{
 "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": "piston-stop-bumper",
 "name": "Piston stop bumper Ø30 and Ø40 × 6",
 "kind": "hardware",
 "category": "Stirling engines",
 "group": "Free-piston",
 "year": null,
 "question": "What stops a free-piston Stirling piston from hitting the end of its cylinder?",
 "tags": [
  "free piston",
  "bumper",
  "end stop",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "free piston stirling stop",
  "piston bumper 40mm",
  "end stop bumper"
 ],
 "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/piston-stop-bumper/",
 "sheet_txt": "https://xemstudio.com/objects/piston-stop-bumper/sheet.txt",
 "sheet": {
  "basis": "OD = liner bore (fit: press); recess 0.6 × bore",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "An end stop for the free-piston core: Ø30 or Ø40 (the liner bore, fit: press) × 6 tall, a 2 mm closed face over a 4 mm deep recess of 0.6 × the bore (Ø18 / Ø24), with a 1 mm rounded edge.",
   "One at each end of the stroke; the stroke is kept to 85 % of the free travel between them. Over-stroke, when the alternator load drops, is the usual way a free-piston engine fails."
  ],
  "table": {
   "columns": [
    "file",
    "OD",
    "height",
    "recess",
    "face"
   ],
   "rows": [
    [
     "stop-bumper-30",
     30,
     6,
     "Ø18 × 4",
     2
    ],
    [
     "stop-bumper-40",
     40,
     6,
     "Ø24 × 4",
     2
    ]
   ]
  }
 },
 "sheet_text": "Piston stop bumper Ø30 and Ø40 × 6 — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/piston-stop-bumper/  (CC BY 4.0)\nBasis: OD = liner bore (fit: press); recess 0.6 × bore\nCatalog version: 2026-10-20\n\nSizes: see table below\n- An end stop for the free-piston core: Ø30 or Ø40 (the liner bore, fit: press) × 6 tall, a 2 mm closed face over a 4 mm deep recess of 0.6 × the bore (Ø18 / Ø24), with a 1 mm rounded edge.\n- One at each end of the stroke; the stroke is kept to 85 % of the free travel between them. Over-stroke, when the alternator load drops, is the usual way a free-piston engine fails.\n\nfile  OD  height  recess  face\nstop-bumper-30  30  6  Ø18 × 4  2\nstop-bumper-40  40  6  Ø24 × 4  2\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/piston-stop-bumper/index.json\nAPI: https://xemstudio.com/api/geometry?object=piston-stop-bumper\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",
  "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": "free-piston-power-piston",
     "why": "at each end of its travel"
    }
   ]
  }
 ],
 "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": "stop bumper OD",
   "exact": "Ø30 / Ø40",
   "mates": "liner bore",
   "fit": "press",
   "files": "stop-bumper-"
  }
 ],
 "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 stops a free-piston engine from over-stroking?",
   "a": "The alternator load first; bumpers at both ends of the stroke for when the load drops, with the stroke kept to 85 % of the free travel."
  },
  {
   "q": "How big is the stop bumper for a 40 mm bore?",
   "a": "Ø40 × 6 with a Ø24 × 4 recess."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "stop-bumper-30",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/free-piston-core/stop-bumper-30.step",
   "stl": "https://xemstudio.com/files/hardware/free-piston-core/stop-bumper-30.stl",
   "note": "end-stop bumper Ø30 OD (= the Ø30 bore) × 6"
  },
  {
   "file": "stop-bumper-40",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/free-piston-core/stop-bumper-40.step",
   "stl": "https://xemstudio.com/files/hardware/free-piston-core/stop-bumper-40.stl",
   "note": "end-stop bumper Ø40 OD (= the Ø40 bore) × 6"
  }
 ],
 "markdown": "https://xemstudio.com/objects/piston-stop-bumper/index.md"
}