Screws and holes for panel mounting with extruded rails

Most of you I guess use wood screws and wooden rails to mount your modules, and if so, you can safely read no further.

For those who use extruded rails with nuts (or threaded inserts) and machine screws:

Mostly I’ve used either pan head or socket head cap screws. But Befaco sells “knurlies”, these lovely things:

I have a few (like about 6) of these and they’re wonderful. They’re also 38,70 € per hundred, or $40 at SynthCube. (The link is to the M3 version but they also come in M2.5, which is what I’m chiefly interested in.)

I figured these must be something that’s available in the non-Eurorack market for less, but haven’t found quite the same thing. Have you?

About the closest I’ve found are these

Head diameter looks to be about 6 mm, compared to about 4 or 5 mm for Befaco knurlies. Lot of 190 (seriously? 190?) for $34.25. I don’t see these in smaller lots. I do see many vendors with these:

5 for $1.70, so about the same price each. There the head size is 9 mm for M2.5. I have some of these, both M2.5 and M3, and they’re pretty good.

Problem is: the Kosmo Specification says:

That means any head diameter above 6 mm won’t work on adjacent modules (unless you leave a gap). Knurlies will fit although they’re very close together. By the way, I note M3 square nuts are 6 mm wide, so just barely work for Kosmo modules.

In Eurorack it’s common to have slotted mounting holes which allow for some flexibility in mounting. Typically I think they’re 1HP (0.2" ~ 5 mm) wide and centered 1.5HP (0.3" ~ 7.5 mm) from the edge. So there’s a good deal more room for wide heads (and nuts) there.

I’m thinking of going with slotted mounting holes for my future FR4 panels, with one end coinciding with the Kosmo spec position but extending inward a few mm from there. (Pretty sure JLCPCB won’t do plated through slots, but I would think grounded pads on each side would be possible.) For homemade panels I may just switch to drilling holes further from each edge. For Kosmo panels from LMNC or others, well, I may just be stuck with not being able to use wide heads everywhere I’d like (or with drilling new holes).

Any further suggestions or ideas?

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On Apr 22, Sam said :

Wait and see…
But I too would like slotted holes.

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I’ve been using M3/8mm knurlies with a 11 mm head like the ones in the aliexpress picture in a euro rack configuration and they may clash with the ones used to fasten modules above and below them. I still use them (because I have lots of them) but I use them mostly in the top and bottom rails of combined 19 inch racks (see pic below). But even the smaller headed brass ones I use in the center rails sometimes clash. I think that explains that the Befaco ones have a really small head.

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I would prefer slotted holes on all Kosmo modules. Especially with a wood rail system where it would be nice to reuse existing holes.

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Even genuine Knurlies sometimes clash with angled rows. I also suspect your choices are limited by the M2.5 size required by Vector rails.

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In germany they called “Rändelschrauben” or “Handschrauben” maybe you find something? In 3mm its no problem.

Maybe this LINK helps you?!?

Iguess this LINK is better, they are m2,5

Sure, it’s not hard to find screws like that, with 8 mm heads. But those are too large for standard Kosmo.

Thumb screw is a common English name. Digikey etc has lots of variants, and places like McMaster-Carr even more, but they’re usually pretty expensive.


The Knurlies from Befaco really are a great product. They work with thumbs, Phillips head, flat heat, and hex wrench. They have a built in washer and they’re 7mm, so they work for the two depths of rail that are common in my experience where you need 6mm for one and 8mm for the other. They are costly for sure, but if you’re going down the rail route and can swing it, I think they’re worth it.


I’ve thought about it, and that was before I heard your sales pitch!


Oh, they will:


Which EDA do you use? Anyone know how to specify this in KiCad?

Added: Apparently at least as of 2018, KiCad’s supported rendering of plated through slots was as elongated holes to be “cut with a single translation of a milling cutter with a given diameter” and specified in the drill file, whereas such a hole “is not supported by any manufacturer as they all or > 90% support only Slots and cut-outs by doing: SMD pads on both top and bottom layers and then drawing the Slots/Cutouts on Edge.Cuts(Board Outline) layer”.

EasyEDA. It’s right in the name :stuck_out_tongue_winking_eye:

Email from JLCPCB, they say they can go in either the drill file or the outline layer but they suggest the drill file.


For the little I know about the zillions steps of modern industrial PCB fabrication, drilling happens before copper plating, so if they go in the drill file, they should be plated.


There’s such a thing as unplated holes and slots. See


Working on some more panel designs