Normally when you mount an AT board in an ATX case, the keyboard port is near the bottom/right side of the I/O shield area. In this configuration it won't fit in an mATX case. But if you shift the board up/left as far as possible it might. Most of the mounting holes will not line up though. You would need to drill new ones.

I have fit them in full ATX cases and even had a replacement cover plate I could screw the onboard serial/parallel/ etc to said plate

Doubt it. Maybe you have a modern atx case where all holes are present, then you could make a real world test without buying a new case.

This is something I thought of too 😀

It's one of the things I spent time on since I got my 3d printer a couple of years ago. I've successfully fit a 486 and 386 baby AT motherboards into two different mATX cases.
You can see pictures of my adapters here: Re: Bought these (retro) hardware today

My 486 case is an unknown make full-height mATX desktop case with an SFX PSU. I bought it specially because I could see that unlike many mATX cases, the standoffs could just be unscrewed, so adapters can sit much lower with no risk of screws causing shorts.
My 386 build is in a Yeong Yang A200 case, which was my reference 3d model as someone shared very detailed design files for it. It has pressed in standoffs and works great with the 3mm 'riser' to stop the board shorting on the standoffs. Somehow the case was sold with no front panel, so of course I made one with the 3d printer and made it a desktop instead of a tower:

The files can be had here if you've got a 3d printer: download/file.php?id=92248
You can make some adapters that sit the motherboard 3 or 4mm higher than the original standoffs, then there's a separate riser for the card slots. I recommend to print them in PETG since PLA can warp at quite low temperatures.

The zip file includes:

• The backplate for ATX > AT with that micro ATX position, which has a built-in card slot for PS/2 / serial / parallel ports. That secures in place with a square m3 nut that presses into the screw hole to make a strong thread.
• An adapter to raise up the card slots mounting surface in-line with the mainboard's new height. This will need longer screws to secure the cards.
• Front & back mATX to AT adapters. The screw threads designed into these are UNC 6/32 fitting.

Beware that the adapter bracket screws attaching to the original standoff locations might be very close to the back of the board. Use small hot swap caddy screws (UNC 6/32 countersunk) if the original risers use UNC 6/32 (coarse thread) or countersunk M3 screws (fine thread) . And / Or put insulation tape on top of the screws.

For the ATX power, I just use TFX or SFX power supplies with an ATX > AT adapter, sometimes with a -5v adapter modded in. My power switch also is just converted from a regular momentary switch to a latching switch that connects into the green + black wire of the cheap ATX > AT adapter cables.

Interesting. There are two different different sets of mATX to ATX adapters - why if I may ask?

Ah I didn't realise that I'd shared both types. The 3mm version is for use where the standoffs can't be removed, so the ATX to AT adapter will sit on top of those standoffs, raising the motherboard in the case by 3mm. Then the 3mm card bracket riser is needed to screw the cards in securely.

The Quarter-Inch version is for where the standoffs are removable in the case, so the ATX to AT bracket screws directly into where the standoffs would've gone. This one's more secure, but I don't seem to have many Micro ATX cases that have removable standoffs. This type of fitting means that the cards sit at their original height too, so no bracket riser required.

I see. The NetVista case has unremovable standoffs; more like raised parts of tray thann/ standoffs really, so the 3mm version for me. But wouldn't that also change the IO plate?

The DIN plug has lots of clearance to work with either position 😁

Thermalwrong wrote on 2022-01-18, 20:25:

Ah I didn't realise that I'd shared both types. The 3mm version is for use where the standoffs can't be removed, so the ATX to AT adapter will sit on top of those standoffs, raising the motherboard in the case by 3mm. Then the 3mm card bracket riser is needed to screw the cards in securely.

The Quarter-Inch version is for where the standoffs are removable in the case, so the ATX to AT bracket screws directly into where the standoffs would've gone. This one's more secure, but I don't seem to have many Micro ATX cases that have removable standoffs. This type of fitting means that the cards sit at their original height too, so no bracket riser required.

Thanks a lot mate! Recently got a 3D printer but yet to be unboxed and assembled.
So, if I get a case with removable standoffs, I can only use ATX-to-AT-v5-quarterinch + AT IO shield (ATmotherboardinMATXcase-v1-flat.stl ?) ?

What is the difference between these two files ?
ATX-to-AT-v5-quarterinch-casefront-flat.stl
ATX-to-AT-v5-quarterinch-caserear-flat.stl

Plasma wrote on 2022-01-17, 18:05:

Normally when you mount an AT board in an ATX case, the keyboard port is near the bottom/right side of the I/O shield area. In this configuration it won't fit in an mATX case. But if you shift the board up/left as far as possible it might. Most of the mounting holes will not line up though. You would need to drill new ones.

that will work, but then the top 3(ish) expansion slots will no longer align to the case and will become un-usable.

Like other users suggested, 3d printing is an option - remove the I/O + expansion card holder from the case (if possible - on some cases the whole expansion slot + i/o side is one piece, riveted to the rear panel, on others it's part of the rear) and 3d-print a new one that conforms to the AT standard. The rest is easy (drilling new holes).

I have a couple of AT and mATX cases that are identical and from the same manufacturer (some no-name brand - possibly inter tech) Same plastic bezel, same covers, same "skeleton", the only difference is the motherboard tray holes and the i/o + expansion slots. I suppose the manufacturer wanted to use one set of tools for most of the work on both AT and ATX cases - smart move from an economic point of view. As such the rear i/o and expansion is riveted to the back of the skeleton and can be rather easily removed and swapped out. The AT version has 7 cutouts for expansion cards, a larger hole for a DIN connector + a smaller one right under for a mini-din, while the mATX version only has 5 and a rectangular cutout for the I/O shield.

It's been a while since I've thought about this 😀
The casefront and caserear files are two separate parts. One connects the screw holes near the front of the case to AT motherboard locations, the other is for the bit at the rear. The 3d printed version I've shared doesn't need to remove any existing case parts or have holes drilled.

If you're doing this with an ISA only board it's fine and it should also be good for a PCI 486 board where you'd have maybe 3 ISA ports and the only accessible PCI port would be used for video.
However if you're putting a Vesa Local Bus motherboard in there, you have to find one that's got VLB slots nearer to the bottom slots if you want to use more than one VLB card.

Thermalwrong wrote on 2023-06-02, 01:11:

It's been a while since I've thought about this :) The casefront and caserear files are two separate parts. One connects the scr […]
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It's been a while since I've thought about this 😀
The casefront and caserear files are two separate parts. One connects the screw holes near the front of the case to AT motherboard locations, the other is for the bit at the rear. The 3d printed version I've shared doesn't need to remove any existing case parts or have holes drilled.

If you're doing this with an ISA only board it's fine and it should also be good for a PCI 486 board where you'd have maybe 3 ISA ports and the only accessible PCI port would be used for video.
However if you're putting a Vesa Local Bus motherboard in there, you have to find one that's got VLB slots nearer to the bottom slots if you want to use more than one VLB card.

Thanks a lot mate! Actually I am going to use it with Pentium MMX era motherboards. I think couple of PCI slots will be covered on 2 & 3 ones

1 = 1 PCI & 1 ISA
2 = 4 PCI & 3 ISA
3 = 3 PCI & 2 ISA