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. A case built on M2.5 IEC subrack hardware, or on #4-40 groove nuts in a Vector T-strut, is not an EIS row, though a module's Ø3.2 mm hole (PANEL-010) clears those screws.

A mounting screw shall engage at least three full threads through a 2.0 mm panel (RAIL-008). Its length L shall fall inside this window, 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 upper bound keeps the tip off the cavity floor through a 1.6 mm panel, the thinnest EIS recommends.

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 (the Vector T-strut, half its 7.391 mm height), so a head of that size bears on the panel and on the rail face of every catalog rail; an ISO 7045 pan head or a 7 mm washer fits.

An enclosure shall publish the mounting-screw length, and shall supply those screws or name them (RAIL-011). A module need not ship with screws unless its panel is thinner than 1.6 mm (PANEL-014).

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 throat 2.92 mm: M3 does not pass Gie-Tec 125010 4.70 to 6.52 Tiptop Audio Z-rail at most 9.12 Schroff 34560 L-KD 8.70 to 10.60 Schroff 34560 L-OD at most 10.75 Schroff 34560 H-KD at most 10.60 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 EIS picks for that rail: the shortest stock length inside the window, or the longest that does not bottom out.

EIS · Mounting screws mounting face t, 1.6 to 2.0 mm seat insert D, screw cavity L, under the head Thread Lengths under the head M3 6 mm, 8 mm, 10 mm 2.0 mm + seat + 1.5 mm ≤ L ≤ 1.6 mm + D (RAIL-009) The throat clears 3.0 mm (RAIL-009) Pan head, at most 7.0 mm across (RAIL-010) Seat and D are measured behind the mounting face. Through a 2.0 mm panel, at least three 0.5 mm threads engage (RAIL-008).

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. EIS recommends the following for a new rail; a rail built another way conforms if it meets the requirements above.

Two vendor figures recur in EIS. The Vector T-strut height (drawing 5280006) is the small end of the recommended height below. The inner face of the Schroff 34560-184 section, with the mounting hole on the cavity center, sets the board opening in 5.2.2: 112.35 mm at 3U, 1.175 mm more on each side than a 110 mm board.

Constant Value Source
T-strut height across the panel 7.391 mm Vector, drawing 5280006
Inner face, in from the mounting hole 5.075 mm Schroff, 34560-184

A new rail inside these limits takes the 5.0 mm × 2.0 mm M3 strip most Eurorack cases use, and a 6, 8, or 10 mm screw. The limits are the envelope of the two extrusions EIS measured, rounded: the Vector T-strut at the small end, the Schroff L-KD at the large. A rail built another way conforms if it meets 5.3.

Rail section millimeters
Height, across the panel 7.0 to 10.0
Depth, front face to back face (F + D + G) 25.0 to 28.5
Lip, if present, across the panel at most 1.5
Lip, if present, in front of the mounting face (F) at most 3.1
Groove, across at least 5.0
Groove, into the rail at least 2.0

A lip is optional; its two limits include its corner radius. The groove admits a rectangular insert 5.0 mm across and 2.0 mm thick; a larger groove is fine if that rectangle enters and stays. The throat in front of the insert clears 3.0 mm (RAIL-009). EIS recommends that a rail maker publish the insert seat, the screw-cavity depth D, and a screw length that fits the window (RAIL-009).

EIS · The recommended rail section groove ≥ 5 × 2 mm mounting face smallest: 25 mm deep largest: 28.5 mm deep smallest: 7 mm high largest: 10 mm high the smallest section the largest section A rail whose section lies between the two takes the 5.0 mm × 2.0 mm insert and a 6, 8, or 10 mm screw. A lip is optional: at most 1.5 mm across the panel and 3.1 mm into the case.

Threaded strip millimeters
Across the groove 5.0
Thickness 1.5 to 2.0
Hole pitch 5.08 (RAIL-001)
First hole, from one end 3.58
Margin past the last hole 6.08

An N HP strip is N × 5.08 + 4.58 mm long with N holes, or is cut from a longer strip; the first-hole and end-margin figures are measured on the Schroff 34561-484 model. A slide nut fits the same 5.0 mm × 2.0 mm section. M3 slide nuts for Eurorack rails are made to the groove; a DIN 562 M3 square nut, 5.5 mm across flats, does not fit it.

EIS · Rail inserts End sections, to scale. Dashed: the 5.0 mm × 2.0 mm groove. An EIS insert is M3 (RAIL-003) and at least 1.5 mm thick (RAIL-004); the M2.5 parts, in grey, are subrack hardware of the same section. Thread Form nVent Schroff 34561-484, tap 2.5 mm M3 strip nVent Schroff 34561-3xx and 34565-196, tap 2.05 mm M2.5 strip Gie-Tec 138049 M3 strip apra M2.5, 5 mm × 1.5 mm M2.5 strip DIN 562 M2.5, 5.0 mm × 1.6 mm M2.5 slide nut

Screw
Lengths 6, 8, or 10 mm, measured under the head; each is the pick for at least one catalog rail (eis-reference)
Insert tap drill 2.5 mm, the Schroff figure
Head Pan head with an H, Z, or hexalobular recess (ISO 7045:2011, ISO 14583:2011); at most 7.0 mm across (RAIL-010)
Finish Zinc-plated steel, black-finished steel, or A2 stainless steel
Pick The shortest length inside the window; where a catalog rail's seat is not published, the longest that does not bottom out through a 1.6 mm panel, then confirm three threads engage (RAIL-008)

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. The catalog rail sections and inserts measured against these limits, and the screw that fits each, are in the eis-reference repository.