BobVila.com

shower

You'll have to reach two fittings thru the surround .....the first tightens packing aound the stemm........the secound larger nut holds the stem to the valve body.......use the hollow plumbing wrenches to remove.....

Here's a typical setup:

http://www.terrylove.com/pp_3.htm

"...measure once.....cut twice....throw that one away and cut a new one...."