# Syringe pistons: ring-less printed pistons with a con-rod fork for 10, 20 and 30 mL syringe barrels

> How do I connect a syringe to a crank for a Stirling engine without the rubber plunger's friction?

- Category: Stirling engines › Drives & mechanisms
- Kind: Hardware
- Basis: Barrel IDs from the plastic-syringes entry (10 mL 14.5, 20 mL 19.1, 30 mL 21.7 mm); same two-land, print-and-lap method as the tri-clamp power pistons
- Page: https://xemstudio.com/objects/syringe-pistons/ · JSON: https://xemstudio.com/api/geometry?object=syringe-pistons · Plain text: https://xemstudio.com/objects/syringe-pistons/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

**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.

## Notes

- The rubber tip of a syringe plunger seals by squeezing the wall: far too much friction for a Stirling. Throw the plunger away and run a printed piston in the barrel instead, lapped to fit like the tri-clamp pistons (sinks slowly under its own weight, holds with the tip capped).
- Each piston has a 5 mm fork slot (exact: the con-rod thickness; see Mating features) open to the bottom and a Ø3 pin hole across it at 0.45 L: the Ross yoke kit's conrod-45 / conrod-60 (5 mm thick, Ø3 ends) or the crank kit's con-rods drop straight in on a 3 × 12 dowel. The fork keeps the con-rod inside the barrel's footprint, so the piston can enter the barrel past the pin.
- Fits: f008 / f015 are 0.08 / 0.15 mm under the barrel; lap is 0.08 over, to lap down. Plastic barrels vary by brand, so measure yours with calipers and pick the closest.
- Used by: the Ross yoke alpha (cylinder block for two 10 mL barrels), the alpha 90° V-block, the gamma power head (10 mL socket) and the can engine.
- LES: the syringe engines' pistons.

## Mating features (exact size; open press / slide / run / socket by your process clearance, leave designed and clamp as drawn)

| feature | exact | mates with | fit | files |
|---|---|---|---|---|
| piston OD in the barrel | barrel ID − 0.08 / − 0.15, lap +0.08 | syringe barrel (14.5 / 19.1 / 21.7) | designed | all |
| pin hole | Ø3 | Ø3 pin | press | all |
| fork slot | 5 | 5 mm thick con-rod (turns in it) | run | all |
| land relief | OD − 0.5 between the lands | barrel (no contact) | designed | all |

## Sizes

| syringe | barrel_id_mm | piston_od_mm | length_mm | offset_mm | pin_z_mm |
|---|---|---|---|---|---|
| 10ml | 14.5 | 14.58 | 19 | +0.08 | 8.6 |
| 10ml | 14.5 | 14.42 | 19 | -0.08 | 8.6 |
| 10ml | 14.5 | 14.35 | 19 | -0.15 | 8.6 |
| 20ml | 19.1 | 19.18 | 25 | +0.08 | 11.2 |
| 20ml | 19.1 | 19.02 | 25 | -0.08 | 11.2 |
| 20ml | 19.1 | 18.95 | 25 | -0.15 | 11.2 |
| 30ml | 21.7 | 21.78 | 28 | +0.08 | 12.6 |
| 30ml | 21.7 | 21.62 | 28 | -0.08 | 12.6 |
| 30ml | 21.7 | 21.55 | 28 | -0.15 | 12.6 |

## Where the files are

- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (9 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/syringe-pistons/index.json

- syringe-piston-10ml-lap: [STEP](https://xemstudio.com/files/hardware/syringe-pistons/syringe-piston-10ml-lap.step) · [STL](https://xemstudio.com/files/hardware/syringe-pistons/syringe-piston-10ml-lap.stl) — 10ml syringe piston Ø14.58 × 19 (lapping stock, +0.08), two 5 mm lands, 5 mm fork slot (for a 5 mm thick con-rod), Ø3 pin hole at 8.6 mm
- syringe-piston-10ml-f008: [STEP](https://xemstudio.com/files/hardware/syringe-pistons/syringe-piston-10ml-f008.step) · [STL](https://xemstudio.com/files/hardware/syringe-pistons/syringe-piston-10ml-f008.stl) — 10ml syringe piston Ø14.42 × 19 (0.08 under the 14.5 barrel), two 5 mm lands, 5 mm fork slot (for a 5 mm thick con-rod), Ø3 pin hole at 8.6 mm
- syringe-piston-10ml-f015: [STEP](https://xemstudio.com/files/hardware/syringe-pistons/syringe-piston-10ml-f015.step) · [STL](https://xemstudio.com/files/hardware/syringe-pistons/syringe-piston-10ml-f015.stl) — 10ml syringe piston Ø14.35 × 19 (0.15 under the 14.5 barrel), two 5 mm lands, 5 mm fork slot (for a 5 mm thick con-rod), Ø3 pin hole at 8.6 mm
- syringe-piston-20ml-lap: [STEP](https://xemstudio.com/files/hardware/syringe-pistons/syringe-piston-20ml-lap.step) · [STL](https://xemstudio.com/files/hardware/syringe-pistons/syringe-piston-20ml-lap.stl) — 20ml syringe piston Ø19.18 × 25 (lapping stock, +0.08), two 5 mm lands, 5 mm fork slot (for a 5 mm thick con-rod), Ø3 pin hole at 11.2 mm
- syringe-piston-20ml-f008: [STEP](https://xemstudio.com/files/hardware/syringe-pistons/syringe-piston-20ml-f008.step) · [STL](https://xemstudio.com/files/hardware/syringe-pistons/syringe-piston-20ml-f008.stl) — 20ml syringe piston Ø19.02 × 25 (0.08 under the 19.1 barrel), two 5 mm lands, 5 mm fork slot (for a 5 mm thick con-rod), Ø3 pin hole at 11.2 mm
- syringe-piston-20ml-f015: [STEP](https://xemstudio.com/files/hardware/syringe-pistons/syringe-piston-20ml-f015.step) · [STL](https://xemstudio.com/files/hardware/syringe-pistons/syringe-piston-20ml-f015.stl) — 20ml syringe piston Ø18.95 × 25 (0.15 under the 19.1 barrel), two 5 mm lands, 5 mm fork slot (for a 5 mm thick con-rod), Ø3 pin hole at 11.2 mm
- syringe-piston-30ml-lap: [STEP](https://xemstudio.com/files/hardware/syringe-pistons/syringe-piston-30ml-lap.step) · [STL](https://xemstudio.com/files/hardware/syringe-pistons/syringe-piston-30ml-lap.stl) — 30ml syringe piston Ø21.78 × 28 (lapping stock, +0.08), two 5 mm lands, 5 mm fork slot (for a 5 mm thick con-rod), Ø3 pin hole at 12.6 mm
- syringe-piston-30ml-f008: [STEP](https://xemstudio.com/files/hardware/syringe-pistons/syringe-piston-30ml-f008.step) · [STL](https://xemstudio.com/files/hardware/syringe-pistons/syringe-piston-30ml-f008.stl) — 30ml syringe piston Ø21.62 × 28 (0.08 under the 21.7 barrel), two 5 mm lands, 5 mm fork slot (for a 5 mm thick con-rod), Ø3 pin hole at 12.6 mm
- syringe-piston-30ml-f015: [STEP](https://xemstudio.com/files/hardware/syringe-pistons/syringe-piston-30ml-f015.step) · [STL](https://xemstudio.com/files/hardware/syringe-pistons/syringe-piston-30ml-f015.stl) — 30ml syringe piston Ø21.55 × 28 (0.15 under the 21.7 barrel), two 5 mm lands, 5 mm fork slot (for a 5 mm thick con-rod), Ø3 pin hole at 12.6 mm

## How it fits

### Goes in or on
- [Plastic syringes 1–60 mL as pistons and cylinders](https://xemstudio.com/objects/plastic-syringes/): runs in the barrel; plunger discarded

### Connects to
- [Alpha V-twin frame for two 10 mL syringes: frame, crank disc and con-rods sized so it actually runs](https://xemstudio.com/objects/alpha-v-twin-frame/): two pistons in the alpha frame
- [Ross yoke alpha for two parallel 10 mL syringes: frame, yoke, rocking link, crank and con-rods, solved for 90°](https://xemstudio.com/objects/ross-yoke-kit/): conrod-45 in the fork
- [Alpha V-twin: 90° two-pin crank discs, V block, transfer tee](https://xemstudio.com/objects/alpha-twin-crank-90/): V-block alpha
- [Crank discs, connecting rods and flywheels for model engines](https://xemstudio.com/objects/crank-conrod-flywheel-kit/): con-rods
- [Can-engine printed kit: can caps, diaphragm clamps, displacer guides, crank stands, CD/platter hubs, syringe cradles](https://xemstudio.com/objects/can-engine-printed-kit/): can engine power piston

### Alternatives
- [Tri-clamp power pistons and crank linkage: full-bore ring-less pistons, con-rods, crank webs and a clamp-on crank frame for 1.5, 2 and 3 in spools](https://xemstudio.com/objects/tri-clamp-power-pistons/): full-bore tri-clamp pistons
- [Power cylinder liners, piston blanks, crosshead and guide (the non-syringe power piston)](https://xemstudio.com/objects/power-cylinder-piston-crosshead/): Ø16–30 blanks in a printed liner

### Also needed
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/): Ø3 × 12 piston pin
- [Printer calibration coupons: hole, pin, nut-trap, T-slot tongue and bearing-pocket gauges](https://xemstudio.com/objects/printer-calibration-coupons/): set the slicer first

## Questions

**Why replace the syringe plunger in a Stirling engine?**

Its rubber seal's friction is larger than the force a small Stirling makes.

**How does the con-rod attach to a syringe piston?**

The piston has a 5 mm fork slot and a Ø3 pin hole (both exact; see Mating features): drop a 5 mm thick con-rod (Ross conrod-45 or crank-kit rods) in and push a 3 × 12 dowel through.

**Which syringe piston do I print for a 10 mL syringe?**

Measure the barrel bore; for the usual 14.5 mm print f008 (Ø14.42), lap to fit, and keep f015 as a spare if the barrel is tight.

## Fits with

- [Plastic syringes 1–60 mL as pistons and cylinders](https://xemstudio.com/objects/plastic-syringes/)
- [Ross yoke alpha for two parallel 10 mL syringes: frame, yoke, rocking link, crank and con-rods, solved for 90°](https://xemstudio.com/objects/ross-yoke-kit/)
- [Alpha V-twin: 90° two-pin crank discs, V block, transfer tee](https://xemstudio.com/objects/alpha-twin-crank-90/)
- [Crank discs, connecting rods and flywheels for model engines](https://xemstudio.com/objects/crank-conrod-flywheel-kit/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)
- [Tri-clamp displacers and engine heads: 1.5, 2 and 3 in spools](https://xemstudio.com/objects/tri-clamp-displacers-and-heads/)
- [Can-engine printed kit: can caps, diaphragm clamps, displacer guides, crank stands, CD/platter hubs, syringe cradles](https://xemstudio.com/objects/can-engine-printed-kit/)