{
 "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": "free-piston-displacer",
 "name": "Free-piston Stirling displacer Ø29 × 75 and Ø39 × 100",
 "kind": "hardware",
 "category": "Stirling engines",
 "group": "Free-piston",
 "year": null,
 "question": "What size displacer goes in a free-piston Stirling engine?",
 "tags": [
  "free piston",
  "displacer",
  "nasa",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "free piston stirling displacer",
  "displacer design free piston",
  "free piston displacer size"
 ],
 "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/free-piston-displacer/",
 "sheet_txt": "https://xemstudio.com/objects/free-piston-displacer/sheet.txt",
 "sheet": {
  "basis": "Beta free-piston layout (Sunpower RE-1000 / NASA): displacer = bore − 1.0 (0.5 mm radial gap, designed), length 2.5 × bore",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "The displacer of the free-piston core: a hollow cylinder with a 1.2 mm wall and a 2 mm rounded top edge, Ø29 × 75 for the Ø30 bore or Ø39 × 100 for the Ø40 bore.",
   "The 0.5 mm radial gap to the liner is designed: it is a shuttle, not a seal.",
   "A Ø9 × 8 rod boss with a Ø5 bore (fit: slide) takes the Ø5 displacer rod. Its spring (flexure plus gas spring) is tuned so it leads the piston by about 60–90°."
  ],
  "table": {
   "columns": [
    "file",
    "diameter",
    "length",
    "bore it runs in",
    "radial gap",
    "wall",
    "rod bore"
   ],
   "rows": [
    [
     "displacer-30x75",
     29,
     75,
     30,
     0.5,
     1.2,
     5
    ],
    [
     "displacer-40x100",
     39,
     100,
     40,
     0.5,
     1.2,
     5
    ]
   ]
  }
 },
 "sheet_text": "Free-piston Stirling displacer Ø29 × 75 and Ø39 × 100 — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/free-piston-displacer/  (CC BY 4.0)\nBasis: Beta free-piston layout (Sunpower RE-1000 / NASA): displacer = bore − 1.0 (0.5 mm radial gap, designed), length 2.5 × bore\nCatalog version: 2026-10-20\n\nSizes: see table below\n- The displacer of the free-piston core: a hollow cylinder with a 1.2 mm wall and a 2 mm rounded top edge, Ø29 × 75 for the Ø30 bore or Ø39 × 100 for the Ø40 bore.\n- The 0.5 mm radial gap to the liner is designed: it is a shuttle, not a seal.\n- A Ø9 × 8 rod boss with a Ø5 bore (fit: slide) takes the Ø5 displacer rod. Its spring (flexure plus gas spring) is tuned so it leads the piston by about 60–90°.\n\nfile  diameter  length  bore it runs in  radial gap  wall  rod bore\ndisplacer-30x75  29  75  30  0.5  1.2  5\ndisplacer-40x100  39  100  40  0.5  1.2  5\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/free-piston-displacer/index.json\nAPI: https://xemstudio.com/api/geometry?object=free-piston-displacer\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-cylinder-liner",
  "free-piston-power-piston",
  "e-clip-grooved-rods-5mm",
  "spiral-flexure-bearing",
  "nasa-fpse-references"
 ],
 "fits": [
  {
   "role": "Goes in or on",
   "items": [
    {
     "slug": "free-piston-cylinder-liner",
     "why": "in the Ø30 / Ø40 bore, 0.5 mm radial gap"
    }
   ]
  },
  {
   "role": "Connects to",
   "items": [
    {
     "slug": "e-clip-grooved-rods-5mm",
     "why": "Ø5 × 150 rod in the boss"
    },
    {
     "slug": "spiral-flexure-bearing",
     "why": "rod suspended on a flexure stack"
    }
   ]
  },
  {
   "role": "Alternatives",
   "items": [
    {
     "slug": "tri-clamp-displacer",
     "why": "tri-clamp displacers"
    }
   ]
  }
 ],
 "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": "displacer in the liner",
   "exact": "liner bore − 1.0 (0.5 radial)",
   "mates": "cylinder liner bore",
   "fit": "designed",
   "files": "displacer-"
  }
 ],
 "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 does a free-piston Stirling keep its phase without a crank?",
   "a": "The displacer is a tuned mass on a spring (flexure and gas spring) that resonates ahead of the piston, typically 60–90°."
  },
  {
   "q": "What gap does the free-piston displacer have?",
   "a": "0.5 mm radial: Ø29 in the Ø30 liner, Ø39 in the Ø40."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "displacer-30x75",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/free-piston-core/displacer-30x75.step",
   "stl": "https://xemstudio.com/files/hardware/free-piston-core/displacer-30x75.stl",
   "note": "displacer Ø29 (0.5 mm radial gap in a Ø30 bore) × 75, hollow 1.2 wall, Ø5 rod boss"
  },
  {
   "file": "displacer-40x100",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/free-piston-core/displacer-40x100.step",
   "stl": "https://xemstudio.com/files/hardware/free-piston-core/displacer-40x100.stl",
   "note": "displacer Ø39 (0.5 mm radial gap in a Ø40 bore) × 100, hollow 1.2 wall, Ø5 rod boss"
  }
 ],
 "markdown": "https://xemstudio.com/objects/free-piston-displacer/index.md"
}