This is the most critical step and why you really should have a drill press. Because you are literally drilling and tapping a piece of 5/8" fiberglass or epoxy sheet, the threaded holes MUST be perfectly centered in the flange holes, and they must be 100% vertical.


I made my spacer square, and wider than the head of the thru-hull, so I was able to clamp it in my drill press vice. I can not stress enough how important it is to make perfectly vertical holes that are perfectly centered in the holes of the flange or seacock. There is NO wiggle room here. Mess this up and you just wrecked the backing block..