To replace piano bass piano strings, there're many sizes to measure. You need to measure the diameter of wire, with a micrometer or caliper, and the length from the hitch pin loop to the start of the copper wrap.

Instructions for installing piano bass piano strings:

1.  Line up replacement piano string with other piano strings in the piano. Place the loop end of the wire over the hitch pin and across the bass bridge. Measure 1/4" from the point where the windings end on other piano strings toward the bass bridge. Cut through the copper winding of the new piano string at this point.

2.  Stretch the replacement piano string across to the upper plate bearing or agraffe. Line up the replacement piano string with the windings of the other piano 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 piano string at this point.

3.  Stretch the replacement piano 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 piano 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 piano string and unwind the copper winding at both ends up to the scored sections.  Then install as you would any custom made piano string.