# 3 in crankcase caps: blind bearing cap, seal cap and 3-port cap

> How do I put bearings and a shaft seal in a 3 inch tri-clamp cross?

- Category: Stirling engines › Tri-clamp engine parts
- Kind: Hardware
- Basis: Fabrication spec: 3 in cap profile with a Ø76.2 neck (the 20° back cone runs to the tube OD, then straight), bored and ported; exact geometry
- Page: https://xemstudio.com/objects/htd-crankcase-caps-3in/ · JSON: https://xemstudio.com/api/geometry?object=htd-crankcase-caps-3in · Plain text: https://xemstudio.com/objects/htd-crankcase-caps-3in/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

**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

- Blind cap (−X branch): a 6001 at the end of a Ø36 boss that reaches 20 into the branch. Seal cap (+X): a 6001 at the boss end and another near the outer face, the shaft seal's housing (12 × 22 × 7) at the outer face, three M4 holes for the generator bracket.
- Port cap (top of the cross): three 1/4 NPT ports for the fill valve, the gauge and the relief valve. The crankcase is the bounce space: it reaches the working space's mean pressure through the piston's fit and the rod bore.
- The shaft seal is the one leak path to the outside of a charged engine: it has to be a seal rated for the relief pressure in that housing size, and a charged engine will still lose helium slowly through it.

## 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 |
|---|---|---|---|---|
| faces | ISO 2852 3 in (neck Ø76.2) | gasket-3in + 3 in clamp | clamp | all |
| bearing seats | Ø28 × 8 | bearing-6001 outer ring | press | bearing-cap-* |
| bearing seats | Ø28 × 8 (two) | bearing-6001 outer rings | press | seal-cap-* |
| seal housing | Ø22 × 7 | radial shaft seal 12 × 22 × 7 | press | seal-cap-* |
| shaft clearance | Ø14 | Ø12 shaft (no contact) | designed | all |
| bracket holes | 3 × M4 (drawn Ø4 × 10) on r 30 | shcs-m4x12 | designed | seal-cap-* |
| ports | 3 × 1/4-18 NPT, tap drill Ø11.1 × 11 | fill valve, gauge, relief valve (1/4 NPT male) | designed | port-cap-* |

## Sizes

| part | thickness_mm | seats |
|---|---|---|
| bearing-cap-3in-blind-6001 | 28.0 | 6001 |
| seal-cap-3in-6001-seal12 | 30.0 | 2 × 6001 + seal 12 × 22 × 7 |
| port-cap-3in-3x-npt14 | 16.0 | 3 × 1/4 NPT |

## Where the files are

- ASME: [ASME B1.20.1 Pipe Threads (NPT): 1/8-27 and 1/4-18, tap drills Ø8.6 / Ø11.1 as on this site's thread envelopes](https://www.asme.org/codes-standards/find-codes-standards/b1-20-1-pipe-threads-general-purpose-inch) — standard — ASME; dimensions are facts (verify at link)
- Tameson (product listing): [Rotary shaft seal 12 × 22 × 7 mm (DIN 3760 sizes)](https://tameson.com/products/s2avq-x20-nbr-design-a-rotary-shaft-seal-12x22x7mm-20-pieces) — web — vendor page; dimensions are facts (verify at link)
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (3 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/htd-crankcase-caps-3in/index.json

- bearing-cap-3in-blind-6001: [STEP](https://xemstudio.com/files/hardware/htd-crankcase-caps-3in/bearing-cap-3in-blind-6001.step) · [STL](https://xemstudio.com/files/hardware/htd-crankcase-caps-3in/bearing-cap-3in-blind-6001.stl) — 3 in blind bearing cap: ISO 2852 face, Ø76.2 neck to 28, inner boss Ø36 × 20 with a 6001 seat
- seal-cap-3in-6001-seal12: [STEP](https://xemstudio.com/files/hardware/htd-crankcase-caps-3in/seal-cap-3in-6001-seal12.step) · [STL](https://xemstudio.com/files/hardware/htd-crankcase-caps-3in/seal-cap-3in-6001-seal12.stl) — 3 in seal cap: face, neck to 30, inner boss with a 6001 seat, a second 6001 seat 14..22, 12 × 22 × 7 shaft-seal housing at the outer face, 3 × M4 tapped on r 30
- port-cap-3in-3x-npt14: [STEP](https://xemstudio.com/files/hardware/htd-crankcase-caps-3in/port-cap-3in-3x-npt14.step) · [STL](https://xemstudio.com/files/hardware/htd-crankcase-caps-3in/port-cap-3in-3x-npt14.stl) — 3 in port cap: neck to 16, three 1/4 NPT ports on r 20 (fill valve, gauge, relief valve)

## Questions

**Where do the fill valve, gauge and relief valve go?**

In the 3 in port cap on top of the crankcase cross; the working space follows through the piston's fit.

## Fits with

- [Build list: HTD engine, 10 in tri-clamp (high-temperature-differential flat engine)](https://xemstudio.com/objects/list-htd-10in/)
- [HTD crank and linkage: webs, link web, con-rods, clevis and Ø12 shafts for a 3 in cross crankcase](https://xemstudio.com/objects/htd-crank-linkage/)
- [Radial shaft seal 12 × 22 × 7 (envelope)](https://xemstudio.com/objects/radial-shaft-seal-12x22x7/)
- [NPT male thread envelopes 1/8 to 1 in (ASME B1.20.1)](https://xemstudio.com/objects/npt-thread-envelopes/)