I have a cast iron pipe that I am trying to get inside of to clean out. It is an end cap and I can't seem to break it loose. The pipe is 50 years old and I don't want to shatter it by getting to agressive, so are ther any tips or tricks to getting these old caps off? I have already tried penetraiting oil with no luck.

Nowhere near being a professional, but have you tried heat?

And, if you bust the cap, Home Depot has tons of them. get one of the rubber ones this time

its all about leverage. use 2 pipe wrenches going in opposite directions.
if it still wont budge you should slip a piece of pipe over one of the wrench handles
(to make it longer) for more leverage.

As a last resort, gently warm the cap with a propane or MAPP torch. As evenly as possible. The cap may break loose while warm, or as/after it cools.

Is it an end cap (thread over the pipe), or pipe plug (thread inside pipe)? They're typically plugs. If it'* a plug, replace it with PVC or rubber. They sell both at Home Depot or Lowe'*.

If you have a dremel you could cut the cap off.

It'* a plug in cast iron (old skool thick a$$ stuff). I just nought a MAPP/OXY torch setup and I'll try heat on it first. All else fails, I'll just beat the hell out of it and then replace it with a standard off the shelf home depot speacial.