5.3 Rails, inserts, and mounting screws

A front panel screws to a pair of rails through a threaded rail insert in each. Five things make a module fit any conforming row:

What Value Requirement
Hole span 122.5 mm at 3U, 33.65 mm at 1U, between the inserts of a row's two rails PANEL-009
Cover At most 9.25 mm of the panel face at each edge PANEL-012
Hole pitch 5.08 mm along the insert RAIL-001
Thread M3, in the insert and on the screw RAIL-003, RAIL-005
Screw length Inside the length window below RAIL-009

Rails and inserts

The holes in a rail insert shall be 5.08 mm apart (RAIL-001), so that every panel hole meets one. A rail for an N HP row shall give at least N × 5.08 mm of mounting length (RAIL-002).

An insert shall be threaded M3 (RAIL-003) and shall be at least 1.5 mm thick, three thread pitches (RAIL-004).

Mounting screws

A mounting screw shall be M3 (RAIL-005): its thread pitch shall be 0.5 mm (RAIL-006) and its major diameter 3.0 mm (RAIL-007), as ISO 261:1998 gives them. M3 is the thread of the Doepfer A-100 frame, of nearly every Eurorack case on sale, and of the screws modules ship with. IEC subrack hardware is often M2.5, and the Vector T-strut takes #4-40 groove nuts; a case built on such rails is not an EIS row, though a module's Ø3.2 mm hole (PANEL-010) still clears those screws.

The screw shall engage at least three full threads through a 2.0 mm panel (RAIL-008). Its length L shall fall inside this window, which keeps the tip off the cavity floor through a 1.6 mm panel, the thinnest EIS recommends, and the throat in front of the insert shall clear the 3.0 mm major diameter (RAIL-009):

2.0 mm + seat + 1.5 mm  ≤  L  ≤  1.6 mm + D

Seat is the depth of the insert's front face behind the rail's mounting face, and D is the depth of the screw cavity behind it (Terms). The screw head shall be at most 7.0 mm across (RAIL-010): the narrowest catalog rail face reaches 3.7 mm past the hole center (Annex A), so a head of that size bears on the panel and on the rail face of every catalog rail, and an ISO 7045 pan head or a 7 mm washer fits.

The enclosure shall state the mounting-screw length, and shall supply those screws or name them (RAIL-011). A module need not ship with screws.

EIS · The M3 screw length window of each catalog rail 3 4 5 6 7 8 9 10 11 12 screw length under the head, mm 6 mm 8 mm 10 mm Vector TS100 5.53 to 7.44 Gie-Tec 125010 4.7 to 6.52 TipTop Z-rail at most 9.12 Schroff 34560 L-KD 8.7 to 10.6 Schroff 34560 L-OD at most 10.75 Schroff 34560 H-KD at most 10.6 HEITEC 9908.721 at most 10.45 Shortest: 2.0 mm panel + seat + three 0.5 mm threads; longest: 1.6 mm panel + D (RAIL-009). A dashed bar is a rail whose insert seat is not published, so only the longest length is known. The dot is the screw of Annex A.3: the shortest stock length inside the window, or the longest that does not bottom out.

EIS · Mounting screws t, 1.6 to 2.0 seat insert D, screw cavity L, under the head mounting face ThreadLengths under the head M36 mm, 8 mm, 10 mm M2.56 mm, 8 mm, 10 mm #4-40 UNC1/4 in, 5/16 in, 3/8 in Pan head, at most 7.0 mm across t max + seat + 3 × pitch ≤ L ≤ t min + D The throat clears the major diameter

Everything else about a rail, an insert, and a screw is construction: the extrusion section and its lip, the groove, the insert width and ends, the screw lengths, the tap drill, the head form, and the finish. Annex A gives the section EIS recommends for a new rail, the 5.0 mm × 2.0 mm insert, the 6, 8, and 10 mm screw lengths, and the screw that fits each catalog rail. The numbers are in data/rail.json, data/insert.json, and data/screw.json; tools/rail.py, tools/insert.py, tools/insert_card.py, and tools/screw.py check them and draw the cards.