Joe wrote:
Getting ready to cut a cast iron vent stack to put in a new vent, now I know
that I must support the weight of the stack and brace it from side movement
I will be renting one of those chain type tools for snapping the pipe. I am
concerned since I never did this, what is going to happen since the weight
of the stack is still there all though supported from dropping how is it
going react, I really don't want this pipe to crack the wrong way. Any one
who has done this and can let me know what to expect would be appreciated.

How big, how tall, how supported? Nowhere near enough information to
be able to tell anything about your particular installation.