Also, VERY IMPORTANT... you see those four little punch marks around the perimeter? A sharp punch or awl was used to "stake" the retainer into position. Use a small drill bit (I used a 3/16", 1/8" would probably work) and drill a shallow hole (maybe 1/8" deep) to relieve the "staked" portion of the threads. The staking process has imparted a slight deformity in the threads at the stake point to prevent the retainer from backing out. If you do not relieve this deformity you will likely destroy the retainer trying to remove it. If you don't damage it beyond reuse you will at least have exerted a great deal more effort to remove it than if you had drilled out the stake points to begin with. I drilled out the "stakes" and was able to unthread my retainer by tapping with a punch, spun right out. The right tool would make it even easier.
