401. Are Head and Arm Tefillin Fungible?

Tefillin, Mezuzah v’Sefer Torah 3:17

Head tefillin may not be turned into arm tefillin but arm tefillin may be turned into head tefillin, which are of a greater degree of sanctity. Similarly, the straps from head tefillin may not be used for arm tefillin. This only applies once the tefillin have been worn; if head tefillin was never worn, it may be turned into arm tefillin. This is done by wrapping it in leather until it becomes a single compartment, after which they may be used on the arm.

Tefillin, Mezuzah v’Sefer Torah 3:18

If two adjacent stitches or three non-adjacent stitches get torn, the tefillin are invalid. This is the case with old tefillin but when it comes to new tefillin, as long as the base is intact, they are valid. The definition of “new” is as long as the leather is strong and won’t tear when grasping where the stitch tore and hanging the tefillin. If one cannot do this because the leather will tear, the tefillin are deemed “old.”