I've found 1/2" -20 which is fine thread for the bolts and nuts holding it to the frame. That's on several AD trucks. Forget if the one tapped in the box is coarse or fine, easy enough to look though.
Grigg
I concur. But, I believe there are 2 each 1/2-20x1.5", 1 each 1/2-20x1.75" (all three with 1/2-20 nuts) and one 1/2-13x1". But, I see no reason not to go with all 1/2-13 (if worried about bolts loosening, then a little blue thread cement should do the trick).
