{
 "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": "fpse-fin-inserts",
 "name": "Free-piston internal fin inserts: slotted heater and cooler sleeves for a 2 in tri-clamp bore (optional)",
 "kind": "hardware",
 "category": "Hardware",
 "group": "Stirling & thermoacoustic",
 "year": null,
 "question": "Do I need internal fins in a tri-clamp Stirling heater and cooler?",
 "tags": [
  "heat exchanger",
  "fins",
  "insert",
  "heater",
  "cooler",
  "optional",
  "tri-clamp",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "stirling heater insert",
  "internal fins stirling cooler",
  "slotted heat exchanger sleeve"
 ],
 "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/fpse-fin-inserts/",
 "sheet_txt": "https://xemstudio.com/objects/fpse-fin-inserts/sheet.txt",
 "sheet": {
  "basis": "Design values of the free-piston generator: 36 axial slots 1.5 × 2.5 in a sleeve whose OD is the 2 in bore (Ø47.5); heat-transfer and flow numbers from the generator's simple analysis",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "Two slotted sleeves that slide into the plain tri-clamp parts: the heater insert on the liner's lower end inside the closed-end spool, the cooler insert pressed into the 3 in spool under the water jacket. The gas runs along the 36 slots, between fins that touch the wall.",
   "Optional: without them the heater is the plain 3.0 mm annulus round the liner and the cooler the 4.05 mm annulus round the displacer. The engine still runs; my analysis gives 6.6 W instead of 43.2 W at the same stroke (helium, 10 bar), because the gas-side heat transfer falls and the cooler annulus adds dead volume.",
   "Fits: the heater insert slides on the liner (press) and its OD against the hot spool's bore (slide); the cooler insert's OD is pressed into S3, its Ø40 bore is the displacer's 0.30 gap."
  ],
  "table": {
   "columns": [
    "insert",
    "id_mm",
    "od_mm",
    "length_mm",
    "slots",
    "slot_mm"
   ],
   "rows": [
    [
     "heater",
     41.5,
     47.5,
     68.05,
     36,
     "1.5 × 2.5"
    ],
    [
     "cooler",
     40,
     47.5,
     73.2,
     36,
     "1.5 × 2.5"
    ]
   ]
  }
 },
 "sheet_text": "Free-piston internal fin inserts: slotted heater and cooler sleeves for a 2 in tri-clamp bore (optional) — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/fpse-fin-inserts/  (CC BY 4.0)\nBasis: Design values of the free-piston generator: 36 axial slots 1.5 × 2.5 in a sleeve whose OD is the 2 in bore (Ø47.5); heat-transfer and flow numbers from the generator's simple analysis\nCatalog version: 2026-10-20\n\nSizes: see table below\n- Two slotted sleeves that slide into the plain tri-clamp parts: the heater insert on the liner's lower end inside the closed-end spool, the cooler insert pressed into the 3 in spool under the water jacket. The gas runs along the 36 slots, between fins that touch the wall.\n- Optional: without them the heater is the plain 3.0 mm annulus round the liner and the cooler the 4.05 mm annulus round the displacer. The engine still runs; my analysis gives 6.6 W instead of 43.2 W at the same stroke (helium, 10 bar), because the gas-side heat transfer falls and the cooler annulus adds dead volume.\n- Fits: the heater insert slides on the liner (press) and its OD against the hot spool's bore (slide); the cooler insert's OD is pressed into S3, its Ø40 bore is the displacer's 0.30 gap.\n\ninsert  id_mm  od_mm  length_mm  slots  slot_mm\nheater  41.5  47.5  68.05  36  1.5 × 2.5\ncooler  40.0  47.5  73.2  36  1.5 × 2.5\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- Ohio University (Urieli): Stirling cycle machine analysis: ideal adiabatic and simple analysis (after Urieli & Berchowitz, Stirling Cycle Engine Analysis, 1984) — 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/fpse-fin-inserts/index.json\nAPI: https://xemstudio.com/api/geometry?object=fpse-fin-inserts\n",
 "sources": [
  {
   "publisher": "Ohio University (Urieli)",
   "title": "Stirling cycle machine analysis: ideal adiabatic and simple analysis (after Urieli & Berchowitz, Stirling Cycle Engine Analysis, 1984)",
   "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": [
  "fpse-displacer-cylinder",
  "tri-clamp-clamp-on-water-jacket",
  "tri-clamp-hot-end-fin-sleeve",
  "list-fpse-generator-2in"
 ],
 "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": "bore",
   "exact": "Ø41.5",
   "mates": "liner tube OD",
   "fit": "press",
   "files": "fpse-heater-insert"
  },
  {
   "feature": "OD",
   "exact": "Ø47.5",
   "mates": "hot spool bore (hot wall)",
   "fit": "slide",
   "files": "fpse-heater-insert"
  },
  {
   "feature": "OD",
   "exact": "Ø47.5",
   "mates": "S3 spool bore",
   "fit": "press",
   "files": "fpse-cooler-insert"
  },
  {
   "feature": "bore",
   "exact": "Ø40.00",
   "mates": "displacer Ø39.4 (0.30 radial gap, no contact)",
   "fit": "designed",
   "files": "fpse-cooler-insert"
  },
  {
   "feature": "slots",
   "exact": "36 × 1.5 wide × 2.5 deep",
   "mates": "gas passages (no contact)",
   "fit": "designed",
   "files": "all"
  }
 ],
 "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 if I leave them out?",
   "a": "The engine is complete without them; expect about 6.6 W instead of 43.2 W at 10 bar helium."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "fpse-heater-insert-41.5x47.5",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/fpse-fin-inserts/fpse-heater-insert-41.5x47.5.step",
   "stl": "https://xemstudio.com/files/hardware/fpse-fin-inserts/fpse-heater-insert-41.5x47.5.stl",
   "note": "heater insert (optional): Ø41.5 / Ø47.5 × 68.05, 36 slots 1.5 × 2.5 deep in the OD, full length; on the liner's lower end, OD against the hot wall"
  },
  {
   "file": "fpse-cooler-insert-40x47.5",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/fpse-fin-inserts/fpse-cooler-insert-40x47.5.step",
   "stl": "https://xemstudio.com/files/hardware/fpse-fin-inserts/fpse-cooler-insert-40x47.5.stl",
   "note": "cooler insert (optional): Ø40 / Ø47.5 × 73.2, 36 slots 1.5 × 2.5 deep in the OD; pressed into the 3 in spool S3, its bore continues the displacer's bore"
  }
 ],
 "markdown": "https://xemstudio.com/objects/fpse-fin-inserts/index.md"
}