Tri-clamp piston plug: tube-rod socket for the -rod piston, 1.5, 2 and 3 in — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/tri-clamp-piston-plug/  (CC BY 4.0)
Basis: Disc Ø = piston skirt bore (bore − 3.2: 31.6 / 44.3 / 69.7); legs pinned at the wrist-pin height 17.6 / 23.5 / 37.0; Ø5 × 15 tube socket; Ø3.4 rod passage
Catalog version: 2026-10-20

Sizes: see table below
- The plug that turns a -rod power piston into a free piston on a tube rod: it drops into the open skirt and is held by the two Ø3 wrist dowels through its legs.
- Disc Ø31.6 / Ø44.3 / Ø69.7 × 6 mm (the piston skirt bore, exact), two 4 × 9 legs up to the wrist-pin height with Ø3 pin holes at 17.6 / 23.5 / 37.0 mm.
- Below the disc: an Ø8 × 15 stub with a Ø5 × 15 socket for the 5 mm tube piston rod. A Ø3.4 passage runs through on centre so the Ø3 displacer rod clears it (the rod runs in the piston crown, not here).
- Overall height 38.6 / 44.5 / 58.0 mm.

spool_in  disc_od_mm  pin_z_mm  tube_socket_mm  rod_passage_mm  overall_h_mm
1.5  31.6  17.6  Ø5 × 15  3.4  38.6
2  44.3  23.5  Ø5 × 15  3.4  44.5
3  69.7  37.0  Ø5 × 15  3.4  58.0

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- NASA NTRS: Free-piston Stirling convertor flexure bearing development and life testing — pdf — US Government work — https://ntrs.nasa.gov/citations/20050182933
- Long Earth Society: Tri-clamp Stirling engine builds — web, video — LES site — https://longearthsociety.com/engineering/
- Wikipedia: Free-piston engine — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Free-piston_engine

Free files (CC BY 4.0, Xem Studio; STEP + STL, 3 parts): https://xemstudio.com/files/hardware/tri-clamp-piston-plug/index.json
API: https://xemstudio.com/api/geometry?object=tri-clamp-piston-plug