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If I find a Rabeynu Tamm tefillin and I do not have another one , can I take it appart , and I know how to , and change the ROSH from Rabeynu Tam to a Rashi tefillin ?

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Welcome to Judaism.SE, and thanks very much for the fascinating question! Please consider registering your account, to help the site keep track of your contributions. – Isaac Moses May 30 '11 at 14:19
Are you asking whether it is legal or whether it is physically possible/feasible? – WAF May 30 '11 at 15:46

You are not allowed to. This is because the two opinions mutually invalidate one another and therefore you would be taking Kosher Tefillin and making it not Kosher according to Rabbeinu Tam. This is considered to be Mezalzel in the shita of Rabbeinu Tam. See the Ridvaz Chelek-6 Siman 2286.

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The Ridvaz says he first ruled it was permissible, then had a bad dream so he changed his mind. Weird. IAE this would only seem to apply to someone who treats RT's opinion as not nidche lehalacha. – Double AA Feb 14 '13 at 5:00

This is Asur because it goes under the category "Maalim BikDusha WeLo Moridin." (Od Yosef Hai Wayera #8. see Oserot Yosef 3:6).

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Your answer is unclear. Are you stating that the temporary posul status when the tefilla is disassembled is a yerida? Or that Rashi tefillin are somehow "less kadosh" than RT? – Isaac Kotlicky Jun 30 '15 at 20:54

Rabbeinu Tam and Rashi argue about the order in which the parshios are written and placed. According to Rashi, they are written according to the order of appearance in the Torah. The Rema O.C. 32:1 writes that a deviation from the order written invalidates the tefilin that they comprise (see MB 3 there for drasha).

So properly written R"T tefilin cannot be converted.

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All opinions agree on the order of writing the parshiot. The debate is only about the order they are inserted in the shel rosh and the format in shel yad. According to r.t., in shel yad, you skip a space, write shema, and then go back and write vehayah in the empty space. So the shel rosh would be kosher if switched from rashi rt. Only it is prohibited to do so. – Aaron Shaffier May 30 '11 at 18:12
I defer to the expert! – YDK May 30 '11 at 20:18
Yup, black and white in the Tur: hebrewbooks.org/pdfpager.aspx?req=14265&st=&pgnum=80 – YDK May 30 '11 at 23:44
Is there a reason you haven't deleted this? – Double AA Feb 14 '13 at 4:53

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