Italian threaded 19mm bottom brackets
 

have a megaxo crankset with 19mm spindle which will work on one bike but not my Italian bike which uses Italian threading for the bottom bracket. Who makes 19mm bottom brackets with Italian threading?

I'm not sure anyone makes/made a MegaExo 19 (Omega) bottom bracket for an Italian threaded shell. I don't see any reference on FSA's site. You may want to contact FSA and ask.

https://www.fullspeedahead.com/en/su...tomer-services

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...additionally, the BB shell in an Italian bike is wider than the 68mm standard for which your crank was designed and manufactured.
Even if you could find outboard bearings to swap out, I doubt you'd get a proper fit of the crank to your frame.

It would be nice if there were more component interchangeability in the bicycle world, but it's just not the reality.