# Pistons Ø29.9 × 30 and Ø39.9 × 40

> What piston fits a 30 or 40 mm cylinder liner?

- Category: Hardware › Pistons, cylinders & rods
- Kind: Hardware
- Basis: Beta free-piston layout; piston drawn at liner bore − 0.1 (0.05 mm radial clearance-seal gap as drawn)
- Page: https://xemstudio.com/objects/pistons-30-40/ · JSON: https://xemstudio.com/api/geometry?object=pistons-30-40 · Plain text: https://xemstudio.com/objects/pistons-30-40/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

**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 ring-less power piston of the free-piston core: Ø29.9 × 30 for the Ø30 liner, Ø39.9 × 40 for the Ø40.
- As drawn it is 0.1 mm under the bore, a 0.05 mm radial gap: the seal is that gap, with no rings.
- Hollow with 2 mm walls, open at the top, with a Ø5 bore (fit: slide) for the Ø5 rod in its closed end.

## 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 |
| power piston in the liner | Ø30 / Ø40 (= liner bore) | cylinder liner bore Ø30 / Ø40 | run | power-piston-* |

## Sizes

| file | diameter | length | liner bore | radial gap | rod bore |
|---|---|---|---|---|---|
| power-piston-30x30 | 29.9 | 30 | 30 | 0.05 | 5 |
| power-piston-40x40 | 39.9 | 40 | 40 | 0.05 | 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/pistons-30-40/index.json

- power-piston-30x30: [STEP](https://xemstudio.com/files/hardware/pistons-30-40/power-piston-30x30.step) · [STL](https://xemstudio.com/files/hardware/pistons-30-40/power-piston-30x30.stl) — power piston Ø30 (= the Ø30 liner bore; clearance-seal running fit) × 30, Ø5 rod bore
- power-piston-40x40: [STEP](https://xemstudio.com/files/hardware/pistons-30-40/power-piston-40x40.step) · [STL](https://xemstudio.com/files/hardware/pistons-30-40/power-piston-40x40.stl) — power piston Ø40 (= the Ø40 liner bore; clearance-seal running fit) × 40, Ø5 rod bore

## How it fits

### Goes in or on
- [Cylinder liners Ø30 × 120 and Ø40 × 160](https://xemstudio.com/objects/cylinder-liners-30-40/): in the Ø30 / Ø40 bore

### Connects to
- [Ø5 rods with E-clip grooves, 120 and 150 mm](https://xemstudio.com/objects/e-clip-grooved-rods-5mm/): Ø5 × 120 rod
- [Linear alternator mover: Ø35 carrier for 16 Ø10 × 3 magnets](https://xemstudio.com/objects/linear-alternator-mover/): mover on the same rod
- [Piston stop bumpers Ø30 and Ø40 × 6](https://xemstudio.com/objects/piston-stop-bumper/): bumpers at both ends of its travel

### Alternatives
- [Pistons Ø16, 20, 25 and 30 with a Ø3 wrist-pin boss](https://xemstudio.com/objects/power-piston/): flanged-liner pistons Ø16–30

## Questions

**What is a clearance seal?**

A piston with no rings that seals by running very close to its bore; leakage through the gap over one cycle is kept to a few percent of the swept volume.

**What size is the 40 mm free-piston power piston?**

Ø39.9 × 40, hollow, Ø5 rod bore, for the Ø40 liner.

## Fits with

- [Cylinder liners Ø30 × 120 and Ø40 × 160](https://xemstudio.com/objects/cylinder-liners-30-40/)
- [Hollow pistons Ø29 × 75 and Ø39 × 100](https://xemstudio.com/objects/hollow-pistons-30-40/)
- [Small pistons Ø14.9 × 15 and Ø19.9 × 20](https://xemstudio.com/objects/gas-spring-piston/)
- [Ø5 rods with E-clip grooves, 120 and 150 mm](https://xemstudio.com/objects/e-clip-grooved-rods-5mm/)
- [Linear alternator mover: Ø35 carrier for 16 Ø10 × 3 magnets](https://xemstudio.com/objects/linear-alternator-mover/)