# Tri-clamp power piston: ring-less full-bore pistons Ø34.8, Ø47.5 and Ø72.9

> What size power piston fits a 2 inch tri-clamp spool?

- Category: Stirling engines › Tri-clamp engine parts
- Kind: Hardware
- Basis: 16 ga ASME BPE bores 34.8 / 47.5 / 72.9 mm; piston drawn at the bore diameter; length 1.2 × bore
- Page: https://xemstudio.com/objects/tri-clamp-power-piston/ · JSON: https://xemstudio.com/api/geometry?object=tri-clamp-power-piston · Plain text: https://xemstudio.com/objects/tri-clamp-power-piston/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

**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 ring-less full-bore piston: no ring and no O-ring, the seal is the fit in the spool bore. The piston is drawn at exactly the bore, Ø34.8 / 47.5 / 72.9; its running fit to the bore (0.02–0.05 mm on diameter for a clearance seal) is listed under Mating features.
- Length 42 / 56 / 88 mm (1.2 × bore). Two 8 mm full-diameter lands at the ends carry the fit; between them the body is relieved 0.4 mm on radius. Crown 2.5 mm, skirt wall 1.6 mm, open skirt.
- Wrist-pin bosses cross the skirt at 0.42 L (17.6 / 23.5 / 37.0 mm from the open end) with a Ø3 pin bore; the gap between the bosses is 16 mm (1.5 in) or 18 mm (2 and 3 in) for the con-rod small end.
- The -rod pistons add a Ø11 × 6 boss under the crown with a Ø3 bore through crown and boss (8.5 long): the Ø3 displacer rod runs through it (beta, rhombic, free-piston layouts).
- The 2 in piston is about 16.8 cm³ of material.

## 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 |
|---|---|---|---|---|
| piston OD in the spool bore | Ø34.8 / 47.5 / 72.9 (= bore) | tri-clamp-spool bore (ring-less clearance seal) | run | piston-* |
| wrist-pin bore | Ø3 | Ø3 pin (stirling-shafts-and-rods pin-3x…) | press | piston-* |
| wrist-pin boss gap | 16 / 18 | con-rod small end | designed | piston-* |
| crown rod bore (-rod pistons) | Ø3 × 8.5 (crown + boss) | Ø3 displacer rod (slides through it) | run | piston-* |

## Sizes

| spool_in | bore_mm | piston_od_mm | length_mm | wrist_pin_z_mm | files |
|---|---|---|---|---|---|
| 1.5 | 34.8 | 34.8 | 42 | 17.6 | piston-1.5in, piston-1.5in-rod |
| 2 | 47.5 | 47.5 | 56 | 23.5 | piston-2in, piston-2in-rod |
| 3 | 72.9 | 72.9 | 88 | 37.0 | piston-3in, piston-3in-rod |

## Where the files are

- Long Earth Society: [Tri-clamp Stirling engine builds](https://longearthsociety.com/engineering/) — web, video — LES site
- James R. Senft: [An Introduction to Stirling Engines (piston fit and leakage)](https://www.stirlingengine.com/) — book — Moriya Press (verify at link)
- ASME: [BPE tube dimensions (16 ga sanitary tube)](https://www.asme.org/codes-standards/find-codes-standards/bpe-bioprocessing-equipment) — standard — ASME (verify at link)

## Free STEP/STL (6 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/tri-clamp-power-piston/index.json

- piston-1.5in: [STEP](https://xemstudio.com/files/hardware/tri-clamp-power-pistons/piston-1.5in.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-power-pistons/piston-1.5in.stl) — 1.5 in full-bore piston Ø34.8 × 42 (the spool bore, exact), two 8 mm lands with 0.4 relief between, Ø3 wrist pin at 17.6 mm, open skirt
- piston-1.5in-rod: [STEP](https://xemstudio.com/files/hardware/tri-clamp-power-pistons/piston-1.5in-rod.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-power-pistons/piston-1.5in-rod.stl) — 1.5 in piston Ø34.8 as above with a Ø3 bore through the crown and a Ø11 × 6 boss under it: the Ø3 displacer rod runs through it (beta, rhombic, free-piston)
- piston-2in: [STEP](https://xemstudio.com/files/hardware/tri-clamp-power-pistons/piston-2in.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-power-pistons/piston-2in.stl) — 2 in full-bore piston Ø47.5 × 56 (the spool bore, exact), two 8 mm lands with 0.4 relief between, Ø3 wrist pin at 23.5 mm, open skirt
- piston-2in-rod: [STEP](https://xemstudio.com/files/hardware/tri-clamp-power-pistons/piston-2in-rod.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-power-pistons/piston-2in-rod.stl) — 2 in piston Ø47.5 as above with a Ø3 bore through the crown and a Ø11 × 6 boss under it: the Ø3 displacer rod runs through it (beta, rhombic, free-piston)
- piston-3in: [STEP](https://xemstudio.com/files/hardware/tri-clamp-power-pistons/piston-3in.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-power-pistons/piston-3in.stl) — 3 in full-bore piston Ø72.9 × 88 (the spool bore, exact), two 8 mm lands with 0.4 relief between, Ø3 wrist pin at 37.0 mm, open skirt
- piston-3in-rod: [STEP](https://xemstudio.com/files/hardware/tri-clamp-power-pistons/piston-3in-rod.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-power-pistons/piston-3in-rod.stl) — 3 in piston Ø72.9 as above with a Ø3 bore through the crown and a Ø11 × 6 boss under it: the Ø3 displacer rod runs through it (beta, rhombic, free-piston)

## How it fits

### Parts that fit it
- [Crankshafts Ø3, Ø5 and Ø8 with full-length D-flat](https://xemstudio.com/objects/stirling-crankshafts/): Ø8 shaft in the crank frame's 608s

### Goes in or on
- [Tri-clamp spool (two ferrules on a tube), 1.5 and 2 inch](https://xemstudio.com/objects/tri-clamp-spool/): piston OD in the 34.8 / 47.5 / 72.9 bore
- [Tri-clamp cups: closed-end spools (ferrule on a capped tube), 1.5, 2 and 3 in](https://xemstudio.com/objects/tri-clamp-cups/): same bore, closed end

### Connects to
- [Tri-clamp con-rod: 58, 70 and 95 mm centres with 623 pockets](https://xemstudio.com/objects/tri-clamp-con-rod/): 623 in the rod small end on the Ø3 wrist pin
- [Tri-clamp piston plug: tube-rod socket for the -rod piston, 1.5, 2 and 3 in](https://xemstudio.com/objects/tri-clamp-piston-plug/): plug in the -rod piston's skirt on the wrist pins

### Alternatives
- [Flanged power cylinder liner Ø16, 20, 25, 30](https://xemstudio.com/objects/power-cylinder-liner/): small-bore pistons in their own liner

### Also needed
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/): Ø3 wrist pin

## Questions

**What clearance does a ring-less Stirling piston need?**

A running fit of 0.02–0.05 mm on diameter. The piston is drawn at the bore diameter; that fit is its mating feature, set by whoever makes it.

**Which piston fits a 2 inch spool?**

The 2 in piston: Ø47.5 (the bore), 56 mm long, with the Ø3 wrist pin 23.5 mm from the open end.

**Why two lands with a relief between?**

Only the two 8 mm end lands touch the bore; the 0.4 mm relief between them halves the rubbing area while the full length keeps the piston square.

**What is the -rod piston?**

The same piston with a Ø3 bore through the crown and an Ø11 crown boss, so the Ø3 displacer rod runs through the piston.

## Fits with

- [Tri-clamp con-rod: 58, 70 and 95 mm centres with 623 pockets](https://xemstudio.com/objects/tri-clamp-con-rod/)
- [Tri-clamp crank frame: ISO 2852 cap with 608 towers for a Ø8 crankshaft, 1.5, 2 and 3 in](https://xemstudio.com/objects/tri-clamp-crank-frame/)
- [Tri-clamp piston plug: tube-rod socket for the -rod piston, 1.5, 2 and 3 in](https://xemstudio.com/objects/tri-clamp-piston-plug/)
- [Tri-clamp spool (two ferrules on a tube), 1.5 and 2 inch](https://xemstudio.com/objects/tri-clamp-spool/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)