Just how strong are rivnuts? I'd never used them before building the Indy and I'm never really sure when they're suitable to use.

For example, I'm going to do the exhaust mounting shortly and wondered if they're strong enough to bolt the mount into the underneath of the chassis??

They just dont seem like they wouldn't hold very well with such a small top lip??

They come in different sizes and materials.

For what youre after, i would say that they should be ok as long as they are suitably spaced, and not under too much load.

Ive used them on mine (8mm thread) and they are solid.
The steel ones are stronger still.

Not on the exhaust imho they will work loose and just spin when you want to undo it

They structure is more than strong enough, they are simply bigger than the hole, so aren't going anywhere. However, if you are using aluminium rivnuts, its worth remembering the threads are only aluminium and you can, if applying loads of force, rip the threads out of them..... I found out the hard way.

My exhaust bracket is held on with 2 M6 steel rivnuts and has been fine. Just make sure they are squashed up nice and tight

David

To be sure of stopping them spin in the hole there's a few things you can do when fitting them.
Take a small file & make a couple of notches in the edge of the hole so as the rivnut tightens it squashes into the notches which will help resist turning.
Coat the outside of the rivnut in some epoxy resin when you fit it.
Buy the rivnuts with splines on, the plain ones are far less resistant to spinning.

I've made a bracket that is fastened to the chassis via the bolts that hold the passenger seat in place, and thu attached to the flat steel cross member.

Maybe it's overkill, but I don't have any worries about it working loose on day.

Depends whether they're steel or ali rivnuts as well. Ali ones are quick weak, strip easily etc etc. Steel ones are pretty strong and as said, with a bit of epoxy job's a good'un...

buy the correct type, steel, serrated and deffo not countersunk type. and a tight fit in the hole