WHOA-UP there coltshooter2.
As luck would have it, I picked up another cap & ball (Hawes 1851 Sheriff) today, and was putting in an order with Thunder Ridge for some nipples, when I noticed they offer two different sets for Italian made revolvers.
M (metric) 6x.75 for Pietta /old Italian revolvers, And 12x28 for Uberti /new Italian revolvers.
THEY ARE COMPLETELY DIFFERENT !!!!
The two I switched nipples on for my reply to you, was a Pietta I bought 3 years ago (metric), and an Uberti I have had since the 70s or early 80s (also metric).
This sent my to my gun room to try a switch between the Pietta and an Uberti 1860 Army, I bought about 5 years ago. THE NIPPLES DO NOT INTERCHANGE.
The metric (Pietta /old Italian) are larger in diameter, so they won't even start into the Uberti cylinder. However, the nipples for the Uberti (12 x 28) are smaller, so thread right into the Pietta cylinder. Obviously, I didn't try to tighten them, but, if they did tighten, the threaded hole is still TOO BIG, and would be pointed right at your face, if the nipple BLEW OUT !!!!
Richland quit importing revovers in 1986, so, my guess is your 1858 comes under the old Italian category.
When you get your new Uberti nipples, MEASURE THE THREADS. If they are smaller than the diameter on your old nipples, SEND THEM BACK, and order some for Pietta / old Italian revolvers.