They make rivet nuts in larger sizes as well. I bought an aftermarket skid plate for my car, it came with some metric rivet nuts that are whatever metric size is approximately 3/8". They griped very well and seemed very strong and well suited to the application.

The smaller sizes may be common but there are lots of other sizes and variations, probably something suitable for anything you'd want a rivnut for.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-