# Bolt-on counterweight arcs R15, R20 and R25

> What does a bolt-on counterweight for a flywheel look like?

- Category: Stirling engines › Drives & mechanisms
- Kind: Hardware
- Basis: 120° ring segment 6 thick, 8 mm radial width, three Ø3.2 holes at R − 4
- Page: https://xemstudio.com/objects/bolt-on-counterweights/ · JSON: https://xemstudio.com/api/geometry?object=bolt-on-counterweights · Plain text: https://xemstudio.com/objects/bolt-on-counterweights/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

**Exact dimensions.** Every file is drawn at true size with no printing tolerance. The holes and pins that meet another part are listed below under Mating features, with the fit each joint needs; open those by your printer's or process's clearance before making the part.

## 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.

## Mating features (exact size; open press / slide / run / socket by your process clearance, leave designed and clamp as drawn)

| feature | exact | mates with | fit | files |
|---|---|---|---|---|
| bolt holes | Ø3.2 | M3 screw clearance (ISO 273 fine) | designed | bolt-on-counterweight-* |

## Sizes

| file | outer R | inner R | arc | thickness | holes |
|---|---|---|---|---|---|
| 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 |

## Where the files are

- Wikipedia: [Engine balance](https://en.wikipedia.org/wiki/Engine_balance) — web — CC BY-SA 4.0 (text)
- Wikipedia: [Crankshaft](https://en.wikipedia.org/wiki/Crankshaft) — web — CC BY-SA 4.0 (text)
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (3 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/bolt-on-counterweights/index.json

- bolt-on-counterweight-r15: [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) — 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
- bolt-on-counterweight-r20: [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) — 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
- bolt-on-counterweight-r25: [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) — 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

## How it fits

### Goes in or on
- [Spoked flywheels Ø50 to Ø150 × 10](https://xemstudio.com/objects/spoked-flywheels/): bolted to the face opposite the pin

### Alternatives
- [Flywheels Ø100–Ø200 with hex-nut rim pockets](https://xemstudio.com/objects/nut-pocket-flywheels/): nuts in one half of the rim

### Also needed
- [Socket head cap screws, M2–M8 (ISO 4762 / DIN 912)](https://xemstudio.com/objects/socket-head-cap-screws-metric/): 3 × M3

## Questions

**Where does a bolt-on counterweight go?**

On the flywheel or crank disc, centred opposite the crank pin, on three M3 screws.

**What is the hole spacing?**

Three Ø3.2 holes at R − 4, 40° apart.

## Fits with

- [Counterweighted crank discs, Ø5 bore, 10 to 30 mm stroke](https://xemstudio.com/objects/counterweighted-crank-discs/)
- [Spoked flywheels Ø50 to Ø150 × 10](https://xemstudio.com/objects/spoked-flywheels/)
- [Flywheels Ø100–Ø200 with hex-nut rim pockets](https://xemstudio.com/objects/nut-pocket-flywheels/)
- [Socket head cap screws, M2–M8 (ISO 4762 / DIN 912)](https://xemstudio.com/objects/socket-head-cap-screws-metric/)