My basement cast iron 5"? sewer line is probably more than 50 years old. In a 2-foot section, there is a wye of about 2" I.D. with a threaded plus with a square boss. I want to install a washing machine/utility sink drain line into this wye. Can I just unscrew the plug and find an adapter for a 2" copper drain pipe? Is there a danger in trying to unscrew something that old? How can I minimize the danger of breaking something? Thanks.