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Tri-clamp power piston: ring-less full-bore pistons Ø34.8, Ø47.5 and Ø72.9
What size power piston fits a 2 inch tri-clamp spool? · Hardware · basis: 16 ga ASME BPE bores 34.8 / 47.5 / 72.9 mm; piston drawn at the bore diameter; length 1.2 × bore
Long Earth Society build part. Full-bore pistons for the tri-clamp beta and alpha engines. The engines
Exact dimensions. Every file is drawn at true size with no printing tolerance. The holes and pins that meet another part are listed below under Mating features, with the fit each joint needs; open those by your printer's or process's clearance before making the part. Mating features
- sizes
- 3 in table
Notes
- A ring-less full-bore piston: no ring and no O-ring, the seal is the fit in the spool bore. The piston is drawn at exactly the bore, Ø34.8 / 47.5 / 72.9; its running fit to the bore (0.02–0.05 mm on diameter for a clearance seal) is listed under Mating features.
- Length 42 / 56 / 88 mm (1.2 × bore). Two 8 mm full-diameter lands at the ends carry the fit; between them the body is relieved 0.4 mm on radius. Crown 2.5 mm, skirt wall 1.6 mm, open skirt.
- Wrist-pin bosses cross the skirt at 0.42 L (17.6 / 23.5 / 37.0 mm from the open end) with a Ø3 pin bore; the gap between the bosses is 16 mm (1.5 in) or 18 mm (2 and 3 in) for the con-rod small end.
- The -rod pistons add a Ø11 × 6 boss under the crown with a Ø3 bore through crown and boss (8.5 long): the Ø3 displacer rod runs through it (beta, rhombic, free-piston layouts).
- The 2 in piston is about 16.8 cm³ of material.
Sizes
| spool_in | bore_mm | piston_od_mm | length_mm | wrist_pin_z_mm | files |
|---|---|---|---|---|---|
| 1.5 | 34.8 | 34.8 | 42 | 17.6 | piston-1.5in, piston-1.5in-rod |
| 2 | 47.5 | 47.5 | 56 | 23.5 | piston-2in, piston-2in-rod |
| 3 | 72.9 | 72.9 | 88 | 37.0 | piston-3in, piston-3in-rod |
Mating features
Drawn at the exact size. Open press, slide, run and socket features by your process's clearance; leave designed gaps and clamp fits as drawn.
| feature | exact | mates with | fit | files |
|---|---|---|---|---|
| piston OD in the spool bore | Ø34.8 / 47.5 / 72.9 (= bore) | tri-clamp-spool bore (ring-less clearance seal) | run | piston-* |
| wrist-pin bore | Ø3 | Ø3 pin (stirling-shafts-and-rods pin-3x…) | press | piston-* |
| wrist-pin boss gap | 16 / 18 | con-rod small end | designed | piston-* |
| crown rod bore (-rod pistons) | Ø3 × 8.5 (crown + boss) | Ø3 displacer rod (slides through it) | run | piston-* |
Where the files are
Other publishers' files are linked, not hosted. "verify" = check the license at the link before commercial use.
| Long Earth Society | Tri-clamp Stirling engine builds | web, video | LES site |
| James R. Senft | An Introduction to Stirling Engines (piston fit and leakage) | book | Moriya Press verify |
| ASME | BPE tube dimensions (16 ga sanitary tube) | standard | ASME verify |
Free files
★ Free files 6 parts. Generated by Xem Studio from the standard; threads are plain cylinders at major diameter. CC BY 4.0. index.json
| piston-1.5in | 1.5 in full-bore piston Ø34.8 × 42 (the spool bore, exact), two 8 mm lands with 0.4 relief between, Ø3 wrist pin at 17.6 mm, open skirt | STEP · STL |
| piston-1.5in-rod | 1.5 in piston Ø34.8 as above with a Ø3 bore through the crown and a Ø11 × 6 boss under it: the Ø3 displacer rod runs through it (beta, rhombic, free-piston) | STEP · STL |
| piston-2in | 2 in full-bore piston Ø47.5 × 56 (the spool bore, exact), two 8 mm lands with 0.4 relief between, Ø3 wrist pin at 23.5 mm, open skirt | STEP · STL |
| piston-2in-rod | 2 in piston Ø47.5 as above with a Ø3 bore through the crown and a Ø11 × 6 boss under it: the Ø3 displacer rod runs through it (beta, rhombic, free-piston) | STEP · STL |
| piston-3in | 3 in full-bore piston Ø72.9 × 88 (the spool bore, exact), two 8 mm lands with 0.4 relief between, Ø3 wrist pin at 37.0 mm, open skirt | STEP · STL |
| piston-3in-rod | 3 in piston Ø72.9 as above with a Ø3 bore through the crown and a Ø11 × 6 boss under it: the Ø3 displacer rod runs through it (beta, rhombic, free-piston) | STEP · STL |
Plain text: sheet.txt · JSON: /api/geometry?object=tri-clamp-power-piston
How it fits
Parts that fit it
Crankshafts Ø3, Ø5 and Ø8 with full-length D-flatØ8 shaft in the crank frame's 608sGoes in or on
Tri-clamp spool, 1.5 and 2 inchpiston OD in the 34.8 / 47.5 / 72.9 bore
Tri-clamp cupssame bore, closed endConnects to
Tri-clamp con-rod623 in the rod small end on the Ø3 wrist pin
Tri-clamp piston plugplug in the -rod piston's skirt on the wrist pinsAlternatives
Flanged power cylinder liner Ø16, 20, 25, 30small-bore pistons in their own linerAlso needed
Dowel pins, 2–6 mmØ3 wrist pinIn these build lists
Beta Stirling, rhombic drive, 2 in tri-clampDisplacer and piston in one bore, driven by two counter-rotating gears on a rhom…Free-piston Stirling, 2 in tri-clampNo crank: piston and displacer each hang on a stack of spiral flexures, with a t…You might also need
Questions
What clearance does a ring-less Stirling piston need?
A running fit of 0.02–0.05 mm on diameter. The piston is drawn at the bore diameter; that fit is its mating feature, set by whoever makes it.
Which piston fits a 2 inch spool?
The 2 in piston: Ø47.5 (the bore), 56 mm long, with the Ø3 wrist pin 23.5 mm from the open end.
Why two lands with a relief between?
Only the two 8 mm end lands touch the bore; the 0.4 mm relief between them halves the rubbing area while the full length keeps the piston square.
What is the -rod piston?
The same piston with a Ø3 bore through the crown and an Ø11 crown boss, so the Ø3 displacer rod runs through the piston.