Source: A letter of the Rebbe, 12 Adar 5715: Igros Kodesh, vol. 10, no. 3339.
In the Alter Rebbe's Shulchan Aruch: Orach Chaim, Siman 14, seif 12.
1. The Issue of Adjusting Another’s Tefillin Knot
The discussion today focuses on the halachic and minhag considerations regarding changing the knot of the tefillin shel rosh—the head tefillin—when using someone else’s tefillin. This situation often arises when a person borrows tefillin that do not fit their head precisely, requiring an adjustment to the knot in order for the tefillin to sit correctly in the proper spot.
It is important to note that the placement of tefillin shel rosh must be exact, both for halachic reasons and to fulfill the mitzvah properly. The Rebbe was asked about taking someone else’s tefillin to daven with, and about something the questioner had heard in the name of the Avnei Nezer.
2. The Avnei Nezer’s Stringency
The Avnei Nezer, who was not only a great scholar but also a renowned chassidic rebbe, addresses this issue at the end of his responsum, Orach Chaim 183, as the Rebbe quotes it: "להמניח ת' של ראש של אחר, לא שפיר עביד כוון שהבעלים ישנו אח"כ הקשר לפי ראשם ונמצא דלא הוי קשר של קיימא, ולא יצא הראשון ידי חובתו ומצוה לפרסם הדבר" ("One who puts on another person's head tefillin does not do well, since the owners will afterward change the knot to fit their heads, so it turns out not to be a lasting knot, and the first person has not fulfilled his obligation; and it is a mitzvah to publicize the matter").
3. The Custom, and the Proof for It
The Rebbe continues: "אבל כבר נהגו העולם דלא כן" ("But the general custom is already otherwise").
The strong proof for the custom is what is stated plainly in the Shulchan Aruch, Orach Chaim 14:4, and copied in the Alter Rebbe's Shulchan Aruch, seif 12: one may put on another person's tefillin. Many later authorities copy this, and nowhere is it explained that this applies only when the size of the head matches, which in truth happens only rarely.
4. Answers to the Avnei Nezer
Others have already raised difficulties on the Avnei Nezer, and their questions are copied there, in sections 184 and 185.
The Kli Chemdah on the Torah, Parshas Vayeilech, defends the general custom in two ways: (a) tefillin do not require a lasting knot, following the Ramban; (b) a knot tied for the time of the mitzvah counts as a lasting knot. He explains there what at first glance does not seem so in certain places.
5. Summary and Broader Considerations
This discussion highlights how even seemingly technical details in mitzvah observance can carry significant halachic weight.