if the stopcock fitting is allready on the lead pipe and the other end is on the copper pipe" you get the new stopcock, that has the same size lock nut, that is comming off the lead end of the pipe. it should be half inch or 15mm, in other words use the old lock nut thats on the end of the lead pipe and dont use (one )of the new lock nuts on the new stopcock, its all ready there for you to use", turn the water off from out side your home, if this stopcock is for the mains comming into your home,,
mick
February 2010