my mistake. i didn't measure well. i finished working on it at 2am and i got confused when i done the post.
if i remember well, both top and bottom measure 30mm (or 31mm; sorry i forgot it already) on the stock.
on the top i fitted 2 washers of 2mm thickness on each side.
on the bottom i could fit only 3 washers (2 on the right hand side, 1 on the left hand side but) but there was still some space left. i didn't had longer bolts to fit another one but i'll do it today if will find some stainless steel ones after work.
be sure and take some notes about the markings on the side plates which are visible from the right hand side of the bike. i got mine wrong on the first install.
hope it will not snow again today to take my bike out tomorrow :)