Leaf hub collars: Ø12 and Ø18 bore, M3 grub — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/fpse-hub-collars/  (CC BY 4.0)
Basis: Design values of the free-piston generator; M3 grub (ISO 4026) in an Ø2.5 tapped hole
Catalog version: 2026-10-20

Sizes: see table below
- The leaf's hub is clamped axially between a shoulder and a collar (leaf B: the displacer cap's boss and one Ø20 collar) or between two collars (leaves C and D on the Ø18 tube rod). Each collar holds with one M3 × 4 grub.
- Fit: the collar bores slide on the rod or tube and are held by the grub.

collar  od_mm  bore_mm  h_mm
displacer  20  12  5
piston  28  18  5

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- Long Earth Society: Engine builds — web, video — LES site — https://longearthsociety.com/engineering/

Free files (CC BY 4.0, Xem Studio; STEP + STL, 2 parts): https://xemstudio.com/files/hardware/fpse-hub-collars/index.json
API: https://xemstudio.com/api/geometry?object=fpse-hub-collars