{
 "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-power-piston",
 "name": "Free-piston Stirling power piston Ø29.9 × 30 and Ø39.9 × 40",
 "kind": "hardware",
 "category": "Stirling engines",
 "group": "Free-piston",
 "year": null,
 "question": "What size is the power piston of a free-piston Stirling engine?",
 "tags": [
  "free piston",
  "piston",
  "clearance seal",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "free piston stirling piston",
  "clearance seal piston",
  "power piston 40 mm"
 ],
 "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-power-piston/",
 "sheet_txt": "https://xemstudio.com/objects/free-piston-power-piston/sheet.txt",
 "sheet": {
  "basis": "Beta free-piston layout; piston drawn at liner bore − 0.1 (0.05 mm radial clearance-seal gap as drawn)",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "The ring-less power piston of the free-piston core: Ø29.9 × 30 for the Ø30 liner, Ø39.9 × 40 for the Ø40.",
   "As drawn it is 0.1 mm under the bore, a 0.05 mm radial gap: the seal is that gap, with no rings.",
   "Hollow with 2 mm walls, open at the top, with a Ø5 bore (fit: slide) for the Ø5 rod in its closed end."
  ],
  "table": {
   "columns": [
    "file",
    "diameter",
    "length",
    "liner bore",
    "radial gap",
    "rod bore"
   ],
   "rows": [
    [
     "power-piston-30x30",
     29.9,
     30,
     30,
     0.05,
     5
    ],
    [
     "power-piston-40x40",
     39.9,
     40,
     40,
     0.05,
     5
    ]
   ]
  }
 },
 "sheet_text": "Free-piston Stirling power piston Ø29.9 × 30 and Ø39.9 × 40 — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/free-piston-power-piston/  (CC BY 4.0)\nBasis: Beta free-piston layout; piston drawn at liner bore − 0.1 (0.05 mm radial clearance-seal gap as drawn)\nCatalog version: 2026-10-20\n\nSizes: see table below\n- The ring-less power piston of the free-piston core: Ø29.9 × 30 for the Ø30 liner, Ø39.9 × 40 for the Ø40.\n- As drawn it is 0.1 mm under the bore, a 0.05 mm radial gap: the seal is that gap, with no rings.\n- Hollow with 2 mm walls, open at the top, with a Ø5 bore (fit: slide) for the Ø5 rod in its closed end.\n\nfile  diameter  length  liner bore  radial gap  rod bore\npower-piston-30x30  29.9  30  30  0.05  5\npower-piston-40x40  39.9  40  40  0.05  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-power-piston/index.json\nAPI: https://xemstudio.com/api/geometry?object=free-piston-power-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-cylinder-liner",
  "free-piston-displacer",
  "gas-spring-piston",
  "e-clip-grooved-rods-5mm",
  "linear-alternator-mover"
 ],
 "fits": [
  {
   "role": "Goes in or on",
   "items": [
    {
     "slug": "free-piston-cylinder-liner",
     "why": "in the Ø30 / Ø40 bore"
    }
   ]
  },
  {
   "role": "Connects to",
   "items": [
    {
     "slug": "e-clip-grooved-rods-5mm",
     "why": "Ø5 × 120 rod"
    },
    {
     "slug": "linear-alternator-mover",
     "why": "mover on the same rod"
    },
    {
     "slug": "piston-stop-bumper",
     "why": "bumpers at both ends of its travel"
    }
   ]
  },
  {
   "role": "Alternatives",
   "items": [
    {
     "slug": "power-piston",
     "why": "flanged-liner pistons Ø16–30"
    }
   ]
  }
 ],
 "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": "power piston in the liner",
   "exact": "Ø30 / Ø40 (= liner bore)",
   "mates": "cylinder liner bore Ø30 / Ø40",
   "fit": "run",
   "files": "power-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": "What is a clearance seal?",
   "a": "A piston with no rings that seals by running very close to its bore; leakage through the gap over one cycle is kept to a few percent of the swept volume."
  },
  {
   "q": "What size is the 40 mm free-piston power piston?",
   "a": "Ø39.9 × 40, hollow, Ø5 rod bore, for the Ø40 liner."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "power-piston-30x30",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/free-piston-core/power-piston-30x30.step",
   "stl": "https://xemstudio.com/files/hardware/free-piston-core/power-piston-30x30.stl",
   "note": "power piston Ø30 (= the Ø30 liner bore; clearance-seal running fit) × 30, Ø5 rod bore"
  },
  {
   "file": "power-piston-40x40",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/free-piston-core/power-piston-40x40.step",
   "stl": "https://xemstudio.com/files/hardware/free-piston-core/power-piston-40x40.stl",
   "note": "power piston Ø40 (= the Ø40 liner bore; clearance-seal running fit) × 40, Ø5 rod bore"
  }
 ],
 "markdown": "https://xemstudio.com/objects/free-piston-power-piston/index.md"
}