[diamond]

I have read numerous accounts about how these large #8 tank screw holes tend to crack a bit when dimpled. Some have suggested drilling them first with a larger bit, like a #17 or #16. I'm wondering if there is any merit to heating the metal up a little before dimpling to reduce the chance of cracking. Has anyone tried this? If so, could you share your technique?

 

[rvbuilder2002]

There are dimpling machines that actually heat the dimple dies to improve the dimpling process.
The issue with doing it as a DIY process is there is a very fine line between the temp that will help, and the temp that will have annealed (softened) the aluminum. Without a way to control the temp precisely, it would be a bad idea.

If you are concerned, the best compromise would be to finish all of the holes with a reamer, then carefully deburr before dimple countersinking.