Gas-spring piston Ø14.9 × 15 and Ø19.9 × 20 — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/gas-spring-piston/  (CC BY 4.0)
Basis: Gas spring k = γ·p·A² / V; piston drawn at bounce-space bore − 0.1 (0.05 mm radial gap as drawn)
Catalog version: 2026-10-20

Sizes: see table below
- The small piston on the rod that compresses a sealed bounce space and makes a gas spring: Ø14.9 × 15 for a Ø15 bore, Ø19.9 × 20 for a Ø20 bore.
- Solid, with a Ø5 bore (fit: slide) for the rod and 0.5 mm chamfers. As drawn it is 0.1 mm under its bore, a 0.05 mm radial gap.
- Stiffness k = γ·p·A²/V: the Ø20 piston at 1 bar with 20 cm³ of bounce space gives about 690 N/m; a smaller volume is stiffer.

file  diameter  length  bore it runs in  rod bore  k at 1 bar, 20 cm³
gas-spring-piston-15x15  14.9  15  15  5  ≈ 220 N/m
gas-spring-piston-20x20  19.9  20  20  5  ≈ 690 N/m

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/gas-spring-piston/index.json
API: https://xemstudio.com/api/geometry?object=gas-spring-piston