Dear Shimshon Lonner, Sholom U-Berocho,

The key lies in RASHI NIDAH 22a DH Le-Maimra. Rashi says there that anything which has a Shiur must automatically be Negiah, i.e. when the flesh of the Ever touches the Shichvas Zera when it emerges. Rashi writes that all Tumos of Re'iyah do not require a Shiur.

This is in fact just what RASHI 42a DH u'le'Tamei writes. Therefore one must say that when Shmuel states(below 43b) that a Zav requires Chasimas Pi Ama , he is therefore saying that a Zav is a Tumah of Negiah. When the Gemara challenges Shmuel from the Mishnah (above 40a) that Zav is Metamei without any Shiur, this means it is contradicting Shmuel and saying that a Zav has a din of Re'iyah and this is in fact the dispute between Rebbi Nasson - who maintains that Zav is Negiah - and between Chachomim, who maintain that Zav is Re'iyah.

Later on, on page 43b, Rav Chanilai states that there is no Shiur for Re'iyah of Shichvas Zera, but the Shiur for Negiah is that of a lentil -"be'ke'Adasha". (Note that the shiur for Shichvas Zera is not the same as for Zav- the latter is Chasimas Pi Ama. Therefore the first opinion that Rashi gives on 42a- ke'Adasha, seems to be referring to Shichvas Zera, whilst the second opinion- Chasimas Pi Ama- seems to be referring to Zav).

Therefore the answer to your question is that the Gemara does not say (below 43b) that Chasimas Pi Ama is a Shiur for seeing Shichvas Zera. Rather, Shmuel said there that it is a Shiur for touching Ziva and the Gemara challenged this from the Mishnah, that is referring to Re'iyah and answered that Shmuel agrees with Rebbi Nasson, who discusses Negiah, whilst the Mishnah maintains that Zav is a din of Re'iyah.

Kol Tuv

R. Dovid Bloom