Free-piston displacer rod, plug and stinger: Ø12 rod through the power piston, Ø3 stinger to the gas spring — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/fpse-displacer-rod/  (CC BY 4.0)
Basis: Beale free-piston layout (rod area drive), Redlich & Berchowitz linear dynamics; stinger stiffness from Euler beam relations
Catalog version: 2026-10-20

Sizes: see table below
- The rod's area (Ø12) is the displacer's drive: the working gas pushes on it from below and the bounce space from above, and that difference, swinging with the piston, pushes the displacer ahead of the piston (the Beale free-piston principle).
- It runs up through leaf B's hub (clamped there by the cap's boss and a collar) and the piston's Ø12 hub bore, a running fit 76 mm long that is also the seal between working and bounce space, to the plug at its top.
- The Ø3 stinger joins the plug to the gas-spring piston: stiff along its length, soft sideways (87 mm free), so the gas spring's own cylinder does not fight the rod's alignment through the piston.

part  size
rod  Ø12 / Ø10 × 260.45
plug  Ø10 × 8 spigot, Ø12 × 8 head
stinger  Ø3 × 87.05

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- Proc. IMechE Part A: Redlich & Berchowitz (1985), Linear dynamics of free-piston Stirling engines — PDF — journal; method cited [verify license at link] — https://journals.sagepub.com/doi/pdf/10.1243/PIME_PROC_1985_199_025_02
- 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/fpse-displacer-rod/index.json
API: https://xemstudio.com/api/geometry?object=fpse-displacer-rod