Rolling-diaphragm piston and head retainers, 2 in power head — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/tri-clamp-rd-piston/  (CC BY 4.0)
Basis: Piston diameter, corner radius and retainer rule from the published 1.50 in rolling-diaphragm size; screw sizes ISO 7380 / ISO 273
Catalog version: 2026-10-20

Sizes: see table below
- 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.

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

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- Marsh Bellofram: Rolling diaphragm design manual (2022): nomenclature, height formula, standard-size tables, hardware radii — PDF — publisher's manual; dimensions cited, nothing copied — https://damapi.marshbellofram.com/uploads/Bellofram_Diaphragm_Design_Manual_2022_7a922642c3.pdf
- Long Earth Society: Engine builds — web, video — LES site — https://longearthsociety.com/engineering/

Free files (CC BY 4.0, Xem Studio; STEP + STL, 3 parts): https://xemstudio.com/files/hardware/tri-clamp-rd-piston/index.json
API: https://xemstudio.com/api/geometry?object=tri-clamp-rd-piston