bathtub faucet help

I've got a bathtub faucet that's been dripping. I'm trying to fix it while I have some time this weekend. I pulled the handle and unscrewed a small plastic ring I found underneath it, but the next step is a mystery. I can't seem to pull the next small piece of plastic, or the little valve stem, and the large metal ring doesn't seem to unscrew (It seems like all one big piece), or at least I got nervous about squeezing it with the pliers before it loosened. Does anyone recognize this or know what I should do? A couple pictures here: http://www.flickr.com/photos/15217781@N02/5212041609/ Thanks much!

That large knurled ring should unscrew, it probably has crud built up on it, they normally come loose with hand power.

Whoa nelly, because you can have a VERY BAD DAY real quick here.

The plastic ring is a tension adjustment. The knurled dome is the cap which gives you access to the seats and springs. BUT...it is often stuck or frozen. AND, if you look behind there and see three small copper tubes supporting the body where that dome is.....you can snap those off so quick it will make your head spin. Then its cut-a-hole-in-the-wall time.
What to do: first, spray and soak that dome nut with white vinegar, often, and overnight. See if it will turn. DO NOT turn too hard. Next step is to slice across the dome very carefully with a fine tooth hacksaw blade..try not to damage the threads in the body. Another technique is to heat the dome nut gently with a hair dryer, or even a torch.
Dont worry about the nut...parts for that Delta valve are readily available and not expensive.

I have had luck lightly tapping the knurl on the cap nut many, many, many times. Eventually I can take it off by hand, which is what you want to do. You do not want to mess up the three copper tubes mentioned above.

Thanks folks!