Here is the situation. I am installing a bathroom in the basement. Recently cut a section out of my 4 inch main line under the basement slab which is cast iron. The cast iron was installed in the 1930's and of course is old and brittle. Unfortunately, my first cut was made with a reciprocating saw... which did a decent job of cutting... but it made a hairline fracture in the cast iron on the bottom side of the pipe. Now, I am not certain that this hairline fracture didn't already exist. I went back another foot and this time used the appropriate tool (Angle Grinder). Cut through the pipe like butter, However I still see that there is a very minute hairline fracture in the pipe.

#1 Question Should I keep breaking concrete and digging out that pipe and cutting sections off until I don't see a hairline fracture? I noticte there is a 1/2 inch of "sludge" in the pipes anyway... I imiagine this hairline fracture would barely leak out anything and in time would probably be sealed by "sludge" My fear is this pipe is so old, it might have always had this fracture and I am just making more trouble for myself and end up replacing the whole pipe. Is it common for these old systems to have hairline fractures and still work properly... I realistically can't start going all over my basement and replacing the my entire waste system... just keep in mind that literally it is a hairline fracture, not like a chunk missing or anything.

#2 Question... if you think I should keep digging out, how long of sections did they install these old cast iron pipes in? If this fracture goes all the way to the next hub, I might have a lot of digging to go.

"#1 Question Should I keep breaking concrete and digging out that pipe and cutting sections off until I don't see a hairline fracture? I noticte there is a 1/2 inch of "sludge" in the pipes anyway...I imiagine this hairline fracture would barely leak out anything and in time would probably be sealed by "sludge" My fear is this pipe is so old, it might have always had this fracture and I am just making more trouble for myself and end up replacing the whole pipe. Is it common for these old systems to have hairline fractures and still work properly...I realistically can't start going all over my basement and replacing the my entire waste system....just keep in mind that literally it is a hairline fracture, not like a chunk missing or anything."

A small drainage will heaL itself in time. If it were my call I would leave it alone.

"#2 Question....if you think I should keep digging out, how long of sections did they install these old cast iron pipes in? If this fracture goes all the way to the next hub, I might have a lot of digging to go."

Answered above. The cast iron pipes come in 5 foot lengths.
Good luck, Tom