# HTD displacers: Ø240 flat hollow displacers for a 10 in chamber

> How do I make a large light displacer for a flat Stirling engine?

- Category: Stirling engines › Tri-clamp engine parts
- Kind: Hardware
- Basis: Exact geometry: Ø240 shells in the 10 in bore (Ø247.19), 3.6 radial gap
- Page: https://xemstudio.com/objects/htd-displacers-10in/ · JSON: https://xemstudio.com/api/geometry?object=htd-displacers-10in · Plain text: https://xemstudio.com/objects/htd-displacers-10in/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

- A thin closed shell: 0.5 skins top and bottom and a 0.5 rim, a Ø20 hub through the middle with the rod socket, a Ø1 hole in the top skin so the inside follows the mean pressure (the skins never carry the charge pressure, only the flow's few pascals).
- Mass about 0.57 kg at a density of 7.9 g/cm³ (a lighter shell material scales it). Gap to the bore 3.6 radial: the annulus is the only passage between hot and cold space and the shuttle-loss path (0.5 W air, 2.4 W helium, Martini).
- Heat leaks through the displacer: the rim skin about 44 W and radiation between the skins about 71 W (ε 0.3, no shield); a foil shield inside cuts the radiation.

## 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 |
|---|---|---|---|---|
| OD | Ø240 in the Ø247.19 bore | hot cup bore (3.6 radial, no contact) | designed | all |
| rod socket | Ø6 × 30 (M6 as the major diameter) | displacer rod, M6 thread | press | all |
| end clearance | 1.5 to floor and ceiling at the stroke ends | cup floor, cap | designed | all |
| equalising hole | Ø1 | gas (no contact) | designed | all |

## Sizes

| part | diameter_mm | thickness_mm | stroke_mm | socket_mm |
|---|---|---|---|---|
| displacer-240x40-rod6 | 240.0 | 40.0 | 8.0 | 30.0 |

## Where the files are

- NASA (NTRS, public domain): [W. R. Martini, Stirling Engine Design Manual, NASA CR-168088 (1983): shuttle loss](https://ntrs.nasa.gov/citations/19830022057) — PDF — US Government work, public domain
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (1 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/htd-displacers-10in/index.json

- displacer-240x40-rod6: [STEP](https://xemstudio.com/files/hardware/htd-displacers-10in/displacer-240x40-rod6.step) · [STL](https://xemstudio.com/files/hardware/htd-displacers-10in/displacer-240x40-rod6.stl) — Ø240 × 40 hollow displacer: 0.5 skins and rim, Ø20 hub, Ø6 rod socket 30 deep (M6 thread as the plain bore), Ø1 equalising hole

## Questions

**Why is the displacer hollow with a hole?**

Light, and the hole lets its inside follow the mean pressure, so a charged engine does not crush it.

## Fits with

- [Build list: HTD engine, 10 in tri-clamp (high-temperature-differential flat engine)](https://xemstudio.com/objects/list-htd-10in/)
- [HTD hot cup: 10 in tri-clamp closed-end cup (welded floor)](https://xemstudio.com/objects/htd-hot-cup-10in/)
- [HTD displacer rods: Ø6 × 356.7 and M6 × 83](https://xemstudio.com/objects/htd-displacer-rods/)