In getting the frame ready for galvanizing I wanted to have all the holes drilled in it that I would need. There are two extra holes that are required for the YJ/CY front shackle hangers and someday I may put YJ springs on this, so the holes will be there if I do.
You can see that the hanger lines up with the two existing holes and has a clearance hole for the rivet. It needs another clearance hole for the 7/16 threaded hole in the bottom of the hanger. Some conversion hangers have you grind the rivet off. I don't do that with my hangers, you just drill the extra hole. Better idea I think.
So we need a place to measure from in order to get the extra hole in the right location. I am measuring from the furthest forward mounting hole.
Here's a fairly accurate way to find the center of that hole. I measured the i.d. of the threaded hole, which was .375, and divide that by 2. Take a drill bit that size, 3/16, and insert it in the hole and slide your square up against it. That does a nice job of making sure the edge of the square is on the center of the hole. Now just scribe a line.
Now you need to find the center of the hole from the edge of the frame so you can mark that where the new hole goes. Just use the same method and slide an adjustable square up against the drill bit. Now move the square in the general location of the new hole and scribe a mark along the end of the scale.
Now you need to measure from the mounting hole on the hanger that I marked the center of on the frame to the new hole. In this case that distance is 2.427. Now scribe a line at that location. You now have both directions marked on the frame. Center punch it and you're ready to drill.
I drilled the final hole 1/2". I stepped that up in three smaller drills, started about 1/8" then 5/16" then the final 1/2" and chamfered the hole to get rid of the sharp edge.