OK, so here is the write up on the stock center cap install on aftermarket steel wheels.
The tool list;
drill press, to make the fixture with, and a welder
1/2" drill bit
15/64" drill bit
5/8"x82 degree counter sink
Material list;
(2) 1/2"x20 2" long bolts
(1) 1 1/2"x6"x1/8" flat bar(steel)
(2) 1/2"x20 lug nuts
(12) zinc plated sheet metal screws, size 10, 3/4" long(i think, i lost this info in the crash!)
My son wanted a different look to the Crager soft 8's than just chrome caps and the caps rust bad anyway so we went with stock center caps from XJ's and some TJ's so they would be easy to find in the junk yards.
The first thing you will notice is that the center caps have 3 screw holes and 2 locator posts, break the posts off. We only want to drill 3 holes in the wheels.
The next issue I ran into was getting the hole perfectly centered and on the same circumference as the lug holes are, this took some trial and error and some math. I made a drill fixture that would center the hole dead on each time. The fixture has (2) 1/2" holes drilled to match two adjacent lug holes and then a 15/64" hole to match the hole to the center cap. I then welded in two 1/2"x20x3" bolts and used some lug nuts to attach the fixture to the wheel at 3 locations to match the screw holes for the center caps.
Then you need to counter sink the screw holes so the screws will not interfere with the wheel mounting surface. I used a 5/8"x82 degree countersink for this, the screws should fit flush.
Then mount the caps up by securing the 3 screws from the inside of the wheel.
The finished product!