Free-piston Stirling power piston Ø29.9 × 30 and Ø39.9 × 40 — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/free-piston-power-piston/  (CC BY 4.0)
Basis: Beta free-piston layout; piston drawn at liner bore − 0.1 (0.05 mm radial clearance-seal gap as drawn)
Catalog version: 2026-10-20

Sizes: see table below
- The ring-less power piston of the free-piston core: Ø29.9 × 30 for the Ø30 liner, Ø39.9 × 40 for the Ø40.
- As drawn it is 0.1 mm under the bore, a 0.05 mm radial gap: the seal is that gap, with no rings.
- Hollow with 2 mm walls, open at the top, with a Ø5 bore (fit: slide) for the Ø5 rod in its closed end.

file  diameter  length  liner bore  radial gap  rod bore
power-piston-30x30  29.9  30  30  0.05  5
power-piston-40x40  39.9  40  40  0.05  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-power-piston/index.json
API: https://xemstudio.com/api/geometry?object=free-piston-power-piston