# Free-piston internal fin inserts: slotted heater and cooler sleeves for a 2 in tri-clamp bore (optional)

> Do I need internal fins in a tri-clamp Stirling heater and cooler?

- Category: Hardware › Stirling & thermoacoustic
- Kind: Hardware
- Basis: Design values of the free-piston generator: 36 axial slots 1.5 × 2.5 in a sleeve whose OD is the 2 in bore (Ø47.5); heat-transfer and flow numbers from the generator's simple analysis
- Page: https://xemstudio.com/objects/fpse-fin-inserts/ · JSON: https://xemstudio.com/api/geometry?object=fpse-fin-inserts · Plain text: https://xemstudio.com/objects/fpse-fin-inserts/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

- Two slotted sleeves that slide into the plain tri-clamp parts: the heater insert on the liner's lower end inside the closed-end spool, the cooler insert pressed into the 3 in spool under the water jacket. The gas runs along the 36 slots, between fins that touch the wall.
- Optional: without them the heater is the plain 3.0 mm annulus round the liner and the cooler the 4.05 mm annulus round the displacer. The engine still runs; my analysis gives 6.6 W instead of 43.2 W at the same stroke (helium, 10 bar), because the gas-side heat transfer falls and the cooler annulus adds dead volume.
- Fits: the heater insert slides on the liner (press) and its OD against the hot spool's bore (slide); the cooler insert's OD is pressed into S3, its Ø40 bore is the displacer's 0.30 gap.

## 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 |
|---|---|---|---|---|
| bore | Ø41.5 | liner tube OD | press | fpse-heater-insert* |
| OD | Ø47.5 | hot spool bore (hot wall) | slide | fpse-heater-insert* |
| OD | Ø47.5 | S3 spool bore | press | fpse-cooler-insert* |
| bore | Ø40.00 | displacer Ø39.4 (0.30 radial gap, no contact) | designed | fpse-cooler-insert* |
| slots | 36 × 1.5 wide × 2.5 deep | gas passages (no contact) | designed | all |

## Sizes

| insert | id_mm | od_mm | length_mm | slots | slot_mm |
|---|---|---|---|---|---|
| heater | 41.5 | 47.5 | 68.05 | 36 | 1.5 × 2.5 |
| cooler | 40.0 | 47.5 | 73.2 | 36 | 1.5 × 2.5 |

## Where the files are

- Ohio University (Urieli): [Stirling cycle machine analysis: ideal adiabatic and simple analysis (after Urieli & Berchowitz, Stirling Cycle Engine Analysis, 1984)](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/fpse-fin-inserts/index.json

- fpse-heater-insert-41.5x47.5: [STEP](https://xemstudio.com/files/hardware/fpse-fin-inserts/fpse-heater-insert-41.5x47.5.step) · [STL](https://xemstudio.com/files/hardware/fpse-fin-inserts/fpse-heater-insert-41.5x47.5.stl) — heater insert (optional): Ø41.5 / Ø47.5 × 68.05, 36 slots 1.5 × 2.5 deep in the OD, full length; on the liner's lower end, OD against the hot wall
- fpse-cooler-insert-40x47.5: [STEP](https://xemstudio.com/files/hardware/fpse-fin-inserts/fpse-cooler-insert-40x47.5.step) · [STL](https://xemstudio.com/files/hardware/fpse-fin-inserts/fpse-cooler-insert-40x47.5.stl) — cooler insert (optional): Ø40 / Ø47.5 × 73.2, 36 slots 1.5 × 2.5 deep in the OD; pressed into the 3 in spool S3, its bore continues the displacer's bore

## Questions

**What if I leave them out?**

The engine is complete without them; expect about 6.6 W instead of 43.2 W at 10 bar helium.

## Fits with

- [Free-piston displacer liner: Ø40 bore, regenerator inner wall, rim clamped in the tri-clamp joint](https://xemstudio.com/objects/fpse-displacer-cylinder/)
- [Clamp-on water jacket for a 2 in tri-clamp tube, 20 mm, with hose spigots (optional)](https://xemstudio.com/objects/tri-clamp-clamp-on-water-jacket/)
- [Clamp-on hot-end fin sleeve for a 2 in tri-clamp tube (optional)](https://xemstudio.com/objects/tri-clamp-hot-end-fin-sleeve/)
- [Build list: Free-piston Stirling generator, 2 in tri-clamp (pressurised, linear alternator)](https://xemstudio.com/objects/list-fpse-generator-2in/)