# CDs, DVDs and hard-drive platters as flywheels

> What are the dimensions of a CD and its centre hole?

- Category: Hardware › Found & household materials
- Kind: Standard part
- Basis: ECMA-130 / ISO 9660 physical disc: Ø120 × 1.2 mm, Ø15 mm hole; hard-drive platters per the 3.5 in (95 mm) and 2.5 in (65 mm) form factors
- Page: https://xemstudio.com/objects/cd-dvd-and-drive-platters/ · JSON: https://xemstudio.com/api/geometry?object=cd-dvd-and-drive-platters · Plain text: https://xemstudio.com/objects/cd-dvd-and-drive-platters/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

## Notes

- CD/DVD/Blu-ray: Ø120.0 mm, 1.2 mm thick, Ø15.0 mm centre hole, 16 g polycarbonate. Mini CD Ø80. The hole is held to +0.1 mm, so a printed Ø14.9 hub press-fits.
- 3.5 in drive platter: Ø95 × 1.27 mm (some 0.8–1.0), Ø25 mm hole, aluminium (older, 30 g) or glass (newer, do not drill). 2.5 in platter: Ø65 × 0.8, Ø20 hole. Mirror-flat, perfectly balanced: a stack of three 3.5 in platters on the printed hub is a 90 g, Ø95 flywheel.
- The hubs in the can-engine kit take a CD (Ø15 bore) or a 3.5 in platter (Ø25) onto a 5 mm or 8 mm shaft with a set screw. Two CDs glued with a 1 mm spacer are stiffer than one.
- A CD flywheel is 16 g at Ø120: it stores as much energy as a Ø60 × 10 mm steel disc, which is why the can engine runs on it.

## Sizes

| disc | dia_mm | thickness_mm | hole_mm | mass_g |
|---|---|---|---|---|
| CD / DVD | 120 | 1.2 | 15 | 16 |
| mini CD | 80 | 1.2 | 15 | 7 |
| 3.5 in HDD platter | 95 | 1.27 | 25 | 30 |
| 2.5 in HDD platter | 65 | 0.8 | 20 | 7 |

## Where the files are

- Wikipedia: [Compact disc](https://en.wikipedia.org/wiki/Compact_disc) — web — CC BY-SA 4.0 (text)
- Wikipedia: [Hard disk drive platter](https://en.wikipedia.org/wiki/Hard_disk_drive_platter) — web — CC BY-SA 4.0 (text)
- Long Earth Society: [Engine builds (videos and write-ups)](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (4 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/cd-dvd-and-drive-platters/index.json

- cd-dvd-120: [STEP](https://xemstudio.com/files/hardware/cd-dvd-and-drive-platters/cd-dvd-120.step) · [STL](https://xemstudio.com/files/hardware/cd-dvd-and-drive-platters/cd-dvd-120.stl) — CD / DVD / Blu-ray: Ø120 × 1.2, Ø15 hole (Ø15.0 +0.1)
- mini-cd-80: [STEP](https://xemstudio.com/files/hardware/cd-dvd-and-drive-platters/mini-cd-80.step) · [STL](https://xemstudio.com/files/hardware/cd-dvd-and-drive-platters/mini-cd-80.stl) — Mini CD: Ø80 × 1.2, Ø15 hole
- hdd-platter-3-5in: [STEP](https://xemstudio.com/files/hardware/cd-dvd-and-drive-platters/hdd-platter-3-5in.step) · [STL](https://xemstudio.com/files/hardware/cd-dvd-and-drive-platters/hdd-platter-3-5in.stl) — 3.5 in hard-drive platter: Ø95 × 1.27, Ø25 hole (aluminium or glass)
- hdd-platter-2-5in: [STEP](https://xemstudio.com/files/hardware/cd-dvd-and-drive-platters/hdd-platter-2-5in.step) · [STL](https://xemstudio.com/files/hardware/cd-dvd-and-drive-platters/hdd-platter-2-5in.stl) — 2.5 in platter: Ø65 × 0.8, Ø20 hole

## Questions

**What is the diameter of a CD centre hole?**

15.0 mm (+0.1); the disc is Ø120 × 1.2 mm. A printed Ø14.9 mm hub press-fits.

**What are hard drive platter dimensions?**

3.5 in drives: Ø95 mm, 25 mm hole, about 1.27 mm thick (aluminium) or 0.8–1.0 (glass). 2.5 in drives: Ø65, 20 mm hole, 0.8 mm.

**Is a CD a good flywheel?**

For a can or test-tube Stirling, yes: 16 g at Ø120 mm beats a small solid disc; stack two for stiffness and glue a coin at the rim to balance.

## Fits with

- [Can-engine printed kit: can caps, diaphragm clamps, displacer guides, crank stands, CD/platter hubs, syringe cradles](https://xemstudio.com/objects/can-engine-printed-kit/)
- [Crank discs, connecting rods and flywheels for model engines](https://xemstudio.com/objects/crank-conrod-flywheel-kit/)
- [Ball bearings, 683 to 6210 and flanged F6xx (ISO 15)](https://xemstudio.com/objects/ball-bearings-metric/)
- [Aluminium beverage cans: 12 oz standard, 16 oz, 330/500 mL, sleek, slim, mini](https://xemstudio.com/objects/beverage-cans/)