Using a small screwdrive, I removed the o-ring (all while water is pouring in), reassembled the handle and was able to properly insert and secure. This is a terrible design, though I guess it lasted 14 years before the threads wore.
Notice the valve is made in 4 sections, held together by 4 large bolts from the top. There's the threaded thuhull (white, not shown), that screws into the 3 1/2" diameter base, ball valve assembly (middle, where handle is attached), and top assembly.
Notice the white plug on the handle (above)? It turns out this is the proper size to fit the thruhull from the outside of the boat. I guess their idea is if the handle breaks off and the boat is sinking, you can jump in the water and plug the thruhull with this little plug. Right..... I had my diver plug these, the next time he cleaned the bottom.
With the thruhulls plugged, I cautiously took apart the seacock,
One final note, the seacock clearly states to not disassemble the valve,but Forespar's technical support encouraged me to do exactly this! It is great when the design is improved and the parts are interchangeable. It would have been just my luck if the assembly was 1/8" different is size or the screws were moved.
I replaced both the smaller sea water intake seacocks. I'm relieved that these are operational without the fear of the handle coming off and flooding the boat. The fix was very easy. I'm thinking about swapping out the ball-valve on the remaining 3 seacocks in the boat.
Intuition - 3500 - #115