Best way to remove lead cap in cast iron main trap

With showers I have a slight back up in my basement at a slop sink drain next to the main drain. Figure the main trap and/or the pipe to the cesspool is clogged. The lead plugs in the cast iron trap and line out are stuck. I am using an 18" monkey wrench but am afraid to go bigger or to use a hammer and chisel because I may break the pipe.

How much banging can the cast iron pipe take and once out is there a solution so I never go through this again?

Cobra,
Yes, I can remember thinking that they were threaded also, and finding out that they are just ribbed, and banged straight in.

Get thee a large flat head screw driver, bang it in where the edge is exposed right next to the cast iron, and just pry it out.

Just be ready for any extra juice that may be lurking.

HE

I gave them a couple of taps from side to side from separate 90 degree angles then put the pie wrench on. Be careful if the line is holding they tend to open suddenly!

Thanks for the quick responses.
Are they always ribbed and not threaded?
Do I need to be careful not to crack the cast iron pipe if I use it for leverage?

Thanks also for the warning. I think I am safe because sink and occasional toilet use seems OK and I have not been using that bathroom for a few days now. Luckily I have two septic systems with a full bath on the other one. I may drill a small hole first just in case.

plug

They WERE threaded, but some people did hammer them in.