{
 "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": "tri-clamp-free-piston-kit",
 "name": "Tri-clamp free-piston kit: flexure head, stack ring, hub collars and piston plug for 1.5, 2 and 3 in spools",
 "kind": "hardware",
 "category": "Stirling engines",
 "group": "Free-piston",
 "year": null,
 "question": "How do I hang a tri-clamp piston and displacer on flexure bearings to make a free-piston Stirling engine?",
 "tags": [
  "free piston",
  "flexure",
  "tri-clamp",
  "stirling",
  "nasa",
  "sunpower",
  "long earth society",
  "linear alternator"
 ],
 "aliases": [
  "free piston stirling tri clamp",
  "how to connect a piston to a flexure bearing",
  "flexure bearing mount stirling",
  "free piston stirling engine 3d print",
  "fpse piston suspension"
 ],
 "overall_mm": {
  "length": null,
  "width": null,
  "height": null
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": true,
 "free_files": 19,
 "url": "https://xemstudio.com/objects/tri-clamp-free-piston-kit/",
 "sheet_txt": "https://xemstudio.com/objects/tri-clamp-free-piston-kit/sheet.txt",
 "sheet": {
  "basis": "Spiral flexure entry (Ø60/80/100 leaves, 3 rim holes, hub Ø13.2/17.6/22); tri-clamp piston entry (-rod variants); Sunpower / NASA layout: displacer rod through the piston, both suspended on planar spring stacks at the cold end",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "Layout, cold end outward: hot cup → spool with the displacer and the -rod piston in it → flexure head (clamps on the spool's open ferrule with a stock clamp) carrying stack A on the piston's tube rod → three M3 standoffs → flexure ring carrying stack B on the displacer rod. Nothing touches but the clearance seals: piston in the bore, displacer rod in the piston's bushing.",
   "Piston side: print the -rod piston, press a dry 3 × 5 × 6 bushing in its crown, drop the piston plug into the skirt and push the two Ø3 × 16 wrist dowels through bosses and plug legs (the bore retains the dowels). Press a 5 mm K&S brass tube into the plug socket (CA): that is the piston rod. Stack A = hub collar b5 · leaf · hub washer · leaf · hub collar b5 on the tube (Ø100 leaves have a Ø8 hub bore: put the hub-bush-100-b5 on the tube first), rim bolted to the head with the clamp plate and spacer from the flexure entry.",
   "Displacer side: Ø3 rod from the displacer cap (M3 grub) through the piston bushing, up the inside of the tube, to stack B on the ring: hub collar b3 · leaf · washer · leaf · collar b3, all on the hub-bush b3 so the leaves' Ø5/Ø8 hub bores are centred on the Ø3 rod. Rod length = spool + head + standoffs.",
   "Stack B height sets the displacer phase mass: 30–40 mm standoffs. Tune: each moving assembly resonates at f = (1/2π)√(k/m); the displacer should run 60–90° ahead of the piston, which you get by making the displacer resonance a little above the piston's (stiffer stack or lighter displacer).",
   "Power take-off: the linear alternator's housing bolts to the flexure head over stack A, and its two-row magnet mover clamps the same 5 mm tube.",
   "Bounce space: with the head open the piston's back side is atmosphere and the engine is a low-pressure demonstrator. For a sealed bounce space put a tri-clamp cup over the whole drive on a longer spool section; the flexure head is then inside the vessel.",
   "LES: the free-piston test bed in tri-clamp."
  ],
  "table": {
   "columns": [
    "spool",
    "bore_mm",
    "flexure_od",
    "hub_mm",
    "tube_rod_mm",
    "displacer_rod_mm",
    "plate_od",
    "plug_pin_z_mm"
   ],
   "rows": [
    [
     "1.5",
     34.8,
     60,
     13.2,
     5,
     3,
     "76",
     17.6
    ],
    [
     "2",
     47.5,
     80,
     17.6,
     5,
     3,
     "96",
     23.5
    ],
    [
     "3",
     72.9,
     100,
     22,
     5,
     3,
     "116",
     37
    ]
   ]
  }
 },
 "sheet_text": "Tri-clamp free-piston kit: flexure head, stack ring, hub collars and piston plug for 1.5, 2 and 3 in spools — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/tri-clamp-free-piston-kit/  (CC BY 4.0)\nBasis: Spiral flexure entry (Ø60/80/100 leaves, 3 rim holes, hub Ø13.2/17.6/22); tri-clamp piston entry (-rod variants); Sunpower / NASA layout: displacer rod through the piston, both suspended on planar spring stacks at the cold end\nCatalog version: 2026-10-20\n\nSizes: see table below\n- Layout, cold end outward: hot cup → spool with the displacer and the -rod piston in it → flexure head (clamps on the spool's open ferrule with a stock clamp) carrying stack A on the piston's tube rod → three M3 standoffs → flexure ring carrying stack B on the displacer rod. Nothing touches but the clearance seals: piston in the bore, displacer rod in the piston's bushing.\n- Piston side: print the -rod piston, press a dry 3 × 5 × 6 bushing in its crown, drop the piston plug into the skirt and push the two Ø3 × 16 wrist dowels through bosses and plug legs (the bore retains the dowels). Press a 5 mm K&S brass tube into the plug socket (CA): that is the piston rod. Stack A = hub collar b5 · leaf · hub washer · leaf · hub collar b5 on the tube (Ø100 leaves have a Ø8 hub bore: put the hub-bush-100-b5 on the tube first), rim bolted to the head with the clamp plate and spacer from the flexure entry.\n- Displacer side: Ø3 rod from the displacer cap (M3 grub) through the piston bushing, up the inside of the tube, to stack B on the ring: hub collar b3 · leaf · washer · leaf · collar b3, all on the hub-bush b3 so the leaves' Ø5/Ø8 hub bores are centred on the Ø3 rod. Rod length = spool + head + standoffs.\n- Stack B height sets the displacer phase mass: 30–40 mm standoffs. Tune: each moving assembly resonates at f = (1/2π)√(k/m); the displacer should run 60–90° ahead of the piston, which you get by making the displacer resonance a little above the piston's (stiffer stack or lighter displacer).\n- Power take-off: the linear alternator's housing bolts to the flexure head over stack A, and its two-row magnet mover clamps the same 5 mm tube.\n- Bounce space: with the head open the piston's back side is atmosphere and the engine is a low-pressure demonstrator. For a sealed bounce space put a tri-clamp cup over the whole drive on a longer spool section; the flexure head is then inside the vessel.\n- LES: the free-piston test bed in tri-clamp.\n\nspool  bore_mm  flexure_od  hub_mm  tube_rod_mm  displacer_rod_mm  plate_od  plug_pin_z_mm\n1.5  34.8  60  13.2  5  3  76  17.6\n2  47.5  80  17.6  5  3  96  23.5\n3  72.9  100  22.0  5  3  116  37.0\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- NASA NTRS: Free-piston Stirling convertor flexure bearing development and life testing — pdf — US Government work — https://ntrs.nasa.gov/citations/20050182933\n- Long Earth Society: Tri-clamp Stirling engine builds — web, video — LES site — https://longearthsociety.com/engineering/\n- Wikipedia: Free-piston engine — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Free-piston_engine\n\nFree files (CC BY 4.0, Xem Studio; STEP + STL, 19 parts): https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/index.json\nAPI: https://xemstudio.com/api/geometry?object=tri-clamp-free-piston-kit\n",
 "sources": [
  {
   "publisher": "NASA NTRS",
   "title": "Free-piston Stirling convertor flexure bearing development and life testing",
   "url": "https://ntrs.nasa.gov/citations/20050182933",
   "formats": [
    "pdf"
   ],
   "license": "US Government work",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Long Earth Society",
   "title": "Tri-clamp Stirling engine builds",
   "url": "https://longearthsociety.com/engineering/",
   "formats": [
    "web",
    "video"
   ],
   "license": "LES site",
   "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
  }
 ],
 "xem": {
  "status": "free-files",
  "sheet_free": true,
  "mesh_price_usd": null,
  "step_price_usd": null,
  "tolerance_mm": null
 },
 "related": [
  "spiral-flexure-bearings",
  "tri-clamp-power-pistons",
  "tri-clamp-displacers-and-heads",
  "tri-clamp-cups",
  "brass-tube-ks",
  "bearing-housings-and-rod-supports",
  "threaded-standoffs-metric",
  "linear-alternator-kit",
  "free-piston-core",
  "nasa-fpse-references"
 ],
 "fits": [
  {
   "role": "Goes in or on",
   "items": [
    {
     "slug": "tri-clamp-spool",
     "why": "flexure head clamps on its ferrule"
    }
   ]
  },
  {
   "role": "Connects to",
   "items": [
    {
     "slug": "spiral-flexure-bearings",
     "why": "leaves, spacers, clamp plates, hub washers"
    },
    {
     "slug": "tri-clamp-power-pistons",
     "why": "-rod piston takes the plug"
    },
    {
     "slug": "tri-clamp-displacers-and-heads",
     "why": "displacer rod through the piston"
    },
    {
     "slug": "linear-alternator-kit",
     "why": "two-row magnet mover on the tube, stator on the rim screws over stack A"
    }
   ]
  },
  {
   "role": "Alternatives",
   "items": [
    {
     "slug": "free-piston-core",
     "why": "stand-alone liner version"
    }
   ]
  },
  {
   "role": "Also needed",
   "items": [
    {
     "slug": "stirling-shafts-and-rods",
     "why": "Ø3 displacer rod; printed tube rod for a mock-up"
    },
    {
     "slug": "spool-hanger-collars",
     "why": "hang it from a 2020 crossbar"
    },
    {
     "slug": "brass-tube-ks",
     "why": "5 mm tube piston rod"
    },
    {
     "slug": "threaded-standoffs-metric",
     "why": "M3 × 30 standoffs head to ring"
    },
    {
     "slug": "dowel-pins-metric",
     "why": "Ø3 × 16 dowels lock the plug"
    },
    {
     "slug": "tri-clamp-clamps",
     "why": "clamp the head on"
    }
   ]
  }
 ],
 "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": "flexure pocket in the head / ring",
   "exact": "flexure OD",
   "mates": "spiral flexure leaf rim",
   "fit": "slide",
   "files": "fp-flexure-"
  },
  {
   "feature": "hub collar bore (b5 / b3)",
   "exact": "Ø5 / Ø3",
   "mates": "5 mm tube / Ø3 rod",
   "fit": "slide",
   "files": "hub-collar-"
  },
  {
   "feature": "centring bush OD",
   "exact": "leaf hub bore (Ø5 / Ø8)",
   "mates": "flexure leaf hub",
   "fit": "slide",
   "files": "hub-bush-"
  },
  {
   "feature": "centring bush bore",
   "exact": "Ø3 / Ø5",
   "mates": "Ø3 rod / 5 mm tube",
   "fit": "slide",
   "files": "hub-bush-"
  },
  {
   "feature": "piston plug disc in the skirt",
   "exact": "f020 skirt bore (bore − 3.4)",
   "mates": "piston skirt (0.3 loose in the lap-rod piston)",
   "fit": "slide",
   "files": "fp-piston-plug-"
  },
  {
   "feature": "plug wrist-pin holes",
   "exact": "Ø3",
   "mates": "Ø3 pins",
   "fit": "press",
   "files": "fp-piston-plug-"
  },
  {
   "feature": "plug tube socket",
   "exact": "Ø5 × 15",
   "mates": "5 mm tube",
   "fit": "press",
   "files": "fp-piston-plug-"
  },
  {
   "feature": "plug rod passage",
   "exact": "Ø3.4",
   "mates": "Ø3 rod (no contact)",
   "fit": "designed",
   "files": "fp-piston-plug-"
  },
  {
   "feature": "head face",
   "exact": "ISO 2852",
   "mates": "ferrule face + Type B gasket",
   "fit": "clamp",
   "files": "fp-flexure-head-"
  },
  {
   "feature": "rim holes (head, ring)",
   "exact": "Ø3.4",
   "mates": "M3 screw clearance",
   "fit": "designed",
   "files": "fp-flexure-"
  },
  {
   "feature": "hub-collar slit",
   "exact": "0.8",
   "mates": "closes on the tube / rod",
   "fit": "clamp",
   "files": "hub-collar-"
  },
  {
   "feature": "hub-collar pinch hole",
   "exact": "Ø2.5",
   "mates": "M3 tap drill",
   "fit": "designed",
   "files": "hub-collar-"
  }
 ],
 "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 the piston connect to a flexure bearing?",
   "a": "Through the piston plug: it sits in the open skirt, the wrist-pin dowels lock it, and its socket takes a 5 mm brass tube. The flexure leaf hubs clamp on that tube between two split hub collars; the leaf rims bolt to the flexure head."
  },
  {
   "q": "How does the displacer rod get past the piston in a free-piston engine?",
   "a": "Through a dry 3 × 5 × 6 bushing pressed in the crown of the -rod piston, then up the inside of the hollow tube piston rod to its own flexure stack on the ring above."
  },
  {
   "q": "What flexure size for a 2 inch spool?",
   "a": "Ø80 leaves (hub Ø17.6) on the 2 in flexure head; Ø60 for 1.5 in, Ø100 for 3 in."
  }
 ],
 "files_license": null,
 "offers": [
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   "file": "fp-flexure-head-1.5in",
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   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-head-1.5in.stl",
   "note": "1.5 in flexure head: ISO 2852 cap face, Ø38.1 pedestal clearing the clamp, Ø76 plate with a Ø60 × 1 pocket (= flexure OD) and 3 × M3 (Ø3.4) on the Ø60 flexure's rim holes; the piston's stack A bolts on it"
  },
  {
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   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-ring-60.stl",
   "note": "Ø76 ring for stack B (displacer rod) on 3 × M3 standoffs above the head: Ø60 × 1 flexure pocket, 3 × Ø3.4"
  },
  {
   "file": "hub-collar-60-b5",
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   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-60-b5.stl",
   "note": "split hub collar Ø13.2 × 5, Ø5 bore, tapped M3 pinch: clamps the leaf hub on the 5 mm tube rod (two per stack)"
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  {
   "file": "hub-collar-60-b3",
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   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-60-b3.stl",
   "note": "split hub collar Ø13.2 × 5, Ø3 bore, tapped M3 pinch: clamps the leaf hub on the Ø3 displacer rod (two per stack)"
  },
  {
   "file": "hub-bush-60-b3",
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   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-60-b3.stl",
   "note": "centring bush Ø5 OD × Ø3 ID × 14: centres the Ø60 leaves (Ø5 hub bore) on the Ø3 displacer rod; or a 4 mm K&S tube sleeved on the rod"
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   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-piston-plug-1.5in.stl",
   "note": "1.5 in piston plug: drops into the open skirt, two Ø3 × 16 wrist dowels through its legs at 17.6 mm retain it, Ø5 tube socket 15 deep, Ø3.4 clearance for the displacer rod (it runs in the crown bushing, not here)"
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   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-head-2in.stl",
   "note": "2 in flexure head: ISO 2852 cap face, Ø50.8 pedestal clearing the clamp, Ø96 plate with a Ø80 × 1 pocket (= flexure OD) and 3 × M3 (Ø3.4) on the Ø80 flexure's rim holes; the piston's stack A bolts on it"
  },
  {
   "file": "fp-flexure-ring-80",
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   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-ring-80.stl",
   "note": "Ø96 ring for stack B (displacer rod) on 3 × M3 standoffs above the head: Ø80 × 1 flexure pocket, 3 × Ø3.4"
  },
  {
   "file": "hub-collar-80-b5",
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   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-80-b5.stl",
   "note": "split hub collar Ø17.6 × 5, Ø5 bore, tapped M3 pinch: clamps the leaf hub on the 5 mm tube rod (two per stack)"
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  {
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   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-80-b3.stl",
   "note": "split hub collar Ø17.6 × 5, Ø3 bore, tapped M3 pinch: clamps the leaf hub on the Ø3 displacer rod (two per stack)"
  },
  {
   "file": "hub-bush-80-b3",
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   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-80-b3.stl",
   "note": "centring bush Ø5 OD × Ø3 ID × 14: centres the Ø80 leaves (Ø5 hub bore) on the Ø3 displacer rod; or a 4 mm K&S tube sleeved on the rod"
  },
  {
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   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-piston-plug-2in.stl",
   "note": "2 in piston plug: drops into the open skirt, two Ø3 × 16 wrist dowels through its legs at 23.5 mm retain it, Ø5 tube socket 15 deep, Ø3.4 clearance for the displacer rod (it runs in the crown bushing, not here)"
  },
  {
   "file": "fp-flexure-head-3in",
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   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-head-3in.stl",
   "note": "3 in flexure head: ISO 2852 cap face, Ø76.2 pedestal clearing the clamp, Ø116 plate with a Ø100 × 1 pocket (= flexure OD) and 3 × M3 (Ø3.4) on the Ø100 flexure's rim holes; the piston's stack A bolts on it"
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  {
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   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-ring-100.step",
   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-ring-100.stl",
   "note": "Ø116 ring for stack B (displacer rod) on 3 × M3 standoffs above the head: Ø100 × 1 flexure pocket, 3 × Ø3.4"
  },
  {
   "file": "hub-collar-100-b5",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-100-b5.step",
   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-100-b5.stl",
   "note": "split hub collar Ø22 × 5, Ø5 bore, tapped M3 pinch: clamps the leaf hub on the 5 mm tube rod (two per stack)"
  },
  {
   "file": "hub-collar-100-b3",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-100-b3.step",
   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-100-b3.stl",
   "note": "split hub collar Ø22 × 5, Ø3 bore, tapped M3 pinch: clamps the leaf hub on the Ø3 displacer rod (two per stack)"
  },
  {
   "file": "hub-bush-100-b3",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-100-b3.step",
   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-100-b3.stl",
   "note": "centring bush Ø8 OD × Ø3 ID × 14: centres the Ø100 leaves (Ø8 hub bore) on the Ø3 displacer rod; or a 4 mm K&S tube sleeved on the rod"
  },
  {
   "file": "hub-bush-100-b5",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-100-b5.step",
   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-100-b5.stl",
   "note": "centring bush Ø8 × Ø5 × 14: centres the Ø100 leaves on the 5 mm tube piston rod"
  },
  {
   "file": "fp-piston-plug-3in",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-piston-plug-3in.step",
   "stl": "https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-piston-plug-3in.stl",
   "note": "3 in piston plug: drops into the open skirt, two Ø3 × 16 wrist dowels through its legs at 37.0 mm retain it, Ø5 tube socket 15 deep, Ø3.4 clearance for the displacer rod (it runs in the crown bushing, not here)"
  }
 ],
 "markdown": "https://xemstudio.com/objects/tri-clamp-free-piston-kit/index.md"
}