.

The "washer" effect shouldn't cause rattling since it would be tight against the rivnut, which is tight against the frame.  The skidplate will be spaced away from the frame, which has some demerits though.  Probably won't matter in this application, but it is a concern.  

Press-fit nuts are another no-weld option that can have less "stickout" if sized correctly:

https://www.mcmaster.com/products/nuts/press-fit-nuts~/

These require accurate holes drilled however.

Easiest/most secure option would be weld-on nuts:

https://www.mcmaster.com/products/nuts/weld-nuts~/

 
MOST rivnuts arent very strong. Maybe look into weld nuts. Weld them on the backside of skid tabs. Box of 100 3/8” will run you $7 on mcmastercarr.com

 
I don't have an opinion on rivnuts or weld nuts or ???

But one thing I didn't like about my floorpan/skid plates is that sand would accumulate between them and the tube frame.  So I installed a washer between the floorpan/skid plate and the frame tube tabs.  It was little bit more effort to install the washer but I thought it was worth the effort because it did keep the sand from accumulating.

 
+1 what @FOG said.  I only had to do it @ the back where the pan sat against the frame where the main tube was.  the rest self cleaned.

 
Mac said:
Thanks of the inputs.

Not sure I want to tack probably 100 plus of these things.  Plus, I don't have an aluminum welder.
Click to expand...
Sorry, usually things are welded to the chassis side and bolt goes up into the tab to create a smooth floor.  Didn't look at your pic, but reversing the fasteners would be worth the time since your current setup makes thread damage easy.  With nuts/bolts, no big deal, break that focker off.  With rivnuts, tears will ensue.

If you do use rivnuts, I would use backing washers to normalize the existing holes and give some extra support.  Should be fine for what you're doing.

https://www.mcmaster.com/products/rivet-washers/

 
Or use nutplates  
f5000anchornutm.jpg


 
My car has a full belly pan, and after taking that thing off once,  I put nut certs or rivnuts in every tab. It has been that way a few years now, no issues. my bolts are 5/16" and the come up from the bottom. I didnt notice any addtional noise or rattle from the minor thickness they added. 

I also did the rear skid plate, its on and off every trip for changing oil. those are 3/8' bolts and I did mess up one of the nut certs at some point, I just drilled it out and put in a new one. 

Im not in love with the look of the nut cert compared to just seeing a bolt head... but it is way easier to work on!

 
I had nut certs on all the pan bolts of my last two cars. Worked well. I had one that came loose. Just put the tool on and pulled it again and it tightened right up. 

 
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mc22 said:
My car has a full belly pan, and after taking that thing off once,  I put nut certs or rivnuts in every tab. It has been that way a few years now, no issues. my bolts are 5/16" and the come up from the bottom. I didnt notice any addtional noise or rattle from the minor thickness they added. 

I also did the rear skid plate, its on and off every trip for changing oil. those are 3/8' bolts and I did mess up one of the nut certs at some point, I just drilled it out and put in a new one. 

Im not in love with the look of the nut cert compared to just seeing a bolt head... but it is way easier to work on!
Click to expand...
Get a Fumoto,  Amazon.com: Fumoto F-133: STANDARD VALVE WITH 12MM-1.25 THREADS WITH LEVER CLIP Gold : Automotive

You won't have to drop your skid plate for oil changes.  It works great!

 
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