Back in the old days us greasers used brass nuts. The brass is soft and needs to be torqued to the right spec. Never had a problem with exhaust nuts again. The only problem I see is getting a brass nut in metric, but I'm sure its possible. Also, you had to run a chase down the threads of an old exhaust stud. The coarseness of the threads would sand down the threads of the nut. Although I put many on without doing the chase and had no problems. We also looked for the thickest nuts we could find. Theirs a large selection in sae. Metric, I just don't know. A few hours on the world wide waste would reveal plenty, I'm sure