Reference for Mikvaot 4:3
הַחוֹטֵט בְּצִנּוֹר לְקַבֵּל צְרוֹרוֹת, בְּשֶׁל עֵץ, כָּל שֶׁהוּא. וּבְשֶׁל חֶרֶס, רְבִיעִית. רַבִּי יוֹסֵי אוֹמֵר, אַף בְּשֶׁל חֶרֶס, כָּל שֶׁהוּא. לֹא אָמְרוּ רְבִיעִית אֶלָּא בְשִׁבְרֵי כְלֵי חֶרֶס. הָיוּ צְרוֹרוֹת מִתְחַלְחֲלִים בְּתוֹכוֹ, פּוֹסְלִים אֶת הַמִּקְוֶה. יָרַד לְתוֹכוֹ עָפָר וְנִכְבַּשׁ, כָּשֵׁר. סִלּוֹן שֶׁהוּא צַר מִכָּאן וּמִכָּאן וְרָחָב מִן הָאֶמְצַע, אֵינוֹ פוֹסֵל, מִפְּנֵי שֶׁלֹּא נַעֲשָׂה לְקַבָּלָה:
If one carves out [furrows] in a [drain] pipe to collect stones, [a furrow] of any size in one of wood, and of a <i>revi'it</i> [a specific measure of volume, a quarter of a <i>log</i>] in one of earthenware [is sufficient for the pipe to be considered a vessel, and thus for its waters to render a <i>mikveh</i> invalid as drawn waters]. Rabbi Yose says: even for one of earthenware, [a furrow] of any size [is sufficient to invalidate waters flowing through a pipe]; they [the Sages] only said [that the minimum measure is] a <i>revi'it</i> regarding shards of earthenware vessels [that they are considered vessels as long as they can still contain that amount]. If there were stones rolling around inside it [the furrow], they [its waters] render a <i>mikveh</i> invalid. If dust descended into it and was compressed, it is valid. A duct which is narrow on either end and widens in the middle [built that way in order to increase water pressure] does not render [its waters] invalid, since it was not made to be a receptacle.