Thought it was time I asked as I have never used them. What are they and what do they do? Are they a better substitute for rivets?

see here

http://www.memfast.co.uk/shop/Vcat1.asp?cat=2250004506

A rivet nut, also known as a blind nut or nut-sert,[1] is a one-piece internally threaded and counterbored tubular rivet that can be anchored entirely from one side. There are two types: one is designed to form a bulge on the back side of the panel as a screw is tightened in its threads. The other is similarly drawn in using a screw, but is drawn into the sleeve instead of creating a bulge.[2][3]

The first rivet nut was created by BF Goodrich in the 1930s and named the rivnut. It was used to mount rubber de-icing boots to aircraft wings.[4]

Rivet nuts are often made of anodized aluminium, and as such, will not conduct electric current.[citation needed]

In the field of aviation, "Conductive" Rivnuts are often used to attach various items, such as static wicks and inspection access covers, to the surface of an aircraft.[citation needed]

dave

Fantastic, now I know

Blimey jossey that was a very comprehensive answer, well done indeed. Barry

Link to a cheap homemade tool for installing them, with pictures of what they look like installed.

http://www.fjr1300.info/howto/rivnut-tool.html