I broke a water pipe that is under ground under pressure. The pipe is steel or iron. I can fit a plastic pipe over the steel pipe and can cap the plastic at the top. how can
I keep the water from leaking from where the steel pipe and the plastic meet. Believe it or not I've had a few plumbers don't know or don't seen to have any alteratives, I can't find a plumber with a pipe threader to put a metal cap over the metal pipe. anyone have a idea? bob d

If they are accessible at the joining point if the plastic and metal, use a Fernco shielded fitting.

Hello Bob, how "broken" is the pipe ? Can you snap a picture of it and post it for us to see what exactly can be done ?

In order to thread an iron pipe you need enough work space to insert pipe threading tools onto the broken pipe. If you manage do get that tool under slab than you will face another problem: the iron pipe under ground is most-likely too fragile due to corrosion. Therefore, there is a very good possibility that the threader will damage the pipe even more during threading. Best way to approach this problem is to excavate the broken pipe to the first joint and carefully unscrew it from the fitting then insert new section of pipe.
Also, and depending how it was plumbed, you may want to try to by-pass the broken pipe by cutting it off at its point of entry under slab and re-routing it through wall and ceiling. Any experienced plumber should know how to approach this problem w/o causing too much damage to your house. Back to you. Milo