There are times when you are fixing or upgrading pipe in your home for a new kitchen or bathroom and you will need to install new PVC waste line then attach it to your existing Cast Iron waste stack or waste line.
Unfortunately this is not an easy job because cast iron is a relatively difficult pipe to work with. Cutting cast iron pipe usually means using a reciprocating saw or a special tool and making attachments will require either a rubber donut or rubber bushing.
If you do need to replace cast iron pipe you should do so at the fitting. This is your best bet for a good connection.
Normally when two pieces of cast iron are joined the plumber will use oakum which is a rope type material that is stuffed into the joint then the top half inch of the joint has lead poured in to make the connection.
Connections between two pieces of Cast Iron pipe are not considered mechanical to the point where the joint is secure from any movement that might occur. If you have settling in your home or an earth quake or other situation where force is placed on the pipe the lead an oakum connection may crack loose and need to be rejoined.
You should keep this in mind when you are making your connections or transitions between PVC pipe or other metal pipe and cast iron.
PVC pipe is glued and the glue actually melts the two pieces of pipe causing a chemical weld. Brass, Steel and galvanized pipe that are not used in residential waste systems as often are either a threaded or soldered connection which gives a moderate amount of mechanical resistance to movement.
Rubber Plumbing Bushings for Cast Iron Connections
Cast iron is best transitioned at the fitting. When you install cast iron the lower pipes in the home will have a straight end on the bottom that points towards the exit of the waste and a fitting or wide mouth opening that the next pipe can be inserted into.
Cast Iron Rubber Bushings or Donuts are inserted into the wide mouth opening of the installed cast iron pipe and allow you to insert a PVC or Steel pipe into the center of the cast iron pipe without the need to use lead and oakum which would melt the PVC or not join well to the Steel or Brass pipe you are inserting.
When using a rubber bushing you should pick one that matches both the exterior wall thickness of the new pipe and the interior of the cast iron pipe opening.
Normally you place the rubber bushing on the PVC pipe first then force the two parts together. This will depend on the positioning of the pipe and your ability to manipulate them.
Rubber Plumbing Unions for Cast Iron Transitions
As a lesser solution you can join two pipes of different types by using a rubber union that seals tightly around both pipes to seal them together.
This is not an optimal choice for pipes that are horizontal because the seal is dependent almost totally on the rubber union.
First cut your PVC pipe so it is longer and can be inserted a good 6 inches into the cast iron pipe without restricting any other flow in the system.
Place the rubber union on the PVC Pipe and then insert the PVC pipe into the center of the cast iron pipe.
Now tighten down the hose clamps to make the connection.
You should never consider a rubber part a mechanical connection in your plumbing system. They are mainly used to prevent sewer gas leaks and to a very small extent reduce the possibility of water leaks.
You should never try to patch a section of Cast Iron with PVC or other pipe you should either replace the pipe with pvc and make a connection to the old or make the fix with cast iron… and never make a short section patch in cast iron with pvc.
If your PVC pipe can be inserted into the Cast Iron pipe far enough the connection should be safe enough for normal residential use.
Waste lines are not normally under a lot of pressure the only time you may have serious problems is if you live in an area where flooding occurs and water flows back into your home. At that point the rubber will be the weakest part of the system however you will most likely have other serious problems such as overflowing toilets and sinks.