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

Name: Rolling-diaphragm piston and head retainers, 2 in power head dimension sheet
Creator: Xem Studio
License: https://creativecommons.org/licenses/by/4.0/

What piston does a 1.50 in rolling diaphragm need? · Hardware · basis: Piston diameter, corner radius and retainer rule from the published 1.50 in rolling-diaphragm size; screw sizes ISO 7380 / ISO 273

Long Earth Society build part. My rolling-diaphragm piston: it never touches its cylinder. The engines

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. Mating features

sizes: 3 in table

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.

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

Mating features

Drawn at the exact size. Open press, slide, run and socket features by your process's clearance; leave designed gaps and clamp fits 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*

Where the files are

Other publishers' files are linked, not hosted. "verify" = check the license at the link before commercial use.

Marsh Bellofram	Rolling diaphragm design manual (2022): nomenclature, height formula, standard-size tables, hardware radii	PDF	publisher's manual; dimensions cited, nothing copied
Long Earth Society	Engine builds	web, video	LES site

Free files

★ Free files 3 parts. Generated by Xem Studio from the standard; threads are plain cylinders at major diameter. CC BY 4.0. index.json

rd-piston-2in	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	STEP · STL
rd-retainer-26-blind	head retainer Ø26 × 2, 0.5 radius on the clamping edge, 3 × Ø3.4 on Ø18, no centre hole	STEP · STL
rd-retainer-26-gland3	head retainer Ø26 × 2 with an Ø3 gland bore on centre (the beta's displacer rod runs through it)	STEP · STL