Hi Folks, I am trying to change the cartridge in this Moen 2 handle shower as it started dripping at the shower head ( doesnt leak at the handles ). The picture explains how far I am able to get so far. I am not sure what to do next..The circular knob in the front prevents the cover from just sliding over.I do see the threaded pipe on the wall exposed if i loosen the faucet cover till it doesn't move any more after it hits the circular knob. Please help.. Thanks