Free-piston Stirling displacer Ø29 × 75 and Ø39 × 100 — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/free-piston-displacer/  (CC BY 4.0)
Basis: Beta free-piston layout (Sunpower RE-1000 / NASA): displacer = bore − 1.0 (0.5 mm radial gap, designed), length 2.5 × bore
Catalog version: 2026-10-20

Sizes: see table below
- The displacer of the free-piston core: a hollow cylinder with a 1.2 mm wall and a 2 mm rounded top edge, Ø29 × 75 for the Ø30 bore or Ø39 × 100 for the Ø40 bore.
- The 0.5 mm radial gap to the liner is designed: it is a shuttle, not a seal.
- A Ø9 × 8 rod boss with a Ø5 bore (fit: slide) takes the Ø5 displacer rod. Its spring (flexure plus gas spring) is tuned so it leads the piston by about 60–90°.

file  diameter  length  bore it runs in  radial gap  wall  rod bore
displacer-30x75  29  75  30  0.5  1.2  5
displacer-40x100  39  100  40  0.5  1.2  5

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- NASA (NTRS, public domain): Free-piston Stirling engine design and analysis (RE-1000 test results) (1984) — PDF — US Government work, public domain — https://ntrs.nasa.gov/search?q=19840017543
- NASA (NTRS, public domain): Dynamics of free-piston Stirling engines (1987) — PDF — US Government work, public domain — https://ntrs.nasa.gov/search?q=19870011457
- Wikipedia: Free-piston engine — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Free-piston_engine
- Wikipedia: Beale number — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Beale_number
- Ohio University: Stirling cycle machine analysis (Urieli), free online — web — Author's site — https://www.ohio.edu/mechanical/stirling/
- 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/free-piston-displacer/index.json
API: https://xemstudio.com/api/geometry?object=free-piston-displacer