2 BSP is the common gas-fitting thread."
  ],
  "table": {
   "columns": [
    "size",
    "npt_od_mm",
    "npt_tpi",
    "npt_tap_drill_mm",
    "bsp_g_od_mm",
    "bsp_tpi"
   ],
   "rows": [
    [
     "1/8",
     10.29,
     27,
     8.6,
     9.73,
     28
    ],
    [
     "1/4",
     13.72,
     18,
     11.6,
     13.16,
     19
    ],
    [
     "3/8",
     17.15,
     18,
     11.6,
     16.66,
     19
    ],
    [
     "1/2",
     21.34,
     14,
     15,
     20.96,
     14
    ],
    [
     "3/4",
     26.67,
     14,
     15,
     26.44,
     14
    ],
    [
     "1",
     33.4,
     11.5,
     18,
     33.25,
     11
    ]
   ]
  }
 },
 "sheet_text": "Pipe thread envelopes: NPT (ANSI B1.20.1) and BSP (ISO 228 / 7), 1/8 to 1 inch — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/npt-bsp-thread-envelopes/  (CC BY 4.0)\nBasis: ANSI/ASME B1.20.1 NPT major diameters and tap drills; ISO 228-1 (G, parallel) and ISO 7-1 (R/Rc, taper) BSP major diameters\nCatalog version: 2026-10-20\n\nSizes: see table below\n- NPT 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 Schedule pipe ODs). Taper 1:16; hand-tight engagement 1/8: 4 mm, 1/4 and 3/8: 6 mm, 1/2 and 3/4: 8 mm.\n- NPT tap drills: 1/8 → 8.6 mm (11/32), 1/4 → 11.6 (7/16), 3/8 → 15.0 (37/64), 1/2 → 18.0 (23/32)... use the listed drill then the taper tap to 2–3 turns past the first full thread.\n- BSP (G) major OD: 1/8 9.73, 1/4 13.16, 3/8 16.66, 1/2 20.96, 3/4 26.44, 1 in 33.25 mm, 55° thread, parallel (G) seals on a washer or O-ring, taper (R) on the thread. NPT and BSP do not interchange: 1/2 NPT is 14 TPI 60°, 1/2 BSP is 14 TPI 55° with a different OD.\n- In a print: model a plain hole at the tap-drill size and tap it, or use a brass insert; printed pipe threads leak above about 2 bar.\n- Why it's here: 1/8 and 1/4 NPT are the pressure-tap and thermocouple ports on the LES engine heads; 1/2 BSP is the common gas-fitting thread.\n\nsize  npt_od_mm  npt_tpi  npt_tap_drill_mm  bsp_g_od_mm  bsp_tpi\n1/8  10.29  27  8.6  9.73  28\n1/4  13.72  18  11.6  13.16  19\n3/8  17.15  18  11.6  16.66  19\n1/2  21.34  14  15.0  20.96  14\n3/4  26.67  14  15.0  26.44  14\n1  33.4  11.5  18.0  33.25  11\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, 12 parts): https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/index.json\nAPI: https://xemstudio.com/api/geometry?object=npt-bsp-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,
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  "tolerance_mm": null
 },
 "related": [
  "schedule-40-pipe",
  "copper-pipe-type-l-m",
  "o-rings-metric",
  "tri-clamp-end-cap"
 ],
 "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 11.6 mm (7/16 in). 1/8 NPT is 10.29 mm, 27 TPI, 8.6 mm drill."
  },
  {
   "q": "Are NPT and BSP interchangeable?",
   "a": "No. Same nominal names, but NPT is 60° and BSP 55°, the ODs differ (1/2 NPT 21.34 vs G1/2 20.96 mm) and 1/8, 1/4 and 3/8 have different TPI. They will cross-thread and leak."
  },
  {
   "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; add 1.5–2 turns with a wrench."
  }
 ],
 "files_license": null,
 "offers": [
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   "file": "npt-1-8-male",
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   "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, 8.6 mm engagement; plain cylinder at OD (taper not modeled)"
  },
  {
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   "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, 11.6 mm engagement; plain cylinder at OD (taper not modeled)"
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  {
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   "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, 11.6 mm engagement; plain cylinder at OD (taper not modeled)"
  },
  {
   "file": "npt-1-2-male",
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   "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, 15.0 mm engagement; plain cylinder at OD (taper not modeled)"
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  {
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   "note": "3/4 NPT male, major OD 26.67 mm, 14 TPI, 15.0 mm engagement; plain cylinder at OD (taper not modeled)"
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  {
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   "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, 18.0 mm engagement; plain cylinder at OD (taper not modeled)"
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  {
   "file": "bsp-1-8-male",
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   "stl": "https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/bsp-1-8-male.stl",
   "note": "G1/8 BSPP male, major OD 9.73 mm, 28 TPI, 14 mm; plain cylinder at OD"
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   "stl": "https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/bsp-1-4-male.stl",
   "note": "G1/4 BSPP male, major OD 13.16 mm, 19 TPI, 14 mm; plain cylinder at OD"
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   "stl": "https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/bsp-3-8-male.stl",
   "note": "G3/8 BSPP male, major OD 16.66 mm, 19 TPI, 14 mm; plain cylinder at OD"
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   "stl": "https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/bsp-1-2-male.stl",
   "note": "G1/2 BSPP male, major OD 20.96 mm, 14 TPI, 14 mm; plain cylinder at OD"
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   "stl": "https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/bsp-3-4-male.stl",
   "note": "G3/4 BSPP male, major OD 26.44 mm, 14 TPI, 14 mm; plain cylinder at OD"
  },
  {
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   "stl": "https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/bsp-1-male.stl",
   "note": "G1 BSPP male, major OD 33.25 mm, 11 TPI, 14 mm; plain cylinder at OD"
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 ],
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}