{
 "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-rd-piston",
 "name": "Rolling-diaphragm piston and head retainers, 2 in power head",
 "kind": "hardware",
 "category": "Hardware",
 "group": "Stirling & thermoacoustic",
 "year": null,
 "question": "What piston does a 1.50 in rolling diaphragm need?",
 "tags": [
  "piston",
  "rolling diaphragm",
  "retainer",
  "tri-clamp",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "rolling diaphragm piston",
  "rolling diaphragm retainer",
  "diaphragm piston head clamp"
 ],
 "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/tri-clamp-rd-piston/",
 "sheet_txt": "https://xemstudio.com/objects/tri-clamp-rd-piston/sheet.txt",
 "sheet": {
  "basis": "Piston diameter, corner radius and retainer rule from the published 1.50 in rolling-diaphragm size; screw sizes ISO 7380 / ISO 273",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "The piston is Ø33.274 (1.31 in), the published piston for the 1.50 in bore: it runs 2.41 mm clear of the bore all round and never touches it. Its crown edge has the 1/16 in radius the diaphragm wraps over.",
   "The diaphragm's head is clamped between the crown and a Ø26 × 2 retainer by three M3 × 8 button screws into the crown. The retainer is smaller than the crown's flat (Ø30.1) so it never pinches the bend.",
   "The Ø5 tube rod goes in a socket in the centre boss and is held by an M3 grub at 18 mm, below the highest point the diaphragm's inner wall ever reaches on the skirt (13.4 mm), so the access hole is never under the diaphragm.",
   "The blind retainer closes the centre. The gland retainer has an Ø3 bore so a Ø3 displacer rod runs through the piston (beta): with the crown's Ø3 bore it is an 8 mm long close-running gland."
  ],
  "table": {
   "columns": [
    "part",
    "od_mm",
    "length_mm",
    "notes"
   ],
   "rows": [
    [
     "piston",
     33.274,
     22,
     "crown 6, Ø5 socket, M3 grub"
    ],
    [
     "retainer blind",
     26,
     2,
     "3 × Ø3.4"
    ],
    [
     "retainer gland",
     26,
     2,
     "3 × Ø3.4 + Ø3 gland"
    ]
   ]
  }
 },
 "sheet_text": "Rolling-diaphragm piston and head retainers, 2 in power head — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/tri-clamp-rd-piston/  (CC BY 4.0)\nBasis: Piston diameter, corner radius and retainer rule from the published 1.50 in rolling-diaphragm size; screw sizes ISO 7380 / ISO 273\nCatalog version: 2026-10-20\n\nSizes: see table below\n- The piston is Ø33.274 (1.31 in), the published piston for the 1.50 in bore: it runs 2.41 mm clear of the bore all round and never touches it. Its crown edge has the 1/16 in radius the diaphragm wraps over.\n- The diaphragm's head is clamped between the crown and a Ø26 × 2 retainer by three M3 × 8 button screws into the crown. The retainer is smaller than the crown's flat (Ø30.1) so it never pinches the bend.\n- The Ø5 tube rod goes in a socket in the centre boss and is held by an M3 grub at 18 mm, below the highest point the diaphragm's inner wall ever reaches on the skirt (13.4 mm), so the access hole is never under the diaphragm.\n- The blind retainer closes the centre. The gland retainer has an Ø3 bore so a Ø3 displacer rod runs through the piston (beta): with the crown's Ø3 bore it is an 8 mm long close-running gland.\n\npart  od_mm  length_mm  notes\npiston  33.274  22.0  crown 6, Ø5 socket, M3 grub\nretainer blind  26.0  2.0  3 × Ø3.4\nretainer gland  26.0  2.0  3 × Ø3.4 + Ø3 gland\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- Marsh Bellofram: Rolling diaphragm design manual (2022): nomenclature, height formula, standard-size tables, hardware radii — PDF — publisher's manual; dimensions cited, nothing copied — https://damapi.marshbellofram.com/uploads/Bellofram_Diaphragm_Design_Manual_2022_7a922642c3.pdf\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/tri-clamp-rd-piston/index.json\nAPI: https://xemstudio.com/api/geometry?object=tri-clamp-rd-piston\n",
 "sources": [
  {
   "publisher": "Marsh Bellofram",
   "title": "Rolling diaphragm design manual (2022): nomenclature, height formula, standard-size tables, hardware radii",
   "url": "https://damapi.marshbellofram.com/uploads/Bellofram_Diaphragm_Design_Manual_2022_7a922642c3.pdf",
   "formats": [
    "PDF"
   ],
   "license": "publisher's manual; dimensions cited, nothing copied",
   "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": [
  "rolling-diaphragm-top-hat",
  "tri-clamp-rd-power-head",
  "tri-clamp-rd-yokes",
  "button-head-screws-metric"
 ],
 "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": "piston OD",
   "exact": "Ø33.274 (1.31 in)",
   "mates": "Ø38.10 bore, 2.41 clear all round (the diaphragm rolls between)",
   "fit": "designed",
   "files": "rd-piston"
  },
  {
   "feature": "crown edge radius",
   "exact": "R1.5875 (1/16 in)",
   "mates": "diaphragm head bend",
   "fit": "designed",
   "files": "rd-piston"
  },
  {
   "feature": "tube socket",
   "exact": "Ø5 × 12",
   "mates": "Ø5 tube rod (M3 grub)",
   "fit": "slide",
   "files": "rd-piston"
  },
  {
   "feature": "crown gland bore",
   "exact": "Ø3 × 6",
   "mates": "Ø3 displacer rod (beta): close-running gland",
   "fit": "run",
   "files": "rd-piston"
  },
  {
   "feature": "retainer screw holes",
   "exact": "3 × Ø2.5 × 5.5",
   "mates": "M3 tap drill",
   "fit": "designed",
   "files": "rd-piston"
  },
  {
   "feature": "grub hole",
   "exact": "Ø2.5, access Ø3.4 in the skirt",
   "mates": "M3 tap drill / key access",
   "fit": "designed",
   "files": "rd-piston"
  },
  {
   "feature": "retainer clearance holes",
   "exact": "3 × Ø3.4",
   "mates": "M3 button screws",
   "fit": "designed",
   "files": "rd-retainer"
  },
  {
   "feature": "retainer gland bore",
   "exact": "Ø3 × 2",
   "mates": "Ø3 displacer rod: close-running gland with the crown bore",
   "fit": "run",
   "files": "rd-retainer-26-gland3"
  }
 ],
 "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": "Does the piston need a close fit?",
   "a": "No: it is 2.41 mm smaller than the bore on each side. Only the diaphragm touches both."
  },
  {
   "q": "How is the diaphragm fixed to the piston?",
   "a": "Its head sits on the crown and the retainer clamps it with three M3 × 8 button screws."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "rd-piston-2in",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/tri-clamp-rd-piston/rd-piston-2in.step",
   "stl": "https://xemstudio.com/files/hardware/tri-clamp-rd-piston/rd-piston-2in.stl",
   "note": "rolling-diaphragm piston Ø33.274 × 22: crown 6 with a 1/16 in edge radius, Ø3 gland bore through the crown, 3 × M3 tapped (Ø2.5) × 5.5 on Ø18, Ø14 boss with a Ø5 tube socket 8..20 and an M3 grub (Ø2.5) at 18 reached through a Ø3.4 hole in the 2 mm skirt"
  },
  {
   "file": "rd-retainer-26-blind",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/tri-clamp-rd-piston/rd-retainer-26-blind.step",
   "stl": "https://xemstudio.com/files/hardware/tri-clamp-rd-piston/rd-retainer-26-blind.stl",
   "note": "head retainer Ø26 × 2, 0.5 radius on the clamping edge, 3 × Ø3.4 on Ø18, no centre hole"
  },
  {
   "file": "rd-retainer-26-gland3",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/tri-clamp-rd-piston/rd-retainer-26-gland3.step",
   "stl": "https://xemstudio.com/files/hardware/tri-clamp-rd-piston/rd-retainer-26-gland3.stl",
   "note": "head retainer Ø26 × 2 with an Ø3 gland bore on centre (the beta's displacer rod runs through it)"
  }
 ],
 "markdown": "https://xemstudio.com/objects/tri-clamp-rd-piston/index.md"
}