# Free-piston Stirling displacer Ø29 × 75 and Ø39 × 100

> What size displacer goes in a free-piston Stirling engine?

- Category: Stirling engines › Free-piston
- Kind: Hardware
- Basis: Beta free-piston layout (Sunpower RE-1000 / NASA): displacer = bore − 1.0 (0.5 mm radial gap, designed), length 2.5 × bore
- Page: https://xemstudio.com/objects/free-piston-displacer/ · JSON: https://xemstudio.com/api/geometry?object=free-piston-displacer · Plain text: https://xemstudio.com/objects/free-piston-displacer/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 displacer of the free-piston core: a hollow cylinder with a 1.2 mm wall and a 2 mm rounded top edge, Ø29 × 75 for the Ø30 bore or Ø39 × 100 for the Ø40 bore.
- The 0.5 mm radial gap to the liner is designed: it is a shuttle, not a seal.
- A Ø9 × 8 rod boss with a Ø5 bore (fit: slide) takes the Ø5 displacer rod. Its spring (flexure plus gas spring) is tuned so it leads the piston by about 60–90°.

## 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 |
|---|---|---|---|---|
| rod bores | Ø5 | Ø5 rod | slide | all |
| displacer in the liner | liner bore − 1.0 (0.5 radial) | cylinder liner bore | designed | displacer-* |

## Sizes

| file | diameter | length | bore it runs in | radial gap | wall | rod bore |
|---|---|---|---|---|---|---|
| displacer-30x75 | 29 | 75 | 30 | 0.5 | 1.2 | 5 |
| displacer-40x100 | 39 | 100 | 40 | 0.5 | 1.2 | 5 |

## Where the files are

- NASA (NTRS, public domain): [Free-piston Stirling engine design and analysis (RE-1000 test results) (1984)](https://ntrs.nasa.gov/search?q=19840017543) — PDF — US Government work, public domain
- NASA (NTRS, public domain): [Dynamics of free-piston Stirling engines (1987)](https://ntrs.nasa.gov/search?q=19870011457) — PDF — US Government work, public domain
- Wikipedia: [Free-piston engine](https://en.wikipedia.org/wiki/Free-piston_engine) — web — CC BY-SA 4.0 (text)
- Wikipedia: [Beale number](https://en.wikipedia.org/wiki/Beale_number) — web — CC BY-SA 4.0 (text)
- Ohio University: [Stirling cycle machine analysis (Urieli), free online](https://www.ohio.edu/mechanical/stirling/) — web — Author's site
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (2 parts, CC BY 4.0, Xem Studio)

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

- displacer-30x75: [STEP](https://xemstudio.com/files/hardware/free-piston-core/displacer-30x75.step) · [STL](https://xemstudio.com/files/hardware/free-piston-core/displacer-30x75.stl) — displacer Ø29 (0.5 mm radial gap in a Ø30 bore) × 75, hollow 1.2 wall, Ø5 rod boss
- displacer-40x100: [STEP](https://xemstudio.com/files/hardware/free-piston-core/displacer-40x100.step) · [STL](https://xemstudio.com/files/hardware/free-piston-core/displacer-40x100.stl) — displacer Ø39 (0.5 mm radial gap in a Ø40 bore) × 100, hollow 1.2 wall, Ø5 rod boss

## How it fits

### Goes in or on
- [Free-piston cylinder liner Ø30 × 120 and Ø40 × 160](https://xemstudio.com/objects/free-piston-cylinder-liner/): in the Ø30 / Ø40 bore, 0.5 mm radial gap

### Connects to
- [Ø5 rods with E-clip grooves, 120 and 150 mm](https://xemstudio.com/objects/e-clip-grooved-rods-5mm/): Ø5 × 150 rod in the boss
- [Planar spiral flexure bearing leaves: Ø40 to Ø120, 3-arm and 2-arm, 0.5 and 2 mm](https://xemstudio.com/objects/spiral-flexure-bearing/): rod suspended on a flexure stack

### Alternatives
- [Tri-clamp displacer: Ø33.6, Ø46 and Ø70.9 for 1.5, 2 and 3 in spools](https://xemstudio.com/objects/tri-clamp-displacer/): tri-clamp displacers

## Questions

**How does a free-piston Stirling keep its phase without a crank?**

The displacer is a tuned mass on a spring (flexure and gas spring) that resonates ahead of the piston, typically 60–90°.

**What gap does the free-piston displacer have?**

0.5 mm radial: Ø29 in the Ø30 liner, Ø39 in the Ø40.

## Fits with

- [Free-piston cylinder liner Ø30 × 120 and Ø40 × 160](https://xemstudio.com/objects/free-piston-cylinder-liner/)
- [Free-piston Stirling power piston Ø29.9 × 30 and Ø39.9 × 40](https://xemstudio.com/objects/free-piston-power-piston/)
- [Ø5 rods with E-clip grooves, 120 and 150 mm](https://xemstudio.com/objects/e-clip-grooved-rods-5mm/)
- [Planar spiral flexure bearing leaves: Ø40 to Ø120, 3-arm and 2-arm, 0.5 and 2 mm](https://xemstudio.com/objects/spiral-flexure-bearing/)
- [NASA and Sunpower free-piston Stirling references (public domain) and design numbers](https://xemstudio.com/objects/nasa-fpse-references/)