{
 "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": "bolt-on-counterweights",
 "name": "Bolt-on counterweight arcs R15, R20 and R25",
 "kind": "hardware",
 "category": "Stirling engines",
 "group": "Drives & mechanisms",
 "year": null,
 "question": "What does a bolt-on counterweight for a flywheel look like?",
 "tags": [
  "counterweight",
  "balance",
  "flywheel",
  "arc",
  "stirling",
  "long earth society"
 ],
 "aliases": [
  "bolt on counterweight",
  "flywheel balance weight",
  "counterweight arc stl"
 ],
 "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/bolt-on-counterweights/",
 "sheet_txt": "https://xemstudio.com/objects/bolt-on-counterweights/sheet.txt",
 "sheet": {
  "basis": "120° ring segment 6 thick, 8 mm radial width, three Ø3.2 holes at R − 4",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "notes": [
   "A 120° arc 6 thick and 8 mm wide radially: R15/R7, R20/R12, R25/R17 (outer / inner).",
   "Three Ø3.2 holes (M3 clearance) at R − 4 (R11, R16, R21), at the centre and ±40°.",
   "It bolts to a flywheel or crank disc opposite the pin; moving it along matching holes trims the balance."
  ],
  "table": {
   "columns": [
    "file",
    "outer R",
    "inner R",
    "arc",
    "thickness",
    "holes"
   ],
   "rows": [
    [
     "bolt-on-counterweight-r15",
     15,
     7,
     "120°",
     6,
     "3 × Ø3.2 at R11"
    ],
    [
     "bolt-on-counterweight-r20",
     20,
     12,
     "120°",
     6,
     "3 × Ø3.2 at R16"
    ],
    [
     "bolt-on-counterweight-r25",
     25,
     17,
     "120°",
     6,
     "3 × Ø3.2 at R21"
    ]
   ]
  }
 },
 "sheet_text": "Bolt-on counterweight arcs R15, R20 and R25 — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/bolt-on-counterweights/  (CC BY 4.0)\nBasis: 120° ring segment 6 thick, 8 mm radial width, three Ø3.2 holes at R − 4\nCatalog version: 2026-10-20\n\nSizes: see table below\n- A 120° arc 6 thick and 8 mm wide radially: R15/R7, R20/R12, R25/R17 (outer / inner).\n- Three Ø3.2 holes (M3 clearance) at R − 4 (R11, R16, R21), at the centre and ±40°.\n- It bolts to a flywheel or crank disc opposite the pin; moving it along matching holes trims the balance.\n\nfile  outer R  inner R  arc  thickness  holes\nbolt-on-counterweight-r15  15  7  120°  6  3 × Ø3.2 at R11\nbolt-on-counterweight-r20  20  12  120°  6  3 × Ø3.2 at R16\nbolt-on-counterweight-r25  25  17  120°  6  3 × Ø3.2 at R21\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- Wikipedia: Engine balance — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Engine_balance\n- Wikipedia: Crankshaft — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Crankshaft\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/bolt-on-counterweights/index.json\nAPI: https://xemstudio.com/api/geometry?object=bolt-on-counterweights\n",
 "sources": [
  {
   "publisher": "Wikipedia",
   "title": "Engine balance",
   "url": "https://en.wikipedia.org/wiki/Engine_balance",
   "formats": [
    "web"
   ],
   "license": "CC BY-SA 4.0 (text)",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Wikipedia",
   "title": "Crankshaft",
   "url": "https://en.wikipedia.org/wiki/Crankshaft",
   "formats": [
    "web"
   ],
   "license": "CC BY-SA 4.0 (text)",
   "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": [
  "counterweighted-crank-discs",
  "spoked-flywheels",
  "nut-pocket-flywheels",
  "socket-head-cap-screws-metric"
 ],
 "fits": [
  {
   "role": "Goes in or on",
   "items": [
    {
     "slug": "spoked-flywheels",
     "why": "bolted to the face opposite the pin"
    }
   ]
  },
  {
   "role": "Alternatives",
   "items": [
    {
     "slug": "nut-pocket-flywheels",
     "why": "nuts in one half of the rim"
    }
   ]
  },
  {
   "role": "Also needed",
   "items": [
    {
     "slug": "socket-head-cap-screws-metric",
     "why": "3 × M3"
    }
   ]
  }
 ],
 "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": "bolt holes",
   "exact": "Ø3.2",
   "mates": "M3 screw clearance (ISO 273 fine)",
   "fit": "designed",
   "files": "bolt-on-counterweight-"
  }
 ],
 "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": "Where does a bolt-on counterweight go?",
   "a": "On the flywheel or crank disc, centred opposite the crank pin, on three M3 screws."
  },
  {
   "q": "What is the hole spacing?",
   "a": "Three Ø3.2 holes at R − 4, 40° apart."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "bolt-on-counterweight-r15",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/counterweighted-cranks/bolt-on-counterweight-r15.step",
   "stl": "https://xemstudio.com/files/hardware/counterweighted-cranks/bolt-on-counterweight-r15.stl",
   "note": "bolt-on counterweight arc, outer R15 / inner R7, 120°, 6 thick, 3 × M3 clearance (Ø3.2) at R11; add to any flywheel or disc opposite the pin"
  },
  {
   "file": "bolt-on-counterweight-r20",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/counterweighted-cranks/bolt-on-counterweight-r20.step",
   "stl": "https://xemstudio.com/files/hardware/counterweighted-cranks/bolt-on-counterweight-r20.stl",
   "note": "bolt-on counterweight arc, outer R20 / inner R12, 120°, 6 thick, 3 × M3 clearance (Ø3.2) at R16; add to any flywheel or disc opposite the pin"
  },
  {
   "file": "bolt-on-counterweight-r25",
   "formats": [
    "step",
    "stl"
   ],
   "price_usd": 0,
   "license": "CC BY 4.0 (Xem Studio)",
   "step": "https://xemstudio.com/files/hardware/counterweighted-cranks/bolt-on-counterweight-r25.step",
   "stl": "https://xemstudio.com/files/hardware/counterweighted-cranks/bolt-on-counterweight-r25.stl",
   "note": "bolt-on counterweight arc, outer R25 / inner R17, 120°, 6 thick, 3 × M3 clearance (Ø3.2) at R21; add to any flywheel or disc opposite the pin"
  }
 ],
 "markdown": "https://xemstudio.com/objects/bolt-on-counterweights/index.md"
}