{
 "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-power-piston",
 "name": "Free-piston power piston: Ø48 clearance-seal land, 40-magnet carrier, top plate and tube rod",
 "kind": "hardware",
 "category": "Hardware",
 "group": "Stirling & thermoacoustic",
 "year": null,
 "question": "What does the power piston of a moving-magnet free-piston generator look like?",
 "tags": [
  "piston",
  "mover",
  "free piston",
  "magnets",
  "linear alternator",
  "tri-clamp",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "free piston mover magnets",
  "moving magnet piston",
  "clearance seal piston"
 ],
 "overall_mm": {
  "length": null,
  "width": null,
  "height": null
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": true,
 "free_files": 3,
 "url": "https://xemstudio.com/objects/fpse-power-piston/",
 "sheet_txt": "https://xemstudio.com/objects/fpse-power-piston/sheet.txt",
 "sheet": {
  "basis": "Design values of the free-piston generator: running fit in the 48.00 bore over a 30 mm land; four magnet rows S N N S facing out; moving mass from the CAD at 7.9 g/cm³ and 7.5 g/cm³ (magnets)",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "The piston is also the alternator's mover. Its lower 30 mm is the seal land, a running fit in the bored spool (the clearance seal between working space and bounce space); above it a Ø47 carrier (0.5 radial gap, no contact) holds 40 Ø12 × 3 disc magnets in four rows, faces out: S, N, N, S from the crown up.",
   "The carrier's body is the magnetic return between the rows, so it must be magnetically soft; the rest of the piston only has to be stiff and light. Moving mass with tube rod, collars and magnets: 918 g.",
   "The displacer rod runs through the Ø12 hub bore (a running fit 76 mm long, also a seal); the Ø23 crown pocket clears leaf B's collar. The top plate closes the carrier bore, carries the tube rod and vents the piston's inside to the bounce space.",
   "The tube rod carries the piston's two leaves (joints C and D) on four Ø28 collars."
  ],
  "table": {
   "columns": [
    "part",
    "size"
   ],
   "rows": [
    [
     "seal land",
     "Ø48.00 × 30"
    ],
    [
     "carrier",
     "Ø47 × 60, bore Ø30"
    ],
    [
     "magnets",
     "40 × Ø12 × 3, rows S N N S at 14 pitch"
    ],
    [
     "hub bore",
     "Ø12, 76 long"
    ],
    [
     "tube rod",
     "Ø18 / Ø14 × 142.15"
    ],
    [
     "moving mass",
     "918 g"
    ]
   ]
  }
 },
 "sheet_text": "Free-piston power piston: Ø48 clearance-seal land, 40-magnet carrier, top plate and tube rod — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/fpse-power-piston/  (CC BY 4.0)\nBasis: Design values of the free-piston generator: running fit in the 48.00 bore over a 30 mm land; four magnet rows S N N S facing out; moving mass from the CAD at 7.9 g/cm³ and 7.5 g/cm³ (magnets)\nCatalog version: 2026-10-20\n\nSizes: see table below\n- The piston is also the alternator's mover. Its lower 30 mm is the seal land, a running fit in the bored spool (the clearance seal between working space and bounce space); above it a Ø47 carrier (0.5 radial gap, no contact) holds 40 Ø12 × 3 disc magnets in four rows, faces out: S, N, N, S from the crown up.\n- The carrier's body is the magnetic return between the rows, so it must be magnetically soft; the rest of the piston only has to be stiff and light. Moving mass with tube rod, collars and magnets: 918 g.\n- The displacer rod runs through the Ø12 hub bore (a running fit 76 mm long, also a seal); the Ø23 crown pocket clears leaf B's collar. The top plate closes the carrier bore, carries the tube rod and vents the piston's inside to the bounce space.\n- The tube rod carries the piston's two leaves (joints C and D) on four Ø28 collars.\n\npart  size\nseal land  Ø48.00 × 30\ncarrier  Ø47 × 60, bore Ø30\nmagnets  40 × Ø12 × 3, rows S N N S at 14 pitch\nhub bore  Ø12, 76 long\ntube rod  Ø18 / Ø14 × 142.15\nmoving mass  918 g\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- K&J Magnetics: Neodymium magnet specifications: N42 Br 1.30–1.32 T, density 7.4–7.5 g/cm³ — web — maker page; values are facts [verify license at link] — https://www.kjmagnetics.com/specs.asp\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, 3 parts): https://xemstudio.com/files/hardware/fpse-power-piston/index.json\nAPI: https://xemstudio.com/api/geometry?object=fpse-power-piston\n",
 "sources": [
  {
   "publisher": "K&J Magnetics",
   "title": "Neodymium magnet specifications: N42 Br 1.30–1.32 T, density 7.4–7.5 g/cm³",
   "url": "https://www.kjmagnetics.com/specs.asp",
   "formats": [
    "web"
   ],
   "license": "maker page; values are facts",
   "license_verified": false,
   "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": [
  "tri-clamp-bored-spool",
  "fpse-linear-alternator-stator",
  "neodymium-disc-magnets",
  "tri-clamp-flexure-leaf",
  "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": "seal land",
   "exact": "Ø48.00 × 30",
   "mates": "S4 bore Ø48.00 (clearance seal)",
   "fit": "run",
   "files": "fpse-piston-48"
  },
  {
   "feature": "carrier OD",
   "exact": "Ø47.0",
   "mates": "S4 bore (0.5 radial gap, no contact)",
   "fit": "designed",
   "files": "fpse-piston-48"
  },
  {
   "feature": "magnet pockets",
   "exact": "40 × Ø12 × 3 radial",
   "mates": "Ø12 × 3 disc magnets",
   "fit": "press",
   "files": "fpse-piston-48"
  },
  {
   "feature": "hub bore",
   "exact": "Ø12 × 76",
   "mates": "displacer rod (seal and guide)",
   "fit": "run",
   "files": "fpse-piston-48"
  },
  {
   "feature": "carrier bore top",
   "exact": "Ø30 × 4",
   "mates": "top plate",
   "fit": "press",
   "files": "fpse-piston-48"
  },
  {
   "feature": "crown pocket",
   "exact": "Ø23 × 6",
   "mates": "displacer collar Ø20 (no contact)",
   "fit": "designed",
   "files": "fpse-piston-48"
  },
  {
   "feature": "bore",
   "exact": "Ø18",
   "mates": "tube rod",
   "fit": "press",
   "files": "fpse-piston-top"
  },
  {
   "feature": "OD",
   "exact": "Ø18",
   "mates": "top plate bore, leaf hubs, collars",
   "fit": "slide",
   "files": "fpse-tube-rod"
  },
  {
   "feature": "bore",
   "exact": "Ø14",
   "mates": "displacer rod Ø12 (no contact)",
   "fit": "designed",
   "files": "fpse-tube-rod"
  }
 ],
 "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 is the piston the mover?",
   "a": "It keeps the moving mass in one part, needs no separate rod to a mover, and puts the magnets right inside the bored wall, where the stator outside can reach them through 1.9 mm of gap and wall."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "fpse-piston-48-40mag",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/fpse-power-piston/fpse-piston-48-40mag.step",
   "stl": "https://xemstudio.com/files/hardware/fpse-power-piston/fpse-piston-48-40mag.stl",
   "note": "piston body: seal land Ø48.00 × 30 (crown 8 with a Ø23 × 6 pocket, skirt bore Ø44), magnet carrier Ø47 × 60 (bore Ø30) with 4 rows × 10 radial pockets Ø12 × 3 at z 37, 51, 65, 79, hub tube Ø20 / Ø12 (the rod's running fit)"
  },
  {
   "file": "fpse-piston-top-plate",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/fpse-power-piston/fpse-piston-top-plate.step",
   "stl": "https://xemstudio.com/files/hardware/fpse-power-piston/fpse-piston-top-plate.stl",
   "note": "piston top plate Ø30 × 4 in the carrier bore on the hub's end, Ø18 bore for the tube rod, 4 × Ø2 vents"
  },
  {
   "file": "fpse-tube-rod-18x14",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/fpse-power-piston/fpse-tube-rod-18x14.step",
   "stl": "https://xemstudio.com/files/hardware/fpse-power-piston/fpse-tube-rod-18x14.stl",
   "note": "tube rod Ø18 / Ø14 × 142.15: from the piston's hub to above leaf D"
  }
 ],
 "markdown": "https://xemstudio.com/objects/fpse-power-piston/index.md"
}