slow leak at iron pipe threads: how to get it tight when alignment is needed?

Have replaced some of the 1" cast-iron piping on the inlet and outlet of a hot-water rad: new valves and new sections of vertical pipe that come up through the floor, which in ignorance of the proper term I'll call the riser nipples. One is 6.5" long, the other 7". They go into the old elbows just below the subfloor. Those elbows and some of the pipes that feed them are viewable - not really accessible - from below through a hole in the basement ceiling.
So I need to screw the new valves onto the new nipples, and the new nipples into the old elbows, tight enough not to leak, *BUT* the valves have to line up to the rad (which isn't in yet). Alignment is in conflict with tightness. This would seem to be a problem inherent with threaded pipe, wherever a right-angle bend has to align?
I used white plumbing tape on the new nipples (a plumber I trust says it's equivalent to pipe dope) and screwed it all together pretty tight. Now that the system is pressurized, I believe there is a very, very slow leak where one of the new riser nipples goes into the old elbow. There is no dripping water but there is definite moisture on the flange of the elbow around the new nipple.
But if I tighten it, the valve won't line up with the rad. I really don't think I'm going to get a whole turn on the valve and nipple. I could maybe get a quarter turn on the nipple, but then I'd have to back the valve off by that much, which could make it leak. And I really don't want to put a lot of force on any of it being unable to effectively brace the elbow from below.
Am currently trying to think good thoughts all day in hopes that it'll be dry by the time I get home.
What advice?
Chip C Toronto
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Put in one or two unions, and short nipples.
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Chip C wrote:

Good quality pipe dope is better under these conditions than tape. Alternative is Loctite hydraulic thread sealant.
Jim
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Use pipe dope rather than tape.
You can tighten pipe joints HARD. So don't be afraid to tighten it to where you need it.
Bob
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Take either or both of the nipples to a plumbing shop if you don't have that size pipe threader and have a half turn more thread cut in or as much as you judge will correct the problem. Reassemble with a Teflon pipe dope and your project should be leak free. You are using a 24" or better pipe wrench aren't you? HTH
Joe
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