You are right, it was threaded in indeed...and most definitely not fun to get out. Ended up sacrificing a screwdriver and in a counter clockwise fashion tapped the 4 slots that run the perimeter of the cap. Apparently some folks sacrifice a large deep socket instead by grinding it down to have 4 tabs to engage the ring, but I didn't have a socket around that was the right size and i was willing to part with. Once the ring came out, the rest was a cake.
MEV Rocket - zetec with powerworks sc
04 BMW M3 Convertible - bone stock
'02 ZX3 - returning to stock
'99 F350 PWS 4x4 Dually - chipped, 4" exhaust, intake, stg1 injectors