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 - JRSC BBK powered. 1350lbs / 190whp
E46 M3 vert, bone stock daily driver
'99 F350 7.3 diesel 4x4 Dually - chip, 4" exhaust, intake mod, stg1 injectors.
In a past life: '02 ZX3 - Built motor, GT28RS, LSD, 4.06 gear, Spec stg3 clutch