Free-piston internal fin inserts: slotted heater and cooler sleeves for a 2 in tri-clamp bore (optional) — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/fpse-fin-inserts/  (CC BY 4.0)
Basis: Design values of the free-piston generator: 36 axial slots 1.5 × 2.5 in a sleeve whose OD is the 2 in bore (Ø47.5); heat-transfer and flow numbers from the generator's simple analysis
Catalog version: 2026-10-20

Sizes: see table below
- Two slotted sleeves that slide into the plain tri-clamp parts: the heater insert on the liner's lower end inside the closed-end spool, the cooler insert pressed into the 3 in spool under the water jacket. The gas runs along the 36 slots, between fins that touch the wall.
- Optional: without them the heater is the plain 3.0 mm annulus round the liner and the cooler the 4.05 mm annulus round the displacer. The engine still runs; my analysis gives 6.6 W instead of 43.2 W at the same stroke (helium, 10 bar), because the gas-side heat transfer falls and the cooler annulus adds dead volume.
- Fits: the heater insert slides on the liner (press) and its OD against the hot spool's bore (slide); the cooler insert's OD is pressed into S3, its Ø40 bore is the displacer's 0.30 gap.

insert  id_mm  od_mm  length_mm  slots  slot_mm
heater  41.5  47.5  68.05  36  1.5 × 2.5
cooler  40.0  47.5  73.2  36  1.5 × 2.5

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- Ohio University (Urieli): Stirling cycle machine analysis: ideal adiabatic and simple analysis (after Urieli & Berchowitz, Stirling Cycle Engine Analysis, 1984) — 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/fpse-fin-inserts/index.json
API: https://xemstudio.com/api/geometry?object=fpse-fin-inserts