anyone know the size of the frame tube that runs right above the oil filter. i am going to buy a p_clamp to mount my horn under there but need to know the size so i can order the correct part.

it's the part that the stebel is mounted too in this pic

here is a link to the pic:
http://www.gadgetjq.com/shep_horn2.jpg

I know the frame tube up front is 1.35" but not sure if the lower tube is thicker ??