# HTD power pistons: Ø48.00 × 72 for the bored 2 in spool

> What piston goes in a bored 2 inch tri-clamp spool?

- Category: Stirling engines › Tri-clamp engine parts
- Kind: Hardware
- Basis: Exact geometry at the finished bore Ø48.00
- Page: https://xemstudio.com/objects/htd-power-pistons/ · JSON: https://xemstudio.com/api/geometry?object=htd-power-pistons · Plain text: https://xemstudio.com/objects/htd-power-pistons/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

- Ring-less: the seal is the fit. Two 8 lands carry it, 0.4 relief between them, 72 long (1.5 × bore) so it cannot cock. The crown faces the working space (down).
- The con-rod runs beside the axis (x 10.5..15.5) between two cheeks; the displacer rod passes up the centre tube (main piston) without touching the con-rod. The offset puts a small tilting moment on the piston, which the 72 length carries.

## 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 | Ø48.00, two 8 lands | bored spool Ø48.00 (ring-less clearance seal) | run | all |
| centre bore | Ø6 × 72 | displacer rod (also the bleed to the crankcase) | run | piston-48-rod6* |
| pin bore | Ø5 in both cheeks | dowel-5x16 | press | all |
| cheek gap | 6.0 (x 10..16) | 5 thick con-rod (0.5 each side) | designed | all |

## Sizes

| part | od_mm | length_mm | centre_bore_mm | pin |
|---|---|---|---|---|
| piston-48-rod6 | 48.0 | 72.0 | 6.0 | Ø5 at z 86, x 5..21 |

## Where the files are

- 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-power-pistons/index.json

- piston-48-rod6: [STEP](https://xemstudio.com/files/hardware/htd-power-pistons/piston-48-rod6.step) · [STL](https://xemstudio.com/files/hardware/htd-power-pistons/piston-48-rod6.stl) — Ø48.00 × 72 power piston: two 8 lands, 0.4 relief, 8 crown, Ø14 centre tube with the Ø6 displacer-rod bore, cross bar and two cheeks with a Ø5 pin bore 14 above the skirt (con-rod at x 10.5..15.5)

## Questions

**How does the displacer rod get past the piston?**

Through the piston's centre tube (Ø6 running fit, 72 long), which is also the bleed between working space and crankcase.

## Fits with

- [Build list: HTD engine, 10 in tri-clamp (high-temperature-differential flat engine)](https://xemstudio.com/objects/list-htd-10in/)
- [Bored 2 in power spool: 2 in × 5 in spool bored to Ø48.00](https://xemstudio.com/objects/htd-bored-power-spool/)
- [HTD crank and linkage: webs, link web, con-rods, clevis and Ø12 shafts for a 3 in cross crankcase](https://xemstudio.com/objects/htd-crank-linkage/)