# Power piston Ø16, 20, 25, 30 with Ø3 wrist-pin boss

> What are the dimensions of a power piston for a Stirling engine power cylinder?

- Category: Stirling engines › Drives & mechanisms
- Kind: Hardware
- Basis: Piston OD = liner bore (fit: run), length 0.9 × bore; Ø3 rod hole and Ø3 wrist-pin hole
- Page: https://xemstudio.com/objects/power-piston/ · JSON: https://xemstudio.com/api/geometry?object=power-piston · Plain text: https://xemstudio.com/objects/power-piston/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

- The piston for the flanged liners, drawn at the liner bore exactly: Ø16 × 14, Ø20 × 18, Ø25 × 22, Ø30 × 27, with a 0.5 chamfer at the crown.
- Hollow with 2 mm walls; its closed end has a Ø3 hole (fit: slide) for a Ø3 piston rod.
- Inside the -blank piston, a wrist-pin bar across the bore carries a Ø3 hole (fit: press) at 0.4 × length + 4 from the closed end.
- Two kinds per bore: the -blank piston has a Ø3 rod hole in its crown and a solid wrist-pin bar (for a crosshead and piston rod); the -trunk piston has a closed 2 mm crown and two wrist-pin bosses 7 mm apart, so a 5 mm con-rod's small end (R7 eye) goes straight on its Ø3 wrist pin with 1 mm each side.

## 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 |
|---|---|---|---|---|
| piston OD in the liner | Ø16 / Ø20 / Ø25 / Ø30 (= liner bore) | liner bore (lapped clearance seal) | run | piston-* |
| piston rod hole | Ø3 | Ø3 piston rod | slide | piston-* |
| piston wrist-pin hole | Ø3 | wrist-pin-3x16 | press | piston-* |
| trunk-piston boss gap | 7 | 5 mm con-rod small end, R7 eye (no contact) | designed | piston-* |

## Sizes

| file | diameter | length | rod hole | wrist-pin hole |
|---|---|---|---|---|
| piston-16x14-blank | 16 | 14 | 3 | 3 |
| piston-20x18-blank | 20 | 18 | 3 | 3 |
| piston-25x22-blank | 25 | 22 | 3 | 3 |
| piston-30x27-blank | 30 | 27 | 3 | 3 |

## Where the files are

- Wikipedia: [Crosshead](https://en.wikipedia.org/wiki/Crosshead) — web — CC BY-SA 4.0 (text)
- Wikipedia: [Piston](https://en.wikipedia.org/wiki/Piston) — web — CC BY-SA 4.0 (text)
- James R. Senft: [Miniature Stirling engine piston fits (LTD book)](https://www.stirlingengine.com/) — book — reference (verify at link)
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (8 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/power-piston/index.json

- piston-16x14-blank: [STEP](https://xemstudio.com/files/hardware/power-cylinder-piston-crosshead/piston-16x14-blank.step) · [STL](https://xemstudio.com/files/hardware/power-cylinder-piston-crosshead/piston-16x14-blank.stl) — piston Ø16 (= the liner bore; clearance-seal running fit) × 14: Ø3 rod hole and a Ø3 wrist-pin boss
- piston-16x14-trunk: [STEP](https://xemstudio.com/files/hardware/power-cylinder-piston-crosshead/piston-16x14-trunk.step) · [STL](https://xemstudio.com/files/hardware/power-cylinder-piston-crosshead/piston-16x14-trunk.stl) — trunk piston Ø16 (= the liner bore) × 14: closed 2 mm crown, open skirt, two wrist-pin bosses 7 apart for a 5 mm con-rod's small end, Ø3 wrist pin across the bore 9.6 above the crown face
- piston-20x18-blank: [STEP](https://xemstudio.com/files/hardware/power-cylinder-piston-crosshead/piston-20x18-blank.step) · [STL](https://xemstudio.com/files/hardware/power-cylinder-piston-crosshead/piston-20x18-blank.stl) — piston Ø20 (= the liner bore; clearance-seal running fit) × 18: Ø3 rod hole and a Ø3 wrist-pin boss
- piston-20x18-trunk: [STEP](https://xemstudio.com/files/hardware/power-cylinder-piston-crosshead/piston-20x18-trunk.step) · [STL](https://xemstudio.com/files/hardware/power-cylinder-piston-crosshead/piston-20x18-trunk.stl) — trunk piston Ø20 (= the liner bore) × 18: closed 2 mm crown, open skirt, two wrist-pin bosses 7 apart for a 5 mm con-rod's small end, Ø3 wrist pin across the bore 11.2 above the crown face
- piston-25x22-blank: [STEP](https://xemstudio.com/files/hardware/power-cylinder-piston-crosshead/piston-25x22-blank.step) · [STL](https://xemstudio.com/files/hardware/power-cylinder-piston-crosshead/piston-25x22-blank.stl) — piston Ø25 (= the liner bore; clearance-seal running fit) × 22: Ø3 rod hole and a Ø3 wrist-pin boss
- piston-25x22-trunk: [STEP](https://xemstudio.com/files/hardware/power-cylinder-piston-crosshead/piston-25x22-trunk.step) · [STL](https://xemstudio.com/files/hardware/power-cylinder-piston-crosshead/piston-25x22-trunk.stl) — trunk piston Ø25 (= the liner bore) × 22: closed 2 mm crown, open skirt, two wrist-pin bosses 7 apart for a 5 mm con-rod's small end, Ø3 wrist pin across the bore 12.8 above the crown face
- piston-30x27-blank: [STEP](https://xemstudio.com/files/hardware/power-cylinder-piston-crosshead/piston-30x27-blank.step) · [STL](https://xemstudio.com/files/hardware/power-cylinder-piston-crosshead/piston-30x27-blank.stl) — piston Ø30 (= the liner bore; clearance-seal running fit) × 27: Ø3 rod hole and a Ø3 wrist-pin boss
- piston-30x27-trunk: [STEP](https://xemstudio.com/files/hardware/power-cylinder-piston-crosshead/piston-30x27-trunk.step) · [STL](https://xemstudio.com/files/hardware/power-cylinder-piston-crosshead/piston-30x27-trunk.stl) — trunk piston Ø30 (= the liner bore) × 27: closed 2 mm crown, open skirt, two wrist-pin bosses 7 apart for a 5 mm con-rod's small end, Ø3 wrist pin across the bore 14.8 above the crown face

## How it fits

### Goes in or on
- [Flanged power cylinder liner Ø16, 20, 25, 30](https://xemstudio.com/objects/power-cylinder-liner/): in the liner of the same bore (run)

### Connects to
- [Wrist pin Ø3 × 16](https://xemstudio.com/objects/wrist-pin-3x16/): pin pressed in the boss
- [Crosshead Ø8 × 24 for a Ø3 piston rod](https://xemstudio.com/objects/crosshead-8mm/): Ø3 piston rod to the crosshead
- [Crank discs for a Ø5 shaft, 10 to 30 mm stroke](https://xemstudio.com/objects/crank-discs-5mm-bore/): con-rod on the wrist pin (trunk style)

### Alternatives
- [Syringe pistons: ring-less pistons with a con-rod fork for 10, 20 and 30 mL syringe barrels](https://xemstudio.com/objects/syringe-pistons/): syringe pistons
- [Free-piston Stirling power piston Ø29.9 × 30 and Ø39.9 × 40](https://xemstudio.com/objects/free-piston-power-piston/): Ø30 / Ø40 clearance-seal pistons

## Questions

**What size piston goes in the 20 mm liner?**

Ø20 × 18, drawn at the bore; the fit is run (see Mating features).

**Does the piston need a crosshead?**

With a crosshead the piston only moves axially; trunk style, the con-rod goes straight on the wrist pin, fine for rods at least 4 × the stroke.

## Fits with

- [Flanged power cylinder liner Ø16, 20, 25, 30](https://xemstudio.com/objects/power-cylinder-liner/)
- [Wrist pin Ø3 × 16](https://xemstudio.com/objects/wrist-pin-3x16/)
- [Crosshead Ø8 × 24 for a Ø3 piston rod](https://xemstudio.com/objects/crosshead-8mm/)
- [Build list: Gamma Stirling, 2 in tri-clamp](https://xemstudio.com/objects/list-gamma-2in-tri-clamp/)