Tefillin 37

It is a halacha lemoshe misinai that tefillin are stitched close with sinews that come from kosher animals. But not from chickens which are weak and also there is no recognizing them as opposed to meat. It is best to sew it shut with the sinews of an ox. We take the sinews from the heal which are white. If they are hard, we bang them with rocks until they are soft as flax, and we sew and intertwine them and they are really strong. We don't take sinews from a kusi b/c we suspect maybe they are from a non kosher animal. The beis Yosef wrote that in a place without animal sinews you stitch it with this unknows item that is made from klaf until it looks like sinews. End of statement. This is not clear b/c since the halacha is said to stitch with sinews, how can we let klaf be used. We need to say that we hold that the halacha is stated this way that even stitching needs to be done from kosher animals and the way to do it with sinews. But sinews are not an exclusion. Therefore in exigent circumstances, you can rely on the beis yosef and stich with dried intestines. There are those who assur it because it is not from the sinews. We should be machmir because since the halacha might say sinews specifically and that is what caught the poskim's eye.

He should make 3 stitchings on every side. The string that stitches should be surrounded on both sides above and below. The stitching needs to be next to the bayis and not on the other hand and it gets passed back and forth. There needs to be 12 stitchings in the shel yad and also the shel rosh. However, if there were only 10 stitches, it is still kosher. There is someone who says that it all needs to be done with one thread and if it is not long enough, then it is not kosher. This is according to the taz. That is to say until he redoes it with a new a long enough string. In any case. If it is a short thread and he can't do all 12 with it, according to the magen avraham, he can use another thread. It appears to me that if he sees that it is too short then he should complete the threading with another string and we should not change it.