Piston stop bumper Ø30 and Ø40 × 6 — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/piston-stop-bumper/  (CC BY 4.0)
Basis: OD = liner bore (fit: press); recess 0.6 × bore
Catalog version: 2026-10-20

Sizes: see table below
- An end stop for the free-piston core: Ø30 or Ø40 (the liner bore, fit: press) × 6 tall, a 2 mm closed face over a 4 mm deep recess of 0.6 × the bore (Ø18 / Ø24), with a 1 mm rounded edge.
- One at each end of the stroke; the stroke is kept to 85 % of the free travel between them. Over-stroke, when the alternator load drops, is the usual way a free-piston engine fails.

file  OD  height  recess  face
stop-bumper-30  30  6  Ø18 × 4  2
stop-bumper-40  40  6  Ø24 × 4  2

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