The conversion cylinders are in the same ballpark in diameter as the cap & ball cylinders, The Colt Navy by Uberti being about 1.534-1.535. The conversion cylinders I have are 1.535-1.536. All that I have, several, both R&D and Kirst fit without interference in Uberti Colts. All bets are off if your 51 is a Pietta.
If your conversion cylinder is/was 1.535, then it's the revolver that is out of spec. Check the diameter of your cap & ball cylinder, if it is 1.534-1.536, then the conversion cylinder was too large. If it is smaller, then the gun is out of spec.