Rack55: a 5.5-inch rack width specification
Rack55 defines a compact, desk-scale rack interface while preserving core rack conventions (1U spacing, familiar fasteners, rail-mounted components). The defining requirement is 140 mm center-to-center rail hole spacing.
At a glance
The 140 mm rail hole center-to-center spacing and the use of standard EIA-310-D rack unit (U) vertical hole spacing together define the Rack55 mechanical interface. All other dimensions are derived, recommended, or implementation-specific.
Specification (summary)
| Specification name | Rack55 |
|---|---|
| Description | 5.5-inch rack width open hardware mechanical specification for desk-scale infrastructure. |
| Primary defining dimension | 140.0 mm center-to-center rail hole spacing |
| Vertical mounting pattern | EIA-310-D 1U = 44.45 mm; hole pitch per U: 15.88, 15.88, 12.70 mm |
| Typical rail width (informative) | 16.0 mm per rail; hole center 8.0 mm from rail edge |
| Fasteners | #10-32 (primary), M5 (optional) |
| Recommended clearance holes | 5.0 mm (#10-32), 5.3 mm (M5) |
| Interoperability | Any component respecting the 140 mm C-C rail hole spacing and EIA-310-D 1U vertical spacing is Rack55-compatible. |
Mechanical reference diagram
Not a fabrication drawing. This diagram is representative and not to scale. It exists to communicate the interface and the normative dimension.
Design intent
Rack55 exists to bring rack discipline to desk-scale infrastructure: compact switches, SBCs, edge nodes, small power distribution, and modular panels.
- Desk-scale: sized for desks and shelves where 10" racks are still bulky.
- Rack grammar preserved: 1U spacing, familiar mounting logic, predictable scaling.
- Material-agnostic: printed, laser-cut, bent metal, or off-the-shelf rails can coexist.
- Interoperable: the specification defines the interface, not a product ecosystem.
Licensing
Rack55 is an open hardware mechanical specification released under the CERN Open Hardware Licence v2 - Permissive (CERN-OHL-P-2.0).
For the full license text, see CERN-OHL-P v2.0.
Conformance
Rack55 conformance is determined by the mechanical interface. Materials and implementations are unconstrained.
- Required: rail hole centers are 140.0 mm apart (C-C).
- Required: vertical mounting follows EIA-310-D rack unit spacing (1U = 44.45 mm).
- Recommended: #10-32 hardware compatibility (printed clearance holes ~5.0 mm).
- Recommended: use slots or generous clearance when mixing printed and metal rails.
Rack55 does not standardize ear geometry, chassis widths, or rail fabrication details—only the mounting interface.
FAQ
Is Rack55 a product or a standard?
Rack55 is a mechanical interface specification, not an official standard or product. It defines where the holes are, not how parts must be built. Think of it as a community-driven dimensional reference rather than a formal industry standard.
Why 140 mm instead of exactly 139.7 mm?
140 mm is a clean metric reference that is close to 5.5 inches (139.7 mm). Rack55 treats 140 mm rail hole spacing as the normative requirement.
Does Rack55 require printed rails?
No. The intent is material-agnostic: printed, machined, laser-cut, or off-the-shelf metal rails can be used as long as the mounting interface is respected.
Can I use cage nuts or capture nuts?
Yes. Rack55 keeps familiar rack fastener conventions. Parts may use through-holes, heat-set inserts, or captive nut features.