: Cold water tap on bath-tub.Knob connection stripped.May have to buy whole unit (hot&cold : with shower button).Tried new knob, but stem is badly worn too.proper soldering, though appearing simple is a skill that is developed with pratice. soldering inside a wall opening is risky even for professionals. why not take the stem assembly out and go to a local supply center and get a replacement stem. the body of your valve in the wall is probably alright. new parts are available for most valves.