# Tri-clamp piston plug: tube-rod socket for the -rod piston, 1.5, 2 and 3 in

> How do I connect a tri-clamp piston to a flexure bearing?

- Category: Stirling engines › Free-piston
- Kind: Hardware
- Basis: Disc Ø = piston skirt bore (bore − 3.2: 31.6 / 44.3 / 69.7); legs pinned at the wrist-pin height 17.6 / 23.5 / 37.0; Ø5 × 15 tube socket; Ø3.4 rod passage
- Page: https://xemstudio.com/objects/tri-clamp-piston-plug/ · JSON: https://xemstudio.com/api/geometry?object=tri-clamp-piston-plug · Plain text: https://xemstudio.com/objects/tri-clamp-piston-plug/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

- The plug that turns a -rod power piston into a free piston on a tube rod: it drops into the open skirt and is held by the two Ø3 wrist dowels through its legs.
- Disc Ø31.6 / Ø44.3 / Ø69.7 × 6 mm (the piston skirt bore, exact), two 4 × 9 legs up to the wrist-pin height with Ø3 pin holes at 17.6 / 23.5 / 37.0 mm.
- Below the disc: an Ø8 × 15 stub with a Ø5 × 15 socket for the 5 mm tube piston rod. A Ø3.4 passage runs through on centre so the Ø3 displacer rod clears it (the rod runs in the piston crown, not here).
- Overall height 38.6 / 44.5 / 58.0 mm.

## 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 plug disc in the skirt | piston skirt bore (bore − 3.4) | piston skirt | slide | fp-piston-plug-* |
| plug wrist-pin holes | Ø3 | Ø3 pins | press | fp-piston-plug-* |
| plug tube socket | Ø5 × 15 | 5 mm tube | press | fp-piston-plug-* |
| plug rod passage | Ø3.4 | Ø3 rod (no contact) | designed | fp-piston-plug-* |

## Sizes

| spool_in | disc_od_mm | pin_z_mm | tube_socket_mm | rod_passage_mm | overall_h_mm |
|---|---|---|---|---|---|
| 1.5 | 31.6 | 17.6 | Ø5 × 15 | 3.4 | 38.6 |
| 2 | 44.3 | 23.5 | Ø5 × 15 | 3.4 | 44.5 |
| 3 | 69.7 | 37.0 | Ø5 × 15 | 3.4 | 58.0 |

## Where the files are

- NASA NTRS: [Free-piston Stirling convertor flexure bearing development and life testing](https://ntrs.nasa.gov/citations/20050182933) — pdf — US Government work
- Long Earth Society: [Tri-clamp Stirling engine builds](https://longearthsociety.com/engineering/) — web, video — LES site
- Wikipedia: [Free-piston engine](https://en.wikipedia.org/wiki/Free-piston_engine) — web — CC BY-SA 4.0 (text)

## Free STEP/STL (3 parts, CC BY 4.0, Xem Studio)

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

- fp-piston-plug-1.5in: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-piston-plug-1.5in.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-piston-plug-1.5in.stl) — 1.5 in piston plug: drops into the open skirt, two Ø3 × 16 wrist dowels through its legs at 17.6 mm retain it, Ø5 tube socket 15 deep, Ø3.4 clearance for the displacer rod (it runs in the piston's Ø3 crown bore, not here)
- fp-piston-plug-2in: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-piston-plug-2in.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-piston-plug-2in.stl) — 2 in piston plug: drops into the open skirt, two Ø3 × 16 wrist dowels through its legs at 23.5 mm retain it, Ø5 tube socket 15 deep, Ø3.4 clearance for the displacer rod (it runs in the piston's Ø3 crown bore, not here)
- fp-piston-plug-3in: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-piston-plug-3in.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-piston-plug-3in.stl) — 3 in piston plug: drops into the open skirt, two Ø3 × 16 wrist dowels through its legs at 37.0 mm retain it, Ø5 tube socket 15 deep, Ø3.4 clearance for the displacer rod (it runs in the piston's Ø3 crown bore, not here)

## How it fits

### Goes in or on
- [Tri-clamp power piston: ring-less full-bore pistons Ø34.8, Ø47.5 and Ø72.9](https://xemstudio.com/objects/tri-clamp-power-piston/): disc in the -rod piston's skirt, on the Ø3 wrist dowels

### Connects to
- [Crankshafts Ø3, Ø5 and Ø8 with full-length D-flat](https://xemstudio.com/objects/stirling-crankshafts/): 5 mm tube rod in the Ø5 × 15 socket
- [Flexure hub collars: split collars Ø13.2, Ø17.6 and Ø22 for Ø3 rod and Ø5 tube](https://xemstudio.com/objects/flexure-hub-collar/): b5 collars on the same tube

### Also needed
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/): two Ø3 × 16 wrist dowels

## Questions

**How does the piston connect to a flexure bearing?**

Through the piston plug: it sits in the skirt on the wrist dowels and its Ø5 socket takes the 5 mm tube; the flexure hubs clamp on that tube.

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

Through a Ø3.4 passage on centre: the Ø3 rod clears the plug and runs on up inside the tube rod.

## Fits with

- [Tri-clamp power piston: ring-less full-bore pistons Ø34.8, Ø47.5 and Ø72.9](https://xemstudio.com/objects/tri-clamp-power-piston/)
- [Tri-clamp flexure head: ISO 2852 cap with Ø60, Ø80 and Ø100 flexure seats](https://xemstudio.com/objects/tri-clamp-flexure-head/)
- [Flexure hub collars: split collars Ø13.2, Ø17.6 and Ø22 for Ø3 rod and Ø5 tube](https://xemstudio.com/objects/flexure-hub-collar/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)
- [Crankshafts Ø3, Ø5 and Ø8 with full-length D-flat](https://xemstudio.com/objects/stirling-crankshafts/)