Question: If the ‘retzuos’ of the tefillin ‘shel rosh’ or ‘shel yad’ tear, can they be repaired (i.e. sewn) or must they be replaced?
Short Answer: It depends on two factors: 1) at what point along the length of the ‘retzua’ did it tear? 2) are replacement-‘retzuos’ available?
There are three “zones” along the length of the ‘retzua shel yad’ and four “zones” along the length of the ‘retzua shel rosh’, as follows:
Zone #1 - the length of ‘retzua’ which is needed to bind the ‘bayis’ around the upper-arm;
Zone #2 - from the end of the first zone down to the point at which, if one by-passed the wraps around his arm and went immediately down to his hand, he would have enough length to wrap it three times around his middle finger;
Zone #3 - from the end of the second zone to the end of the ‘retzua’
Zone #1 - the “circle” which goes around his head;
Zone #2 – a length of two ‘tefachim’ (hand-breadths) from the knot and downward, on each of the two straps which hang on either side of his head;
Zone #3 - from the end of the second zone down to the ideal minimum ‘shiur’ (measurement) of the ‘retzuos’, namely, on the right side - until his naval, and on the left side - until his chest;
Zone #4 - any extra length of ‘retzua’ beyond the minimum ‘shiur’
So here is the deal: Repairs are definitely not accepted in zone #1 of the ‘shel yad’ and zones #1 and #2 of the ‘shel rosh’. If no other replacement ‘retzuos’ are available, the torn ones may be sewn – with kosher sinew only – in zone #2 of the ‘shel yad’ and zone #3 of the ‘shel rosh’. Repairs may always be made in zone #3 of the ‘shel yad’ and zone #4 of the ‘shel rosh’. (Note: In fact, in the last category, the torn ‘retzuos’ may even be repaired with regular thread or by tying the torn piece on!)