How to fit a union where there is no space for one?

The ¾ inch galvanized main water line is in a hole 4 feet deep and there is less than 10 inches of usable distance in which to put a Tee, a union and necessary nipple. So far I have been unable to force the main pipes apart sufficiently to pry the installed halves of the union past each other into alignment so the union can be tightened. If I try to over-tighten the nipple threads to create sufficient space for the union parts to slip by each other and then loosen the threads, they leak. I’d like to keep a straight-through connection if possible because the ¾ inch pipe does not flow much water as it is. Thanks in advance for your advice. Tom

leak

10" is not much for any kind of connection. You might be able to use a 3" nipple, and a 6" one with a "Dresser" compression coupling, if you can find a short one. Slide the Dresser on to the 6" nipple with only one gland on it. Put the removed gland on to the 3" nipple. Tighten the 6" nipple into the fitting. Slide the Dresser all the way back. Screw in the 3" nipple with the gland on it, the slide that gland back against its fitting. Slide the Dresser and its gland onto the 3" nipple. Insert the bolts and tighten the nuts. You might be able to do the same thing with a PVC "FloCoupling" but sometimes getting the "locknuts" straight and tight is difficult.

Thanks, I didn't know about a Dresser coupling.

Preliminary question: Can a Dresser coupling be buried? Or must it be in a vault?

Because this pipe has been buried 30 years and is slightly corroded on the outside, I would assume the gasket on this type of coupling might not make a good seal. Options:

1. I can use emery cloth and clean up the pipe where the gasket will fit to get bright metal all around and then crank down hard on the coupling.

2. I can enlarge the hole and saw off and re-thread the pipe so I can screw on new ends for the gaskets to contact.

Any idea which option would be best?

Your initial question, implied that the union was going between two fittings, otherwise there should NOT be a size limitation. IF not, then you may NEVER get the pipe smooth enough for anything to seal properly. Leaking steel pipes usually have to be either completely replaced, or replaced in a section between fittings when the old pipe is removed from them.