They’ve been around for a long time but have only recently come down enough in price to make their purchase worthwhile for amateur mechanics. So what are they? Rivnut tools – devices for inserting special threaded inserts into blind holes. Let’s take a look at these tools and their uses.
(Note: rivnuts are also called nutserts, blind rivet nuts and similar.)
Types of rivnut tools
The most common rivnut tool is very much like a pop rivet gun. By operating the handles, an inner mandrel is drawn upwards with great force. In a pop rivet gun, the ‘mandrel’ comprises the shank of the rivet, while with the rivnut tool, the mandrel is a threaded spigot onto which the nut insert is screwed.
This type of tool is fine if you want to insert only the occasional, small diameter rivnut.
However, as with inserting large diameter pop rivets, if you wish to insert larger diameter rivnuts (eg 8mm and above) it helps if the rivnut tool has long handles so that lots of leverage can be applied.
If you are going to be inserting a lot of rivnuts, a compressed air gun is available which massively reduces the amount of effort needed.
However, the best tool for inserting rivnuts for home mechanic use is one that uses a completely different approach. Rather than just pulling on the mandrel through leverage, this type of tool uses a knurled mating surface to stop the body of the rivnut from turning and then uses the threaded mandrel (working as a screw) to pull the rivnut into place.
To give you an idea of how much force these screw tools apply compare with traditional lever-type rivnut tools, I applied a screw-type tool to a rivnut that had already been put into place with a lever tool. Using the screw-type tool, I was able to compress the rivut another 2 or more millimetres – making it immensely more secure.
The only downside of the screw type rivnut tool is that it leaves a mark on the face of the rivnut.
Rivnut tools (all types) use threaded mandrels that match the diameters and thread types of the rivnuts being inserted. So if you are going to be inserting both imperial and metric rivnuts, keep this in mind when selecting the tool.
The rivnut is screwed onto the mandrel.
The rivnut is inserted into the hole and…
…the spanner (arrowed) used to draw the rivnut up tight. Once that is done, the tool can be unscrewed from the captive rivnut.
The finished job.
Rivnuts are available in metric and imperial sizes. Typical metric kits include a tool and rivnuts in M3, M4, M5 and M6 sizes. Note that larger sizes are available (eg M8 and M10) but these become problematic to insert with just a conventional lever-type hand tool – not enough force can be easily applied so instead you’ll need to use a screw-type tool.
Rivnuts are available in steel, stainless steel and aluminium, and most rivnuts have a serrated collar to allow them to better grip the parent material. Some rivnuts are designed to be used just on very thin sheet.
Finally, some rivnuts are designed for near-flush applications and others work with a slightly raised collar.
Advantages and disadvantages
The huge advantage of a rivnut is that it can be used to attach a bolt to a panel where access to the back is difficult or impossible. Rather than trying to get a nut on the back of the panel, simply drill a hole of the right size, insert the rivnut and lock it into place. There’s your captive nut - it’s quick, simple and neat.
However, even if you can access the rear of the surface, rivnuts can also be useful where it might just be tricky to hold the nut in place as you screw-in the bolt.
However, there are also some downsides. Rivnuts, especially those inserted with a lever-type hand tool, do not flare a great deal after being inserted – their hold on the parent material is tenuous, especially when compared with a traditional washer and nut. This means that they should not be used in structural applications, or where failure could cause danger.
I’ve seen seat brackets bolted to rivnuts inserted through the car’s floor panel: this is dangerously stupid. I’ve also seen heavy electric motor controllers held in place with rivnuts inserted into chassis rails. Again, if you consider the mass of the controller might increase by 40 or 50 times in a crash, this is a silly thing to do.
(Note that some car manufacturers, especially those making aluminium cars, use nut inserts quite widely in their structures. However, if you look at these fasteners closely, you’ll see that these machine-inserted rivnuts have a much greater bearing surface on the parent material than a typical hand-applied rivnut.)
The ability of the rivnut to sustain a load also decreases when the insert is made from aluminium rather than steel.
To be able to insert a rivnut, the clearance in front of the panel needs to be sufficient that the tool’s nose can be inserted. In tight confines, this can be impossible. Where there is insufficient clearance, at a pinch you can insert a rivnut by using a well-greased bolt and washer to pull the rear of the rivnut forward, so expanding it. You’ll need to use pliers to stop the rivnut turning initially.
The inserted rivnut is also usually a little longer than a conventional nut – again, in some situations, this rear clearance needs to be taken into account.
This fabricated steel airbox, that takes a long cylindrical air filter, uses rivnuts to hold on the lid (no access possible to the back of the panel when the screws are being inserted)…
… and to secure a solenoid into place (very fiddly to try to hold conventional nuts in place).
Here a rivnut is being used on a tab to clamp a wiring loom in place (can’t access back of tab when pipe is installed).
Rivnuts are the sort of fastener that you think will be used rarely or not at all – and then when you have a kit, you find yourself using them all the time! In short, if you’re fabricating fiddly stuff, they’re a lifesaver.