# Clearance-seal hollow piston: Ø40.00 land, Ø38.50 body, 106 long

> What hollow piston runs in a 40 mm liner with a clearance seal?

- Category: Hardware › Pistons, cylinders & rods
- Kind: Hardware
- Basis: Design values; shuttle loss after Martini; land and gap sizes set by the cycle model
- Page: https://xemstudio.com/objects/clearance-seal-hollow-piston-40/ · JSON: https://xemstudio.com/api/geometry?object=clearance-seal-hollow-piston-40 · Plain text: https://xemstudio.com/objects/clearance-seal-hollow-piston-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 gas must go through the slotted heater, screens and cooler, not round the displacer. A short running land Ø40.00 × 8 at the cold end is a clearance seal in the liner bore; the body behind it is Ø38.50, a designed 0.75 mm radial gap that keeps the shuttle conduction low (the land is cold, the gap is where the temperature changes).
- Stroke 30 mm in both engines; the cold face comes to 10 mm from the hot spool's face and the hot end to 3.4 mm from the floor.
- The cap's Ø0.5 vent lets the inside follow the mean charge slowly, so the shell never carries the charge pressure; the rod screws into the cap's M6 hole and is locked by an M6 nut on the cold face.

## 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 |
|---|---|---|---|---|
| land | Ø40.00 × 8 | liner bore Ø40.00 (clearance seal) | run | displacer-cap* |
| spigot | Ø36.50 × 6 | shell bore Ø36.50 | press | displacer-cap* |
| rod hole | M6 tapped (Ø5.0) × 12 | Ø6 rod, M6 end | designed | displacer-cap* |
| vent | Ø0.5 | lets the inside follow the mean charge | designed | displacer-cap* |
| body | Ø38.50 | liner bore Ø40.00: 0.75 radial gap | designed | displacer-shell* |
| shell bore | Ø36.50 | cap spigot | press | displacer-shell* |

## Sizes

| part | od_mm | length_mm | fit |
|---|---|---|---|
| cap land | 40.0 | 8.0 | run (liner bore) |
| cap spigot | 36.5 | 6.0 | press (shell bore) |
| shell | 38.5 | 98.0 | designed 0.75 gap |

## Where the files are

- NASA (W. R. Martini): [Stirling Engine Design Manual, 2nd edition, NASA CR-168088 (1983): decoupled second-order losses (shuttle, conduction, flow friction, regenerator)](https://ntrs.nasa.gov/citations/19830022057) — PDF — US government report
- 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/clearance-seal-hollow-piston-40/index.json

- displacer-shell-38.5x98: [STEP](https://xemstudio.com/files/hardware/clearance-seal-hollow-piston-40/displacer-shell-38.5x98.step) · [STL](https://xemstudio.com/files/hardware/clearance-seal-hollow-piston-40/displacer-shell-38.5x98.stl) — displacer shell Ø38.50 × 98 (z 8–106), wall 1, closed hot end 1.5 with a 1.0 round
- displacer-cap-40-land: [STEP](https://xemstudio.com/files/hardware/clearance-seal-hollow-piston-40/displacer-cap-40-land.step) · [STL](https://xemstudio.com/files/hardware/clearance-seal-hollow-piston-40/displacer-cap-40-land.stl) — displacer cold cap: running land Ø40.00 × 8, spigot Ø36.50 × 6 into the shell (press), M6 tapped (Ø5.0) × 12, Ø0.5 vent

## Questions

**Why a land only at the cold end?**

A close fit along the whole length would carry heat from hot to cold on every stroke (shuttle loss scales with 1 / gap); the land seals where it is cold.

## Fits with

- [Slotted heat-exchange liner for a 2 in tri-clamp spool, Ø48.00 / Ø40.00](https://xemstudio.com/objects/tri-clamp-slotted-liner-48-40/)
- [Ground rods, hollow rods and crank shafts: Ø6, Ø8 and Ø10 / 7](https://xemstudio.com/objects/precision-rods-and-crank-shafts/)