Five Millimetres
A 3.5-slot GPU leaves 5 mm above the next slot. Every riser on the market needs 8–15. A tour of CAD teardowns, a vendor with no drawings, and the fix that made it all moot.
The plan was simple: keep the RTX 4090 for gaming (VR racing is non-negotiable), add more cards for inference. The problem was geometric. A 3.5-slot 4090 overhangs the adjacent PCIe slot and leaves about 5 mm of clearance above it. Every commercial riser connector I could find stands 8–15 mm tall.
I tried three cables plus right-angle adapters, including one that exits downward specifically for clearance situations. All too tall. This kicked off a week of the kind of engineering archaeology that consumer PC building shouldn’t require but regularly does.
Reading CAD because nobody publishes drawings
The most promising parts came from ADT-Link, whose R33/M33/K33 families include side-exit designs clearly built for tight stacks. What ADT-Link does not publish is a single dimensioned drawing. Their PDFs turned out to be 3D-model wrappers — U3D embedded in a 24×31-point page, zero extractable text. If you want a height number, you download a 93 MB STEP assembly and measure it yourself.
So I did. The M33JM’s connector band measured ~8.2 mm thick and ~10 mm tall off the board:
too tall, confirmed from geometry rather than marketing. The later find was
the K33UF-RR — the RR suffix decodes, via the
Chinese spec sheet, to a right-side ribbon exit — and the CAD shows the construction: a
bare edge-card PCB with the ribbon bonded flat, exiting in-plane. That is the genuinely
sub-5 mm category. Its own 95 MB STEP assembly defeated my
coordinate-filtering approach to extracting a trustworthy height, which is where I stopped
measuring and wrote the vendor an engineering spec instead — including the derived
81.28 mm center-to-center distance my layout needed (four slots below a 3.5-slot card,
at the standard 20.32 mm slot pitch).
Two reusable findings from the detour:
- Vertical mounting makes multi-GPU worse, not better. A vertically-mounted 4090 spans roughly seven slot-widths of case depth versus 3.5 horizontal — the blockage just rotates 90° and grows.
- Connector mechanicals live in vendor presentations, not datasheets: Amphenol’s PCIe card-edge deck was the only source with real dimensions for the mating side.
The resolution was zero millimetres tall
After all that, the answer came from re-reading the motherboard manual. Only two slots on my board carry enough lanes for a GPU — and the 4090’s heatsink covered one of them. No riser at any height could conjure lanes that weren’t there.
The actual fix: change the card order. The 3090 went in slot 1, the 4090 below it — they’re both blower-through designs that tolerate a ~3 mm gap (a thermal claim I’ve now verified under sustained load) — and the mining cards go external on a switch-based chassis, which deserves its own post.
I spent five days engineering around a constraint that dissolved when I questioned the layout instead of the connector. The 5 mm problem was real; it just wasn’t the problem.