3.3.2 Assembly of pipes of unequal thickness
If the difference of thickness between pipes to be buttwelded
exceeds 10% of the thickness of the thinner pipe
plus 1 mm, subject to a maximum of 4 mm, the thicker pipe
is to be thinned down to the thickness of the thinner pipe on
a length at least equal to 4 times the offset, including the
width of the weld if so desired.