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How do I disconect iron sewer pipes at the joints to replace sections of the line without breaking the joint?
It'll help you if you understand how the joint is formed in the first place.
First, "oakum" is packed into the joint. It's a fibrous, ropy material made from hemp or jute and impregnated with tar. When it gets wet, the oakum swells. This forms the watertight seal. Then molten lead is poured over the oakum to form the mechanical joint.
You'll need to first dig out the lead. This can probably be done with a hammer, small chisel, and various picking and prying tools. At this point you will probably be able to wiggle the joint enough to get it apart; if not you'll need to dig out the oakum.
For joining plastic pipe to the bell of the cast iron pipe, you can use a rubber donut made specifically for that purpose. The donut takes the place of the oakum and lead. If you can cut the cast iron pipe, you can join the plastic to it using a rubber coupling, also made for the purpose.
Here's a link to the "proper" tool for cutting cast-iron pipe: http://www.plumbingstore.com/snapcutters.html