T-Slot Nut Dimensions
A T-slot nut has to do four things at once: pass through the slot opening, turn once it is inside, lock against the undercut so it cannot pull out, and hold a thread strong enough for the joint. Only the last of those is about the thread size you asked for — the first three are what actually decide whether the part fits.
That is why a table of thread sizes on its own is not much help. Below are the hardware dimensions, and then the three clearance checks that turn them into a yes or no for each slot series.
These are metric-standard approximate values, and they describe the printed parts you can generate here. Commercial metal T-nuts have their own body designs and do not follow these numbers — always check the supplier's drawing for those.
Hardware dimensions by thread size
Four numbers matter per thread size. The through-hole is what a bolt passes through in a printed part; the insert diameter is the recommended bore for a heat-set brass insert; the across-flats figure is the size of a standard hex nut, which is the part that limits how small a printed nut can be; and the nut thickness is how much depth the hex nut takes up.
| Thread | Through-hole ⌀ | Heat-set insert ⌀ | Hex nut across flats | Hex nut thickness |
|---|---|---|---|---|
| M3 | 3.4 | 4.0 | 5.5 | 2.4 |
| M4 | 4.5 | 5.0 | 7.0 | 3.2 |
| M5 | 5.5 | 6.3 | 8.0 | 4.0 |
| M6 | 6.6 | 8.0 | 10.0 | 5.0 |
| M8 | 8.6 | 10.5 | 13.0 | 6.5 |
All values in millimetres. The through-hole is deliberately larger than the nominal thread so a bolt slides in without binding.
Slot dimensions to check against
| Series | Slot opening | Throat height | Channel width | Channel depth | Undercut per side | Centre bore |
|---|---|---|---|---|---|---|
| 20 series | 6.0 | 1.5 | 8.0 | 4.6 | 1.0 | ⌀4.2 |
| 30 series | 8.0 | 2.0 | 11.0 | 6.2 | 1.5 | ⌀6.8 |
| 40 series | 8.0 | 2.0 | 11.0 | 6.2 | 1.5 | ⌀6.8 |
| 45 series | 10.0 | 2.5 | 14.0 | 7.8 | 2.0 | ⌀8.5 |
Full detail, including how to measure these on your own profile, is on the extrusion size chart.
The three clearance checks
Run these in order. Failing any one of them means the part does not work, no matter how good the thread is.
- Flange versus slot opening. The flange has to fit through the opening to get in, and be wider than the opening to stay in. So it must be narrower than 6.0 mm to enter a 20 series slot, and wider than 6.0 mm to lock. That leaves a narrow band to design in, and it is why the flange is usually a rectangle rather than a circle.
- Body versus channel width. Once inside, the nut turns and its body sits in the channel. The channel is 8.0 mm wide on the 20 series, 11.0 mm on the 30 and 40 series, and 14.0 mm on the 45 series. Anything wider than that cannot turn.
- Thickness versus channel depth. The channel is 4.6 mm deep on the 20 series and 7.8 mm on the 45 series. A nut thicker than the channel stands proud of the profile and the bracket will not sit flat.
The undercut — the gap the flange grabs — is half the difference between the channel width and the opening: 1.0 mm per side on the 20 series, 1.5 mm on the 30 and 40 series, and 2.0 mm on the 45 series. That is the entire engagement you get, so the flange needs to be a good fit rather than an approximate one.
Which thread fits which series
If you are printing the nut and capturing a standard hex nut inside it, the hex nut's across-flats figure is the binding constraint, because that is the widest part of what you are trying to hide in the slot:
| Series | Channel width | Hex nut that fits | Practical choice |
|---|---|---|---|
| 20 series | 8.0 | M3 (5.5), M4 (7.0); M5 is 8.0 and fills the channel completely | M3 or M4 |
| 30 / 40 series | 11.0 | M3, M4, M5, M6 (10.0); M8 is 13.0 and does not fit | M4 or M6 |
| 45 series | 14.0 | M3 through M8 (13.0) all fit | M6 or M8 |
This is why the generator suggests M3 and M4 for the 20 series. It is not that a 5 mm bolt is too weak — it is that an M5 hex nut is exactly as wide as the channel, leaving nothing for clearance or for the walls of the printed part around it.
Commercial metal T-nuts are a different case. They are made with a body shaped to the slot rather than a hex nut dropped inside it, so M5 T-nuts for 2020 profiles are common and work fine. Use the table above for parts you print, and the supplier's drawing for parts you buy.
Heat-set inserts: the one-way trip
An insert is better than a captured hex nut for anything you will disassemble, because the thread is brass and the bolt will not chew it up. But an insert has to get into the nut somehow, and the slot opening decides whether that can happen in place.
| Series | Slot opening | Insert that passes through | Insert that only goes in from the end |
|---|---|---|---|
| 20 series | 6.0 | M3 (4.0), M4 (5.0) | M5 (6.3) and larger |
| 30 / 40 series | 8.0 | M3, M4, M5 (6.3) | M6 (8.0) is a line-to-line fit, so treat it as too tight |
| 45 series | 10.0 | M3 through M6 (8.0) | M8 (10.5) |
If the insert is too wide for the opening, you have two options: melt it into the nut before the nut goes into the slot, or slide the nut in from the end of the profile. Both work, but they change the order of assembly, so decide before you print.
What the centre bore passes
The centre bore runs down the middle of the profile and is used by hidden joiners and by bolts that go right through. It is smaller than most people expect:
| Series | Centre bore | Bolt that passes |
|---|---|---|
| 20 series | ⌀4.2 | M3 only |
| 30 / 40 series | ⌀6.8 | M3 through M6 |
| 45 series | ⌀8.5 | M3 through M6; M8 is 8.6 and does not pass |
A 4.2 mm bore on the 20 series is the reason printed 2020 frames are nearly always bolted with M3 or M4. If you need an M5 bolt through the middle, drill the bore out, or use a T-nut in the slot instead.
Frequently asked questions
What is the difference between a T-nut and a spring nut?
They are the same part with one addition: a spring nut has a leaf spring that pushes the nut against the channel wall so it stays put while you position the profile. Without it the nut slides along the slot and drops to the bottom, which is why assembly is easier with the spring version.
Which thread size should I use in a 2020 slot?
M5 is the usual choice for commercial metal T-nuts in a 2020 slot. If you are printing the nut and holding a hex nut inside it, stay at M3 or M4, because an M5 hex nut is 8 mm across the flats and the 20 series channel is 8 mm wide, so there is no clearance left.
Can I put a heat-set insert into a printed T-nut?
Yes, and it is the better option for anything you will take apart. The catch is whether the insert can pass through the slot opening: on the 20 series the opening is 6 mm, so an M4 insert at 5 mm passes but an M5 insert at 6.3 mm does not, and you have to load the insert into the nut before the nut goes into the slot.
Why does my printed T-nut jam halfway into the slot?
Almost always the throat height. The narrow part of the slot is only 1.5 mm deep on the 20 series and 2 mm on the 30 and 40 series, so the neck of the nut has to be shorter than that before the flange starts. Print a short test piece of the nut alone and try it before printing the whole part.
Do 2020 and 2040 use the same T-nut?
Yes. Both use the 20 series slot, so every nut, bracket and printed part is interchangeable between them. The 30 and 40 series share their slot in the same way.
How much clearance should I leave on a printed nut?
Leave 0.1 to 0.2 mm for a tight fit and 0.3 to 0.5 mm for a loose one, applied to the width and the thickness. Printed parts shrink or swell depending on the material and the printer, so cut a short test piece first rather than committing to a long print.
Print the nut for your slot
The generator takes the slot series and the thread size and produces a T-nut with the right flange, boss and centre hole, with a fit clearance you set yourself. Add the spring leaf if you want it to stay in place during assembly.
Open the aluminum extrusion generator →
Related: designing and printing a T-nut · extrusion size chart · choosing connectors · 2020 vs 2040