In this photo I have just completed the machine work on the lower and upper fittings that will mount on the Delrin end-caps of the 10/80 meter coil-form. The fitting on the left is the lower endcap fitting that couples to the mast. The fitting on the right is the upper endcap fitting that the whip/caphat assembly will couple to. The Stainless-steel 8/32 machine-screws on the top of these two adapters is where the12 gage tinned buss wire of the coil will fasten to.
It should be mentioned that with heavy mobile antennas such as this one, you do not want a incident with the antenna coming off of your mobile and creating a dangerous situation for other vehicles on the highway behind you. I build my antenna masts with 1/2 X 20 TPH where they mount on my mobile, and also at the fitting located at the bottom of the coil (the one on the left in the photo). The normal 3/8ths X 24 TPH which is a standard used by most antenna manufacturers, should be upgraded to a larger diameter on larger and heavier antennas (this antenna now that it is finished weighs in at 11 lbs. I used the normal 3/8ths X 24TPH for the upper fitting of the coil that supports the whip/caphat assembly in the photo (fitting on the right).