Hi, XL83NL
It seems I am not alone with this doubt. I just contacted a flange manufacturer and the designer said they use +/- 1mm for flanges till 4", bigger can be increased to +/- 1,5 otherwise he said that there are no standard for it, just experience. Thanks for your help and your time...
Hello, XL83NL, Sorry for not being specific on the standard description and thanks for your answer.
I read the ASME B16.5, item 6 (and in special 6.5) where the bolt holes tolerance should be, and after that looked for the entire standard and didn`t find the tolerance for flange bolt holes...