Free-piston power piston: Ø48 clearance-seal land, 40-magnet carrier, top plate and tube rod — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/fpse-power-piston/  (CC BY 4.0)
Basis: Design values of the free-piston generator: running fit in the 48.00 bore over a 30 mm land; four magnet rows S N N S facing out; moving mass from the CAD at 7.9 g/cm³ and 7.5 g/cm³ (magnets)
Catalog version: 2026-10-20

Sizes: see table below
- The piston is also the alternator's mover. Its lower 30 mm is the seal land, a running fit in the bored spool (the clearance seal between working space and bounce space); above it a Ø47 carrier (0.5 radial gap, no contact) holds 40 Ø12 × 3 disc magnets in four rows, faces out: S, N, N, S from the crown up.
- The carrier's body is the magnetic return between the rows, so it must be magnetically soft; the rest of the piston only has to be stiff and light. Moving mass with tube rod, collars and magnets: 918 g.
- The displacer rod runs through the Ø12 hub bore (a running fit 76 mm long, also a seal); the Ø23 crown pocket clears leaf B's collar. The top plate closes the carrier bore, carries the tube rod and vents the piston's inside to the bounce space.
- The tube rod carries the piston's two leaves (joints C and D) on four Ø28 collars.

part  size
seal land  Ø48.00 × 30
carrier  Ø47 × 60, bore Ø30
magnets  40 × Ø12 × 3, rows S N N S at 14 pitch
hub bore  Ø12, 76 long
tube rod  Ø18 / Ø14 × 142.15
moving mass  918 g

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- K&J Magnetics: Neodymium magnet specifications: N42 Br 1.30–1.32 T, density 7.4–7.5 g/cm³ — web — maker page; values are facts [verify license at link] — https://www.kjmagnetics.com/specs.asp
- 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-power-piston/index.json
API: https://xemstudio.com/api/geometry?object=fpse-power-piston