I was thinking about dual trussrods but honestly I can't see a reason for it. The only reason to use two trussrods would be to correct twist. But I have built a few 5 and 6 string basses and never seen them twist (in 15 years) so I am not sold. After talking to a few friends who build 8s all the time who reassured me a single trussrod was enough I move ahead with one.
Cleaning up the trussrod slot using a dremel. Then I need to round the bottom of the slot where the allen nut sits. So I use the 1/4" rasp to clean it up.
I try to keep my trussrod slots as tight as possible. The ALLPARTS rods are about .357 in deep on the square ends. You can cut your slot shallow and use a 1/4" chisel to scrape the slot at both ends. This allows you to get the rod really tight (not binding but tight). I check it with my engineer ruler (really straight).
The ALLPARTS trussrods are never really flat. Use a diamond sharpener to square up the ends. Yes I try to fit my trussrods this close... close enough that scrubbing a few thousandths off the ends makes them fit better.
Trussrod done. It feels overly epic but it is really important.
The neck blank is about 4mm too thin for an 8 string. I am adding a few mm to each side and this should result in a seamless add. The extra pieces will all be hidden in the body but I know they are there. This sucks but I will make sure to make blanks wide enough in the future. Lost the picture but I added the headstock wings at the same time.