Originally Posted by Gryphon
It sound like the stem pieces have galled together. Stainless steel is good for resisting rust, but it is prone to thermal warpage and galling.
Applying heat can work, as long as it is applied unequally to both pieces. If they are heated together, they will expand together. The female threads need to be warmer to expand away from the male threads.
Freezing the stem and then pouring boiling water on the stem above the joint should work.
After you get it apart, check the threads for damage. If you see slivers of metal, or bent over threads, clean the threads with a thread file or tap/die. A light application of canola or mineral oil will help prevent it from happening again.
just make sure you dont give it to much of a thermal shock. if its a very high quality stem it should be fine but if its cheap there might be impurities in the metal that would let it crack more easily (also either way look to make sure you dont see any hair line cracks or anything like that before you try the thermal solutions)
while i fully suggest the tap and die (you would need a bottoming tap to make sure the threads are clear) the price of those pieces, unless you have friends with them or work metal, is not worth the cost compared to just buying a new stem (probably spend 20-45 bucks on just the tap and die, if you can find the correct threading)
Gryphon, if a piece galls together is that still the weakest point? or is it kind of at first yes until it is left for a while, also slightly off topic for solutions to use here but would one of the very low strength loctites prevent galling or no