Before you drill and tap the backer block you need to tighten the flanged adapter to the backing block using the thru-hull. Because you are missing the thickness of the hull you will need to create a spacer that takes the hulls place.


I used some PVC exterior trim lumber and drilled the same diameter hole as in the backing block. Then all I did was center & tighten the flange and thru-hull together with the backing plate and spacer sandwiched in between.