How to mount accessories to 4040 extrusion with T-nuts

Most aluminum profile sim rigs are built from 40-series extrusion, usually called 4040 because the profile is 40 by 40 mm. Everything you add to the rig, from a shifter to a phone holder, bolts on the same way. A T-nut sits inside the slot and a bolt goes through your bracket into it. With the wrong T-nut or bolt, the part either won't tighten or slides down mid-race.

Check your slot size first

The outside of 4040 is always 40 mm, but the slot opening on the face is usually either 8 mm or 10 mm, depending on who made it. T-nuts are made for one or the other, so this is the number that matters.

Find the slot on one face of the extrusion.
Measure across the opening with calipers, or a ruler if that's all you have.
About 8 mm means slot 8 T-nuts. About 10 mm means slot 10.

If your rig came with spare T-nuts, keep one as a sample. Matching a new part against something that already fits is easier than going by listings.

The kinds of T-nuts

Roll-in spring T-nuts go in from the front of the slot. You tip them in, roll them a quarter turn, and a small spring or ball holds them where you leave them. They're the easiest kind for adding things to a rig that's already built. You don't have to take anything apart, and the nut doesn't fall to the bottom of the slot while you line up the bracket.

Hammer-head or drop-in T-nuts also go in from the front. They turn into place when you tighten the bolt. They work, but they can turn the wrong way or slide while you're fiddling with the part, and there's nothing holding them up in a vertical slot.

Slide-in T-nuts only go in from the end of the extrusion. They're strong and cheap, but you have to pull the end cap and slide them along, and if something is already bolted in the way, you can't get past it. Fine when you're building a rig, annoying after.

T-nuts also come in different thread sizes, like M5, M6 and M8. Pick the one that matches the bolt your part is made for.

Bolt length

The bolt has to go all the way through the T-nut, but it can't hit the bottom of the slot. If it bottoms out, it stops turning before it clamps anything, and the part wobbles no matter how hard you tighten it.

Start with the thickness of the part where the bolt goes through, add the thickness of a washer, then add enough to pass through the T-nut. If the part's maker lists a bolt length, use that one. If the bolt turns and turns but the part still moves, try one size shorter.

Putting it on

Hold the part where you want it and look at which slot it lines up with.
Drop the T-nut into that slot, roughly where the bolt will land.
Put the washer on the bolt and the bolt through the part.
Start the bolt into the T-nut by hand, a few turns, with the part loose.
Slide the part to exactly where you want it. The T-nut slides with it.
Tighten until it's snug, then sit in the rig and check the position.
Tighten it the rest of the way.

Go easy on printed parts. Tighten until the part doesn't move when you push on it and stop there, because plastic cracks long before steel does. A washer spreads the load so the bolt head doesn't dig in.

Stopping it from turning

With one bolt, a part can rotate around that bolt if you lean on it. You can stop that with a second bolt in the same slot, or with a part that has a small key on the back that sits in the slot. A key the width of the slot keeps a mount pointing the right way even after months of being knocked with your knee.

Mistakes I see a lot

Buying slot 10 T-nuts for a slot 8 rig, or the other way round. Measure first.
Using slide-in nuts after the rig is built, then finding there's a bracket in the way.
A bolt that's too long. It feels tight but the part still moves.
Overtightening a printed part until it cracks.
Losing a drop-in nut down a vertical slot. Roll-in spring nuts stay where you put them.

If you're looking for things that already fit, the Flag Dash mounts this way: one roll-in spring T-nut for an 8 mm slot and one M8 bolt, both in the box.