3 in crankcase caps: blind bearing cap, seal cap and 3-port cap — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/htd-crankcase-caps-3in/  (CC BY 4.0)
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
Catalog version: 2026-10-20

Sizes: see table below
- 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.

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

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- 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 — standard — ASME; dimensions are facts [verify license at link] — https://www.asme.org/codes-standards/find-codes-standards/b1-20-1-pipe-threads-general-purpose-inch
- Tameson (product listing): Rotary shaft seal 12 × 22 × 7 mm (DIN 3760 sizes) — web — vendor page; dimensions are facts [verify license at link] — https://tameson.com/products/s2avq-x20-nbr-design-a-rotary-shaft-seal-12x22x7mm-20-pieces
- Long Earth Society: Engine builds — web, video — LES site — https://longearthsociety.com/engineering/

Free files (CC BY 4.0, Xem Studio; STEP + STL, 3 parts): https://xemstudio.com/files/hardware/htd-crankcase-caps-3in/index.json
API: https://xemstudio.com/api/geometry?object=htd-crankcase-caps-3in