If one has touched his forearm while placing the arm-Tefilin (I see that it's a very common case), is he obligated to wash his hands immediately, as he has to say a Beracha on the head-Tefilin? If so, how can he make a pause between placing the Tefilins?
See Halachically Speaking Volume 3 Issue 4 (page 4):
Although some poskim say one should wash his hands, the minhag is that one does not have to wash his hands in this situation. The reason is because since that part of the arm is revealed when putting on tefillin it is not considered a place which is normally covered.
See there for sources...
I've asked this question one rav posek (Rav Ehuda Gantz, from Ramot Dalet, Jerusalem).
He told me that in order to make any broha (on the head-Tefilin in our case) it is enough to clean the hands on the clothes (מידי דמנקי). After finishing putting the head-Tefilin one can wash his hands before the praying, without any problems of pausing between arm- and head-Tefilin.