# Crank discs, connecting rods and flywheels for model engines

> What stroke and flywheel size does a small Stirling engine need?

- Category: Hardware › Stirling & thermoacoustic
- Kind: Hardware
- Basis: Parametric parts sized for 3 mm crank pins, 623 bearings (3 × 10 × 4) in the rod ends, and 5 / 8 mm shafts; proportions from the Long Earth Society tri-clamp engines
- Page: https://xemstudio.com/objects/crank-conrod-flywheel-kit/ · JSON: https://xemstudio.com/api/geometry?object=crank-conrod-flywheel-kit · Plain text: https://xemstudio.com/objects/crank-conrod-flywheel-kit/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

## Notes

- Crank disc: pin hole at stroke/2 from the bore; 3 mm dowel pin pressed in (2.95 mm hole in metal, 3.2 mm printed then reamed). Set screw locks the disc to a 5 mm shaft.
- Con-rod: 10.2 mm pockets take a 623 bearing at each end so the rod runs on balls, not plastic; centres 30–80 mm. Rod length ≥ 2 × stroke keeps the piston side-load low.
- Flywheel: energy ∝ D⁴, so go wide and thin: Ø100 × 10 mm in PLA with a brass rim insert outruns Ø60 solid. Balance by drilling the heavy side after the first run.
- Long Earth Society: the 2 in tri-clamp engines use a 20 mm stroke, 50 mm rod and Ø100 flywheel on an 8 mm shaft in 608 bearings.

## Sizes

| part | sizes | shaft_or_pin | bearing |
|---|---|---|---|
| crank disc | stroke 10, 15, 20, 25, 30 | Ø5 shaft, Ø3 pin | — |
| con-rod | centres 30, 40, 50, 60, 80 | Ø3 pins | 623 both ends |
| flywheel | Ø50, 80, 100, 120, 150 × 10 | Ø5 (50) / Ø8 | 608 on the shaft |

## Where the files are

- Wikipedia: [Crank (mechanism)](https://en.wikipedia.org/wiki/Crank_(mechanism)) — web — CC BY-SA 4.0 (text)
- Wikipedia: [Flywheel](https://en.wikipedia.org/wiki/Flywheel) — web — CC BY-SA 4.0 (text)
- Wikipedia: [Stirling engine](https://en.wikipedia.org/wiki/Stirling_engine) — web — CC BY-SA 4.0 (text)
- Long Earth Society: [Engine builds that use these parts (videos and write-ups)](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (15 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/index.json

- crank-disc-stroke10: [STEP](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/crank-disc-stroke10.step) · [STL](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/crank-disc-stroke10.stl) — crank disc for 10 mm stroke (pin at 5.0 mm), Ø5 bore, M3 set screw, Ø3 pin hole
- crank-disc-stroke15: [STEP](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/crank-disc-stroke15.step) · [STL](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/crank-disc-stroke15.stl) — crank disc for 15 mm stroke (pin at 7.5 mm), Ø5 bore, M3 set screw, Ø3 pin hole
- crank-disc-stroke20: [STEP](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/crank-disc-stroke20.step) · [STL](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/crank-disc-stroke20.stl) — crank disc for 20 mm stroke (pin at 10.0 mm), Ø5 bore, M3 set screw, Ø3 pin hole
- crank-disc-stroke25: [STEP](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/crank-disc-stroke25.step) · [STL](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/crank-disc-stroke25.stl) — crank disc for 25 mm stroke (pin at 12.5 mm), Ø5 bore, M3 set screw, Ø3 pin hole
- crank-disc-stroke30: [STEP](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/crank-disc-stroke30.step) · [STL](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/crank-disc-stroke30.stl) — crank disc for 30 mm stroke (pin at 15.0 mm), Ø5 bore, M3 set screw, Ø3 pin hole
- conrod-30: [STEP](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/conrod-30.step) · [STL](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/conrod-30.stl) — connecting rod, 30 mm centres, 10.2 mm pockets for 623 bearings both ends, 5 thick
- conrod-40: [STEP](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/conrod-40.step) · [STL](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/conrod-40.stl) — connecting rod, 40 mm centres, 10.2 mm pockets for 623 bearings both ends, 5 thick
- conrod-50: [STEP](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/conrod-50.step) · [STL](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/conrod-50.stl) — connecting rod, 50 mm centres, 10.2 mm pockets for 623 bearings both ends, 5 thick
- conrod-60: [STEP](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/conrod-60.step) · [STL](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/conrod-60.stl) — connecting rod, 60 mm centres, 10.2 mm pockets for 623 bearings both ends, 5 thick
- conrod-80: [STEP](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/conrod-80.step) · [STL](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/conrod-80.stl) — connecting rod, 80 mm centres, 10.2 mm pockets for 623 bearings both ends, 5 thick
- flywheel-50: [STEP](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/flywheel-50.step) · [STL](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/flywheel-50.stl) — spoked flywheel Ø50 × 10, Ø5 bore, M3 set screw
- flywheel-80: [STEP](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/flywheel-80.step) · [STL](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/flywheel-80.stl) — spoked flywheel Ø80 × 10, Ø8 bore, M3 set screw
- flywheel-100: [STEP](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/flywheel-100.step) · [STL](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/flywheel-100.stl) — spoked flywheel Ø100 × 10, Ø8 bore, M3 set screw
- flywheel-120: [STEP](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/flywheel-120.step) · [STL](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/flywheel-120.stl) — spoked flywheel Ø120 × 10, Ø8 bore, M3 set screw
- flywheel-150: [STEP](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/flywheel-150.step) · [STL](https://xemstudio.com/files/hardware/crank-conrod-flywheel-kit/flywheel-150.stl) — spoked flywheel Ø150 × 10, Ø8 bore, M3 set screw

## Questions

**How big should a Stirling engine flywheel be?**

For a 2 in (50 mm bore) displacer engine, Ø100 × 10 mm is a good start; energy stored goes with the fourth power of diameter, so diameter beats thickness.

**What bearings go in a model engine connecting rod?**

623 (3 × 10 × 4) at each end on 3 mm pins; the rod pocket is 10.2 mm in a print.

**How do I set the stroke on a crank disc?**

Stroke = 2 × pin offset. The discs here have the pin at 5, 7.5, 10, 12.5 and 15 mm for 10–30 mm strokes.

## Fits with

- [Spur gears, involute 20°: module 0.5, 1 and 2, 12–60 teeth](https://xemstudio.com/objects/spur-gears-involute/)
- [Ball bearings, 683 to 6210 and flanged F6xx (ISO 15)](https://xemstudio.com/objects/ball-bearings-metric/)
- [608 ball bearing](https://xemstudio.com/objects/608-bearing/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)
- [Tri-clamp spool (two ferrules on a tube), 1.5 and 2 inch](https://xemstudio.com/objects/tri-clamp-spool/)
- [Set screws (grub screws), M3–M8 (ISO 4026)](https://xemstudio.com/objects/set-screws-metric/)