# Tri-clamp free-piston kit: flexure head, stack ring, hub collars and piston plug for 1.5, 2 and 3 in spools

> How do I hang a tri-clamp piston and displacer on flexure bearings to make a free-piston Stirling engine?

- Category: Stirling engines › Free-piston
- Kind: Hardware
- Basis: Spiral flexure entry (Ø60/80/100 leaves, 3 rim holes, hub Ø13.2/17.6/22); tri-clamp piston entry (-rod variants); Sunpower / NASA layout: displacer rod through the piston, both suspended on planar spring stacks at the cold end
- Page: https://xemstudio.com/objects/tri-clamp-free-piston-kit/ · JSON: https://xemstudio.com/api/geometry?object=tri-clamp-free-piston-kit · Plain text: https://xemstudio.com/objects/tri-clamp-free-piston-kit/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

- Layout, cold end outward: hot cup → spool with the displacer and the -rod piston in it → flexure head (clamps on the spool's open ferrule with a stock clamp) carrying stack A on the piston's tube rod → three M3 standoffs → flexure ring carrying stack B on the displacer rod. Nothing touches but the clearance seals: piston in the bore, displacer rod in the piston's bushing.
- Piston side: print the -rod piston, press a dry 3 × 5 × 6 bushing in its crown, drop the piston plug into the skirt and push the two Ø3 × 16 wrist dowels through bosses and plug legs (the bore retains the dowels). Press a 5 mm K&S brass tube into the plug socket (CA): that is the piston rod. Stack A = hub collar b5 · leaf · hub washer · leaf · hub collar b5 on the tube (Ø100 leaves have a Ø8 hub bore: put the hub-bush-100-b5 on the tube first), rim bolted to the head with the clamp plate and spacer from the flexure entry.
- Displacer side: Ø3 rod from the displacer cap (M3 grub) through the piston bushing, up the inside of the tube, to stack B on the ring: hub collar b3 · leaf · washer · leaf · collar b3, all on the hub-bush b3 so the leaves' Ø5/Ø8 hub bores are centred on the Ø3 rod. Rod length = spool + head + standoffs.
- Stack B height sets the displacer phase mass: 30–40 mm standoffs. Tune: each moving assembly resonates at f = (1/2π)√(k/m); the displacer should run 60–90° ahead of the piston, which you get by making the displacer resonance a little above the piston's (stiffer stack or lighter displacer).
- Power take-off: the linear alternator's housing bolts to the flexure head over stack A, and its two-row magnet mover clamps the same 5 mm tube.
- Bounce space: with the head open the piston's back side is atmosphere and the engine is a low-pressure demonstrator. For a sealed bounce space put a tri-clamp cup over the whole drive on a longer spool section; the flexure head is then inside the vessel.
- LES: the free-piston test bed in tri-clamp.

## 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 |
|---|---|---|---|---|
| flexure pocket in the head / ring | flexure OD | spiral flexure leaf rim | slide | fp-flexure-* |
| hub collar bore (b5 / b3) | Ø5 / Ø3 | 5 mm tube / Ø3 rod | slide | hub-collar-* |
| centring bush OD | leaf hub bore (Ø5 / Ø8) | flexure leaf hub | slide | hub-bush-* |
| centring bush bore | Ø3 / Ø5 | Ø3 rod / 5 mm tube | slide | hub-bush-* |
| piston plug disc in the skirt | f020 skirt bore (bore − 3.4) | piston skirt (0.3 loose in the lap-rod piston) | 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-* |
| head face | ISO 2852 | ferrule face + Type B gasket | clamp | fp-flexure-head-* |
| rim holes (head, ring) | Ø3.4 | M3 screw clearance | designed | fp-flexure-* |
| hub-collar slit | 0.8 | closes on the tube / rod | clamp | hub-collar-* |
| hub-collar pinch hole | Ø2.5 | M3 tap drill | designed | hub-collar-* |

## Sizes

| spool | bore_mm | flexure_od | hub_mm | tube_rod_mm | displacer_rod_mm | plate_od | plug_pin_z_mm |
|---|---|---|---|---|---|---|---|
| 1.5 | 34.8 | 60 | 13.2 | 5 | 3 | 76 | 17.6 |
| 2 | 47.5 | 80 | 17.6 | 5 | 3 | 96 | 23.5 |
| 3 | 72.9 | 100 | 22.0 | 5 | 3 | 116 | 37.0 |

## 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 (19 parts, CC BY 4.0, Xem Studio)

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

- fp-flexure-head-1.5in: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-head-1.5in.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-head-1.5in.stl) — 1.5 in flexure head: ISO 2852 cap face, Ø38.1 pedestal clearing the clamp, Ø76 plate with a Ø60 × 1 pocket (= flexure OD) and 3 × M3 (Ø3.4) on the Ø60 flexure's rim holes; the piston's stack A bolts on it
- fp-flexure-ring-60: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-ring-60.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-ring-60.stl) — Ø76 ring for stack B (displacer rod) on 3 × M3 standoffs above the head: Ø60 × 1 flexure pocket, 3 × Ø3.4
- hub-collar-60-b5: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-60-b5.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-60-b5.stl) — split hub collar Ø13.2 × 5, Ø5 bore, tapped M3 pinch: clamps the leaf hub on the 5 mm tube rod (two per stack)
- hub-collar-60-b3: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-60-b3.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-60-b3.stl) — split hub collar Ø13.2 × 5, Ø3 bore, tapped M3 pinch: clamps the leaf hub on the Ø3 displacer rod (two per stack)
- hub-bush-60-b3: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-60-b3.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-60-b3.stl) — centring bush Ø5 OD × Ø3 ID × 14: centres the Ø60 leaves (Ø5 hub bore) on the Ø3 displacer rod; or a 4 mm K&S tube sleeved on the rod
- 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 crown bushing, not here)
- fp-flexure-head-2in: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-head-2in.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-head-2in.stl) — 2 in flexure head: ISO 2852 cap face, Ø50.8 pedestal clearing the clamp, Ø96 plate with a Ø80 × 1 pocket (= flexure OD) and 3 × M3 (Ø3.4) on the Ø80 flexure's rim holes; the piston's stack A bolts on it
- fp-flexure-ring-80: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-ring-80.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-ring-80.stl) — Ø96 ring for stack B (displacer rod) on 3 × M3 standoffs above the head: Ø80 × 1 flexure pocket, 3 × Ø3.4
- hub-collar-80-b5: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-80-b5.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-80-b5.stl) — split hub collar Ø17.6 × 5, Ø5 bore, tapped M3 pinch: clamps the leaf hub on the 5 mm tube rod (two per stack)
- hub-collar-80-b3: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-80-b3.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-80-b3.stl) — split hub collar Ø17.6 × 5, Ø3 bore, tapped M3 pinch: clamps the leaf hub on the Ø3 displacer rod (two per stack)
- hub-bush-80-b3: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-80-b3.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-80-b3.stl) — centring bush Ø5 OD × Ø3 ID × 14: centres the Ø80 leaves (Ø5 hub bore) on the Ø3 displacer rod; or a 4 mm K&S tube sleeved on the rod
- 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 crown bushing, not here)
- fp-flexure-head-3in: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-head-3in.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-head-3in.stl) — 3 in flexure head: ISO 2852 cap face, Ø76.2 pedestal clearing the clamp, Ø116 plate with a Ø100 × 1 pocket (= flexure OD) and 3 × M3 (Ø3.4) on the Ø100 flexure's rim holes; the piston's stack A bolts on it
- fp-flexure-ring-100: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-ring-100.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/fp-flexure-ring-100.stl) — Ø116 ring for stack B (displacer rod) on 3 × M3 standoffs above the head: Ø100 × 1 flexure pocket, 3 × Ø3.4
- hub-collar-100-b5: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-100-b5.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-100-b5.stl) — split hub collar Ø22 × 5, Ø5 bore, tapped M3 pinch: clamps the leaf hub on the 5 mm tube rod (two per stack)
- hub-collar-100-b3: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-100-b3.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-collar-100-b3.stl) — split hub collar Ø22 × 5, Ø3 bore, tapped M3 pinch: clamps the leaf hub on the Ø3 displacer rod (two per stack)
- hub-bush-100-b3: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-100-b3.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-100-b3.stl) — centring bush Ø8 OD × Ø3 ID × 14: centres the Ø100 leaves (Ø8 hub bore) on the Ø3 displacer rod; or a 4 mm K&S tube sleeved on the rod
- hub-bush-100-b5: [STEP](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-100-b5.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-free-piston-kit/hub-bush-100-b5.stl) — centring bush Ø8 × Ø5 × 14: centres the Ø100 leaves on the 5 mm tube piston rod
- 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 crown bushing, not here)

## How it fits

### Goes in or on
- [Tri-clamp spool (two ferrules on a tube), 1.5 and 2 inch](https://xemstudio.com/objects/tri-clamp-spool/): flexure head clamps on its ferrule

### Connects to
- [Planar spiral flexure bearings (Sunpower / NASA free-piston style): Ø40–120, DXF + STEP](https://xemstudio.com/objects/spiral-flexure-bearings/): leaves, spacers, clamp plates, hub washers
- [Tri-clamp power pistons and crank linkage: full-bore ring-less pistons, con-rods, crank webs and a clamp-on crank frame for 1.5, 2 and 3 in spools](https://xemstudio.com/objects/tri-clamp-power-pistons/): -rod piston takes the plug
- [Tri-clamp displacers and engine heads: 1.5, 2 and 3 in spools](https://xemstudio.com/objects/tri-clamp-displacers-and-heads/): displacer rod through the piston
- [Linear alternator for the free-piston Stirling: two-row moving magnet, opposed coils, back-iron sleeve (2 in / 3 in tri-clamp)](https://xemstudio.com/objects/linear-alternator-kit/): two-row magnet mover on the tube, stator on the rim screws over stack A

### Alternatives
- [Free-piston Stirling core: displacer, power piston, liner, gas spring, stops, clearance-seal and tuning tables](https://xemstudio.com/objects/free-piston-core/): stand-alone liner version

### Also needed
- [Stirling shafts and rods: Ø3 rods, Ø5 / Ø8 D-flat crankshafts, Ø5 tube rods, Ø3 pins, thrust spacers](https://xemstudio.com/objects/stirling-shafts-and-rods/): Ø3 displacer rod; printed tube rod for a mock-up
- [Spool hanger collars: clamp a tri-clamp spool to 2020 extrusion over the flame (1.5, 2, 3 in)](https://xemstudio.com/objects/spool-hanger-collars/): hang it from a 2020 crossbar
- [Hobby brass tube (K&S), 1/16–21/32 in and 1–12 mm, telescoping](https://xemstudio.com/objects/brass-tube-ks/): 5 mm tube piston rod
- [Threaded standoffs, M2.5 and M3 (hex and round)](https://xemstudio.com/objects/threaded-standoffs-metric/): M3 × 30 standoffs head to ring
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/): Ø3 × 16 dowels lock the plug
- [Tri-clamp clamps (printable two-bolt), hot-joint gaskets and heat shields: every flange size, ½ to 4 in](https://xemstudio.com/objects/tri-clamp-clamps/): clamp the head on

## Questions

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

Through the piston plug: it sits in the open skirt, the wrist-pin dowels lock it, and its socket takes a 5 mm brass tube. The flexure leaf hubs clamp on that tube between two split hub collars; the leaf rims bolt to the flexure head.

**How does the displacer rod get past the piston in a free-piston engine?**

Through a dry 3 × 5 × 6 bushing pressed in the crown of the -rod piston, then up the inside of the hollow tube piston rod to its own flexure stack on the ring above.

**What flexure size for a 2 inch spool?**

Ø80 leaves (hub Ø17.6) on the 2 in flexure head; Ø60 for 1.5 in, Ø100 for 3 in.

## Fits with

- [Planar spiral flexure bearings (Sunpower / NASA free-piston style): Ø40–120, DXF + STEP](https://xemstudio.com/objects/spiral-flexure-bearings/)
- [Tri-clamp power pistons and crank linkage: full-bore ring-less pistons, con-rods, crank webs and a clamp-on crank frame for 1.5, 2 and 3 in spools](https://xemstudio.com/objects/tri-clamp-power-pistons/)
- [Tri-clamp displacers and engine heads: 1.5, 2 and 3 in spools](https://xemstudio.com/objects/tri-clamp-displacers-and-heads/)
- [Tri-clamp cups: closed-end spools (ferrule on a capped tube), 1.5, 2 and 3 in](https://xemstudio.com/objects/tri-clamp-cups/)
- [Hobby brass tube (K&S), 1/16–21/32 in and 1–12 mm, telescoping](https://xemstudio.com/objects/brass-tube-ks/)
- [Bearing housings and linear supports: KP08, KFL08, SK8, SC8UU, V-wheels, sleeve bushings](https://xemstudio.com/objects/bearing-housings-and-rod-supports/)
- [Threaded standoffs, M2.5 and M3 (hex and round)](https://xemstudio.com/objects/threaded-standoffs-metric/)
- [Linear alternator for the free-piston Stirling: two-row moving magnet, opposed coils, back-iron sleeve (2 in / 3 in tri-clamp)](https://xemstudio.com/objects/linear-alternator-kit/)
- [Free-piston Stirling core: displacer, power piston, liner, gas spring, stops, clearance-seal and tuning tables](https://xemstudio.com/objects/free-piston-core/)
- [NASA and Sunpower free-piston Stirling references (public domain) and design numbers](https://xemstudio.com/objects/nasa-fpse-references/)