I am troubled as to finding out the seatpost size on my trek jack 3 frame. I got the frame as a replacement for a cracked trek bruiser, but the frame came with no seatpost and i have no clue where to find the size. it isn't on the trek website and i cant find it anywhere else either. the diameter is roughlt just under 31.2mm but is it 31,8 or 31,6??? help me!!!