I have a house that has 50 year old galvanized 3/4" pipe. It has started to give quite a bit of rust into my water, I am considering running 1/2" pex inside of the old galvanized so that it has protection from critters it any rats or squirrels find find their way into the attic and to also give the appearance of solid metal pipe with the convenience of a plastic liner.

Has anyone tried this in the past, and if so other than the problems of having to unscrew all of the elbows to feed it through are there any reasons not to consider this combination?

The minimum bend radius is 6x the diameter, no won't work.

http://www.zurn.com/operations/pexrh/pdfs/installation/PEXDesApplGuide.pdf

Thank You for the fast reply... That is exactly the type of information that I needed.

It is too bad they do not make plastic lined pipe instead of it being galvanized, that would seem to be a logical solution since the main problems with steel pipe was the galvanization wearing away.

Hmmm... maybe I have come up with a whole new market!

Besides the bend not working chances are pretty good your old 3/4 gal pipe won't allow a 1/2" PEX through due to the build up of rust, you may have pencil sized passages already