{
 "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": "gas-spring-piston",
 "name": "Gas-spring piston Ø14.9 × 15 and Ø19.9 × 20",
 "kind": "hardware",
 "category": "Stirling engines",
 "group": "Free-piston",
 "year": null,
 "question": "What size is the gas-spring piston in a free-piston Stirling engine?",
 "tags": [
  "free piston",
  "gas spring",
  "piston",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "gas spring stirling",
  "gas spring piston size",
  "bounce space piston"
 ],
 "overall_mm": {
  "length": null,
  "width": null,
  "height": null
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": true,
 "free_files": 2,
 "url": "https://xemstudio.com/objects/gas-spring-piston/",
 "sheet_txt": "https://xemstudio.com/objects/gas-spring-piston/sheet.txt",
 "sheet": {
  "basis": "Gas spring k = γ·p·A² / V; piston drawn at bounce-space bore − 0.1 (0.05 mm radial gap as drawn)",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "The small piston on the rod that compresses a sealed bounce space and makes a gas spring: Ø14.9 × 15 for a Ø15 bore, Ø19.9 × 20 for a Ø20 bore.",
   "Solid, with a Ø5 bore (fit: slide) for the rod and 0.5 mm chamfers. As drawn it is 0.1 mm under its bore, a 0.05 mm radial gap.",
   "Stiffness k = γ·p·A²/V: the Ø20 piston at 1 bar with 20 cm³ of bounce space gives about 690 N/m; a smaller volume is stiffer."
  ],
  "table": {
   "columns": [
    "file",
    "diameter",
    "length",
    "bore it runs in",
    "rod bore",
    "k at 1 bar, 20 cm³"
   ],
   "rows": [
    [
     "gas-spring-piston-15x15",
     14.9,
     15,
     15,
     5,
     "≈ 220 N/m"
    ],
    [
     "gas-spring-piston-20x20",
     19.9,
     20,
     20,
     5,
     "≈ 690 N/m"
    ]
   ]
  }
 },
 "sheet_text": "Gas-spring piston Ø14.9 × 15 and Ø19.9 × 20 — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/gas-spring-piston/  (CC BY 4.0)\nBasis: Gas spring k = γ·p·A² / V; piston drawn at bounce-space bore − 0.1 (0.05 mm radial gap as drawn)\nCatalog version: 2026-10-20\n\nSizes: see table below\n- The small piston on the rod that compresses a sealed bounce space and makes a gas spring: Ø14.9 × 15 for a Ø15 bore, Ø19.9 × 20 for a Ø20 bore.\n- Solid, with a Ø5 bore (fit: slide) for the rod and 0.5 mm chamfers. As drawn it is 0.1 mm under its bore, a 0.05 mm radial gap.\n- Stiffness k = γ·p·A²/V: the Ø20 piston at 1 bar with 20 cm³ of bounce space gives about 690 N/m; a smaller volume is stiffer.\n\nfile  diameter  length  bore it runs in  rod bore  k at 1 bar, 20 cm³\ngas-spring-piston-15x15  14.9  15  15  5  ≈ 220 N/m\ngas-spring-piston-20x20  19.9  20  20  5  ≈ 690 N/m\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- NASA (NTRS, public domain): Free-piston Stirling engine design and analysis (RE-1000 test results) (1984) — PDF — US Government work, public domain — https://ntrs.nasa.gov/search?q=19840017543\n- NASA (NTRS, public domain): Dynamics of free-piston Stirling engines (1987) — PDF — US Government work, public domain — https://ntrs.nasa.gov/search?q=19870011457\n- Wikipedia: Free-piston engine — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Free-piston_engine\n- Wikipedia: Beale number — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Beale_number\n- Ohio University: Stirling cycle machine analysis (Urieli), free online — web — Author's site — https://www.ohio.edu/mechanical/stirling/\n- Long Earth Society: Engine builds — web, video — LES site — https://longearthsociety.com/engineering/\n\nFree files (CC BY 4.0, Xem Studio; STEP + STL, 2 parts): https://xemstudio.com/files/hardware/gas-spring-piston/index.json\nAPI: https://xemstudio.com/api/geometry?object=gas-spring-piston\n",
 "sources": [
  {
   "publisher": "NASA (NTRS, public domain)",
   "title": "Free-piston Stirling engine design and analysis (RE-1000 test results) (1984)",
   "url": "https://ntrs.nasa.gov/search?q=19840017543",
   "formats": [
    "PDF"
   ],
   "license": "US Government work, public domain",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "NASA (NTRS, public domain)",
   "title": "Dynamics of free-piston Stirling engines (1987)",
   "url": "https://ntrs.nasa.gov/search?q=19870011457",
   "formats": [
    "PDF"
   ],
   "license": "US Government work, public domain",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Wikipedia",
   "title": "Free-piston engine",
   "url": "https://en.wikipedia.org/wiki/Free-piston_engine",
   "formats": [
    "web"
   ],
   "license": "CC BY-SA 4.0 (text)",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Wikipedia",
   "title": "Beale number",
   "url": "https://en.wikipedia.org/wiki/Beale_number",
   "formats": [
    "web"
   ],
   "license": "CC BY-SA 4.0 (text)",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Ohio University",
   "title": "Stirling cycle machine analysis (Urieli), free online",
   "url": "https://www.ohio.edu/mechanical/stirling/",
   "formats": [
    "web"
   ],
   "license": "Author's site",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Long Earth Society",
   "title": "Engine builds",
   "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": [
  "free-piston-displacer",
  "free-piston-power-piston",
  "e-clip-grooved-rods-5mm",
  "spiral-flexure-bearing"
 ],
 "fits": [
  {
   "role": "Connects to",
   "items": [
    {
     "slug": "e-clip-grooved-rods-5mm",
     "why": "on the Ø5 rod"
    },
    {
     "slug": "free-piston-displacer",
     "why": "spring for the displacer rod"
    }
   ]
  },
  {
   "role": "Alternatives",
   "items": [
    {
     "slug": "spiral-flexure-bearing",
     "why": "flexure stiffness instead of gas"
    }
   ]
  }
 ],
 "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": [
  {
   "feature": "rod bores",
   "exact": "Ø5",
   "mates": "Ø5 rod",
   "fit": "slide",
   "files": "all"
  },
  {
   "feature": "gas-spring piston in its bore",
   "exact": "Ø15 / Ø20 (= bore)",
   "mates": "Ø15 / Ø20 bounce-space bore",
   "fit": "run",
   "files": "gas-spring-piston-"
  }
 ],
 "fit_classes": {
  "press": "stays put: pin, tube, bearing or magnet held by interference. Typical FDM: hole +0.0 to +0.1 mm on diameter.",
  "slide": "assembles by hand and is held by a grub, pinch or clamp: shaft in a hub, rod in a socket, spigot in a bore. Typical FDM: +0.2 mm.",
  "run": "turns or slides in use: pin in a link, rod through a guide. Typical FDM: +0.3 mm.",
  "socket": "large sliding fit: syringe barrel, sleeve, bobbin in a bore. Typical FDM: +0.3 mm.",
  "clamp": "split part that closes on the mating part with screws: make it at size.",
  "designed": "a gap that is part of the design (piston fit option, displacer gap, air gap): do not change."
 },
 "faq": [
  {
   "q": "How stiff is a gas spring?",
   "a": "K = γ·p·A²/V; Ø20 at 1 bar with 20 cm³ gives about 690 N/m."
  },
  {
   "q": "What keeps a gas spring's mean position from drifting?",
   "a": "Centring ports: two 0.5 mm holes the piston uncovers at mid-stroke."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "gas-spring-piston-15x15",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/free-piston-core/gas-spring-piston-15x15.step",
   "stl": "https://xemstudio.com/files/hardware/free-piston-core/gas-spring-piston-15x15.stl",
   "note": "gas-spring piston Ø15 (= the Ø15 bounce-space bore; clearance-seal running fit) × 15, Ø5 rod bore"
  },
  {
   "file": "gas-spring-piston-20x20",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/free-piston-core/gas-spring-piston-20x20.step",
   "stl": "https://xemstudio.com/files/hardware/free-piston-core/gas-spring-piston-20x20.stl",
   "note": "gas-spring piston Ø20 (= the Ø20 bounce-space bore; clearance-seal running fit) × 20, Ø5 rod bore"
  }
 ],
 "markdown": "https://xemstudio.com/objects/gas-spring-piston/index.md"
}