# Rolling-diaphragm piston and head retainers, 2 in power head

> What piston does a 1.50 in rolling diaphragm need?

- Category: Hardware › Stirling & thermoacoustic
- Kind: Hardware
- Basis: Piston diameter, corner radius and retainer rule from the published 1.50 in rolling-diaphragm size; screw sizes ISO 7380 / ISO 273
- Page: https://xemstudio.com/objects/tri-clamp-rd-piston/ · JSON: https://xemstudio.com/api/geometry?object=tri-clamp-rd-piston · Plain text: https://xemstudio.com/objects/tri-clamp-rd-piston/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 piston is Ø33.274 (1.31 in), the published piston for the 1.50 in bore: it runs 2.41 mm clear of the bore all round and never touches it. Its crown edge has the 1/16 in radius the diaphragm wraps over.
- The diaphragm's head is clamped between the crown and a Ø26 × 2 retainer by three M3 × 8 button screws into the crown. The retainer is smaller than the crown's flat (Ø30.1) so it never pinches the bend.
- The Ø5 tube rod goes in a socket in the centre boss and is held by an M3 grub at 18 mm, below the highest point the diaphragm's inner wall ever reaches on the skirt (13.4 mm), so the access hole is never under the diaphragm.
- The blind retainer closes the centre. The gland retainer has an Ø3 bore so a Ø3 displacer rod runs through the piston (beta): with the crown's Ø3 bore it is an 8 mm long close-running gland.

## 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 | Ø33.274 (1.31 in) | Ø38.10 bore, 2.41 clear all round (the diaphragm rolls between) | designed | rd-piston* |
| crown edge radius | R1.5875 (1/16 in) | diaphragm head bend | designed | rd-piston* |
| tube socket | Ø5 × 12 | Ø5 tube rod (M3 grub) | slide | rd-piston* |
| crown gland bore | Ø3 × 6 | Ø3 displacer rod (beta): close-running gland | run | rd-piston* |
| retainer screw holes | 3 × Ø2.5 × 5.5 | M3 tap drill | designed | rd-piston* |
| grub hole | Ø2.5, access Ø3.4 in the skirt | M3 tap drill / key access | designed | rd-piston* |
| retainer clearance holes | 3 × Ø3.4 | M3 button screws | designed | rd-retainer* |
| retainer gland bore | Ø3 × 2 | Ø3 displacer rod: close-running gland with the crown bore | run | rd-retainer-26-gland3* |

## Sizes

| part | od_mm | length_mm | notes |
|---|---|---|---|
| piston | 33.274 | 22.0 | crown 6, Ø5 socket, M3 grub |
| retainer blind | 26.0 | 2.0 | 3 × Ø3.4 |
| retainer gland | 26.0 | 2.0 | 3 × Ø3.4 + Ø3 gland |

## Where the files are

- Marsh Bellofram: [Rolling diaphragm design manual (2022): nomenclature, height formula, standard-size tables, hardware radii](https://damapi.marshbellofram.com/uploads/Bellofram_Diaphragm_Design_Manual_2022_7a922642c3.pdf) — PDF — publisher's manual; dimensions cited, nothing copied
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

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

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

- rd-piston-2in: [STEP](https://xemstudio.com/files/hardware/tri-clamp-rd-piston/rd-piston-2in.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-rd-piston/rd-piston-2in.stl) — rolling-diaphragm piston Ø33.274 × 22: crown 6 with a 1/16 in edge radius, Ø3 gland bore through the crown, 3 × M3 tapped (Ø2.5) × 5.5 on Ø18, Ø14 boss with a Ø5 tube socket 8..20 and an M3 grub (Ø2.5) at 18 reached through a Ø3.4 hole in the 2 mm skirt
- rd-retainer-26-blind: [STEP](https://xemstudio.com/files/hardware/tri-clamp-rd-piston/rd-retainer-26-blind.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-rd-piston/rd-retainer-26-blind.stl) — head retainer Ø26 × 2, 0.5 radius on the clamping edge, 3 × Ø3.4 on Ø18, no centre hole
- rd-retainer-26-gland3: [STEP](https://xemstudio.com/files/hardware/tri-clamp-rd-piston/rd-retainer-26-gland3.step) · [STL](https://xemstudio.com/files/hardware/tri-clamp-rd-piston/rd-retainer-26-gland3.stl) — head retainer Ø26 × 2 with an Ø3 gland bore on centre (the beta's displacer rod runs through it)

## Questions

**Does the piston need a close fit?**

No: it is 2.41 mm smaller than the bore on each side. Only the diaphragm touches both.

**How is the diaphragm fixed to the piston?**

Its head sits on the crown and the retainer clamps it with three M3 × 8 button screws.

## Fits with

- [Top-hat rolling diaphragm, 1.50 in bore, flat flange: the one that clamps in a 2 in tri-clamp joint](https://xemstudio.com/objects/rolling-diaphragm-top-hat/)
- [Tri-clamp rolling-diaphragm power head, 2 in: standard clamp face, cylinder, rod guide and crank tower, with its diaphragm seat ring](https://xemstudio.com/objects/tri-clamp-rd-power-head/)
- [Rod-end yokes with a side stub pin: Ø5 tube and Ø3 rod](https://xemstudio.com/objects/tri-clamp-rd-yokes/)
- [Button head socket screws, M2–M8 (ISO 7380)](https://xemstudio.com/objects/button-head-screws-metric/)