Nutserts
 

Can somebody help me out? I am making rear corners out of 3/13 and wanted to use nutserts to mount them but don't know what kind to get. 5/16 18 sounds about right but where is a good place to get them? And what are they made of alluminum, brass, or steel ? Thanks

I use these:
http://www.mcmaster.com/#95105a151/=45ddlc

Edited for direct linking to Mcmaster? Lame.

They come in aluminum and steel..

You also have to look at the thickness of metal they are being inserted into..

IMO I would use the aluminum body nutsert that accepts a 1/4 in flat top bolt since it is not a load bearing. Also they are easier to crush down. I think the lenght of nutsert I used with the PSC rocker armor was like 1/2 in to just over.

Dissimilar metals bad. Stick with coated steel.

...and Napa Auto Parts sells several types/sizes....

Is 1/4 20 good enough or do they need to be 5/16?

I use the metric Snap On tool made by Marson all the time in my trade and 99% of the time I'm using M4 nutserts. I'd just use that size because anything bigger is really overkill for what you are doing plus the bigger ones are a pain in the ass to squeeze the tool. And for sure get the aluminum ones the have a nice flange as well. BTW, M4 is the size that the TJ Rubicon used for the OEM diamond plate rocker guards along with some funky little torx screws. You could get those at the dealer. Even sicker looking would be to use the M4 button head torx bolts that Dodge attaches the fender liner to the front bumper on a RAM truck and also uses on the Durango and Dakota to do the same thing on the front and rear.