{
 "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": "stirling-clearance-seal-displacer",
 "name": "Clearance-seal displacer for a Ø40 liner: Ø40.00 running land, Ø38.50 body, 106 long",
 "kind": "hardware",
 "category": "Hardware",
 "group": "Stirling & thermoacoustic",
 "year": null,
 "question": "How should a displacer seal in a Stirling engine with an external regenerator?",
 "tags": [
  "displacer",
  "clearance seal",
  "shuttle",
  "tri-clamp",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "stirling displacer seal land",
  "displacer clearance seal",
  "low shuttle loss displacer"
 ],
 "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/stirling-clearance-seal-displacer/",
 "sheet_txt": "https://xemstudio.com/objects/stirling-clearance-seal-displacer/sheet.txt",
 "sheet": {
  "basis": "Design values; shuttle loss after Martini; land and gap sizes set by the cycle model",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "The gas must go through the slotted heater, screens and cooler, not round the displacer. A short running land Ø40.00 × 8 at the cold end is a clearance seal in the liner bore; the body behind it is Ø38.50, a designed 0.75 mm radial gap that keeps the shuttle conduction low (the land is cold, the gap is where the temperature changes).",
   "Stroke 30 mm in both engines; the cold face comes to 10 mm from the hot spool's face and the hot end to 3.4 mm from the floor.",
   "The cap's Ø0.5 vent lets the inside follow the mean charge slowly, so the shell never carries the charge pressure; the rod screws into the cap's M6 hole and is locked by an M6 nut on the cold face."
  ],
  "table": {
   "columns": [
    "part",
    "od_mm",
    "length_mm",
    "fit"
   ],
   "rows": [
    [
     "cap land",
     40,
     8,
     "run (liner bore)"
    ],
    [
     "cap spigot",
     36.5,
     6,
     "press (shell bore)"
    ],
    [
     "shell",
     38.5,
     98,
     "designed 0.75 gap"
    ]
   ]
  }
 },
 "sheet_text": "Clearance-seal displacer for a Ø40 liner: Ø40.00 running land, Ø38.50 body, 106 long — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/stirling-clearance-seal-displacer/  (CC BY 4.0)\nBasis: Design values; shuttle loss after Martini; land and gap sizes set by the cycle model\nCatalog version: 2026-10-20\n\nSizes: see table below\n- The gas must go through the slotted heater, screens and cooler, not round the displacer. A short running land Ø40.00 × 8 at the cold end is a clearance seal in the liner bore; the body behind it is Ø38.50, a designed 0.75 mm radial gap that keeps the shuttle conduction low (the land is cold, the gap is where the temperature changes).\n- Stroke 30 mm in both engines; the cold face comes to 10 mm from the hot spool's face and the hot end to 3.4 mm from the floor.\n- The cap's Ø0.5 vent lets the inside follow the mean charge slowly, so the shell never carries the charge pressure; the rod screws into the cap's M6 hole and is locked by an M6 nut on the cold face.\n\npart  od_mm  length_mm  fit\ncap land  40.0  8.0  run (liner bore)\ncap spigot  36.5  6.0  press (shell bore)\nshell  38.5  98.0  designed 0.75 gap\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- NASA (W. R. Martini): Stirling Engine Design Manual, 2nd edition, NASA CR-168088 (1983): decoupled second-order losses (shuttle, conduction, flow friction, regenerator) — PDF — US government report — https://ntrs.nasa.gov/citations/19830022057\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/stirling-clearance-seal-displacer/index.json\nAPI: https://xemstudio.com/api/geometry?object=stirling-clearance-seal-displacer\n",
 "sources": [
  {
   "publisher": "NASA (W. R. Martini)",
   "title": "Stirling Engine Design Manual, 2nd edition, NASA CR-168088 (1983): decoupled second-order losses (shuttle, conduction, flow friction, regenerator)",
   "url": "https://ntrs.nasa.gov/citations/19830022057",
   "formats": [
    "PDF"
   ],
   "license": "US government report",
   "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": [
  "stirling-slotted-regenerator-liner",
  "stirling-precision-rods",
  "list-tri-clamp-horizontal-gamma",
  "list-tri-clamp-charged-beta"
 ],
 "fits": [],
 "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": "land",
   "exact": "Ø40.00 × 8",
   "mates": "liner bore Ø40.00 (clearance seal)",
   "fit": "run",
   "files": "displacer-cap"
  },
  {
   "feature": "spigot",
   "exact": "Ø36.50 × 6",
   "mates": "shell bore Ø36.50",
   "fit": "press",
   "files": "displacer-cap"
  },
  {
   "feature": "rod hole",
   "exact": "M6 tapped (Ø5.0) × 12",
   "mates": "Ø6 rod, M6 end",
   "fit": "designed",
   "files": "displacer-cap"
  },
  {
   "feature": "vent",
   "exact": "Ø0.5",
   "mates": "lets the inside follow the mean charge",
   "fit": "designed",
   "files": "displacer-cap"
  },
  {
   "feature": "body",
   "exact": "Ø38.50",
   "mates": "liner bore Ø40.00: 0.75 radial gap",
   "fit": "designed",
   "files": "displacer-shell"
  },
  {
   "feature": "shell bore",
   "exact": "Ø36.50",
   "mates": "cap spigot",
   "fit": "press",
   "files": "displacer-shell"
  }
 ],
 "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": "Why a land only at the cold end?",
   "a": "A close fit along the whole length would carry heat from hot to cold on every stroke (shuttle loss scales with 1 / gap); the land seals where it is cold."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "displacer-shell-38.5x98",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/stirling-clearance-seal-displacer/displacer-shell-38.5x98.step",
   "stl": "https://xemstudio.com/files/hardware/stirling-clearance-seal-displacer/displacer-shell-38.5x98.stl",
   "note": "displacer shell Ø38.50 × 98 (z 8–106), wall 1, closed hot end 1.5 with a 1.0 round"
  },
  {
   "file": "displacer-cap-40-land",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/stirling-clearance-seal-displacer/displacer-cap-40-land.step",
   "stl": "https://xemstudio.com/files/hardware/stirling-clearance-seal-displacer/displacer-cap-40-land.stl",
   "note": "displacer cold cap: running land Ø40.00 × 8, spigot Ø36.50 × 6 into the shell (press), M6 tapped (Ø5.0) × 12, Ø0.5 vent"
  }
 ],
 "markdown": "https://xemstudio.com/objects/stirling-clearance-seal-displacer/index.md"
}