If you want to perform a plumbing task like fitting a copper pipe and cast iron pipe together, then you must know that the biggest problem of this is trying to avoid galvanization of the pipes. When copper and iron pipes are fitted together, they can create a small electrical charge between them, like a battery, which leads to corrosion and damage to both pipes. In order to avoid this, you will need to proceed in fitting the copper pipe and the iron pipe together very carefully.
Step 1 - Fit the Copper Pipe
Begin by fitting the copper pipe into position, Use your wrench to ensure that it is fitted tightly into place and that there are no leaks along the water line thus far.
Step 2 - Fit the Plastic Valve
You can then take the plastic valve which will be used to join the two pipes and place it at the edge of the copper pipe. Using your pliers, hold the pipe steady while pulling the valve on with the wrench.
Step 3 - Add the Cast Iron Pipe
You can then fit the cast iron pipe. The plastic valve will ensure that the two metal pieces don't touch, preventing galvanic action.