# LTD Stirling displacer Ø96 × 8 and Ø146 × 8

> What size displacer goes in a coffee-cup Stirling engine?

- Category: Stirling engines › Can & LTD engines
- Kind: Hardware
- Basis: Senft LTD rule: displacer diameter ≫ stroke; diameter = chamber bore − 4 (2 mm radial gap, designed)
- Page: https://xemstudio.com/objects/ltd-displacer/ · JSON: https://xemstudio.com/api/geometry?object=ltd-displacer · Plain text: https://xemstudio.com/objects/ltd-displacer/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 wide, thin displacer of the LTD engine: a solid disc Ø96 × 8 for the Ø100 chamber or Ø146 × 8 for the Ø150, with a Ø2 hole at x −5 (under the top plate's rod guide) for the 2 mm rod.
- The 2 mm radial gap to the chamber wall is designed: air passes round the edge as the displacer moves.
- In the 20 mm chamber it moves 10 mm; at Ø96 that sweeps about 72 mL.

## 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 |
|---|---|---|---|---|
| displacer rod hole | Ø2 | 2 mm rod | press | ltd-100-displacer* |
| displacer rod hole | Ø2 | 2 mm rod | press | ltd-150-displacer* |
| displacer to chamber gap | 2 radial (Ø D−4 in Ø D) | chamber ring bore | designed | ltd-100-displacer* |
| displacer to chamber gap | 2 radial (Ø D−4 in Ø D) | chamber ring bore | designed | ltd-150-displacer* |

## Sizes

| file | diameter | thickness | chamber | radial gap | rod hole |
|---|---|---|---|---|---|
| ltd-100-displacer | 96 | 8 | 100 | 2 | 2 |
| ltd-150-displacer | 146 | 8 | 150 | 2 | 2 |

## Where the files are

- Wikipedia: [Stirling engine](https://en.wikipedia.org/wiki/Stirling_engine) — web — CC BY-SA 4.0 (text)
- James R. Senft: [An Introduction to Low Temperature Differential Stirling Engines](https://www.stirlingengine.com/) — book — Moriya Press (verify at link)
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (2 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/ltd-displacer/index.json

- ltd-100-displacer: [STEP](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-100-displacer.step) · [STL](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-100-displacer.stl) — displacer Ø96 × 8 with a Ø2 rod hole at x -5 under the top plate's guide (2 mm radial gap to the Ø100 chamber: designed)
- ltd-150-displacer: [STEP](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-150-displacer.step) · [STL](https://xemstudio.com/files/hardware/ltd-coffee-cup-engine-kit/ltd-150-displacer.stl) — displacer Ø146 × 8 with a Ø2 rod hole at x -5 under the top plate's guide (2 mm radial gap to the Ø150 chamber: designed)

## How it fits

### Goes in or on
- [LTD Stirling chamber ring, Ø100 and Ø150 × 20](https://xemstudio.com/objects/ltd-chamber-ring/): inside the Ø100 / Ø150 bore, 2 mm radial gap

### Connects to
- [LTD Stirling top plate with syringe socket, Ø120 and Ø170](https://xemstudio.com/objects/ltd-top-plate/): its 2 mm rod runs in the top plate's Ø2 bore

### Also needed
- [Hobby brass tube (K&S), 1/16–21/32 in and 1–12 mm, telescoping](https://xemstudio.com/objects/brass-tube-ks/): 2 mm rod in the Ø2 hole

## Questions

**What size displacer for a coffee cup Stirling engine?**

Ø96 × 8 in a Ø100 × 20 chamber at 10 mm stroke on the engine here; Ø146 × 8 in the Ø150 chamber.

**Why is an LTD displacer so wide and thin?**

With a 15–20 K difference the pressure swing is small, so it needs a large area moved a short way with little friction.

## Fits with

- [LTD Stirling chamber ring, Ø100 and Ø150 × 20](https://xemstudio.com/objects/ltd-chamber-ring/)
- [LTD Stirling top plate with syringe socket, Ø120 and Ø170](https://xemstudio.com/objects/ltd-top-plate/)
- [LTD Stirling bottom plate, Ø120 and Ø170](https://xemstudio.com/objects/ltd-bottom-plate/)
- [Build list: LTD coffee-cup engine](https://xemstudio.com/objects/list-ltd-coffee-cup/)