Universal Bass Strings are a good solution to enable you to replace single bass strings when you don't need to replace the whole set. There are many sizes to choose from. You would choose the diameter of wire that most closely matches the size that you need. You can determine the diameter of the wire with a micrometer or caliper. There is 3-5/8" from the hitch pin loop to the start of the copper wrap.
Instructions for installing universal bass strings:
1. Place the loop end of the wire over the hitch pin and across the bass bridge. Line up replacement string with other strings in the piano. Measure 1/4" from the point where the windings end on other strings toward the bass bridge. Cut through the copper winding of the new string at this point.
2. Stretch the replacement string across to the upper plate bearing or agraffe. Line up the replacement string with the windings of the other strings. Measure 1/4" from the winding line toward the opposite (hitch pin) side to allow for stretch. Cut through the copper winding of the new string at this point.
3. Stretch the replacement string up to the tuning pin. Allow enough length for 3 coils on the tuning pin. Cut through the winding and wire core to remove excess length of the replacement string.
4. Be sure to cut through the copper winding with a circular motion in the same direction as the winding has been done. This will prevent the windings from loosening their grip on the core wire.
5. Remove the replacement string and unwind the copper winding at both ends up to the scored sections. Then install as you would any custom made string.