Build · 2025-07-23
Designing a displacer for a 3-foot free-piston Stirling engine
What's the simplest displacer for a 6-inch free-piston engine that is light, rigid, and cheap to make?
Two grooved end plates that clamp thin sheet-metal sides between them on a threaded rod. No welding. Quoted cost fell from ~$400 to $66 for the pair across iterations.
- Kind
- Build
- Status
- Built
- Date
- 2025-07-23
- Engine
- 6 in free-piston Stirling
- Plate floor
- 1 mm
- Center hole
- 4.7 mm (threaded rod)
- Cost path
- ~$400 → $66 per pair
Video
What it is
Design walkthrough: seven iterations of the displacer, each asking 'is this over-engineered?' and removing material, until the part became two plates and a sheet of flashing.
Results
- →Final design clamps the sides between plates, removing the need to tack-weld aluminum to stainless.
- →Optional air holes allow steel-wool fill or venting.
- →Center hole sized at 4.7 mm to clear a 4 mm rod with tapping as a fallback.
Lab notes
Iteration path
- Sealed hollow can: how to fill it, and unprintable internals.
- Open cup with 2 mm walls and bracing: over-engineered.
- Cup with side cutouts: still too much material.
- Tack-welded sheet sides: can't weld through the top plate.
- Ring-and-sheet: sheet too thin to carry the ring.
- Grooved plates clamping the sheet: works.
- Remove the solid core between plates: final.
Design principle
Design for the cheapest available stock (rolled sheet metal) rather than machining a cup; keep modular options (holes that can be plugged, a hole that can be tapped) so the part doesn't box in later decisions.
Notes transcribed from the build video.
Related
Questions about this one? Ask in the Discord. It's the fastest way to reach me.