[Regarding] a sereedah that is placed over the mouth of an oven surrounded by a sealed cover, [if] a crack developed between the oven and the sereedah, its measure is the width of the mouth of an ox-goad that cannot be inserted. R’ Yehudah says: [If] it can enter. [If] the sereedah developed a crack, its measure is the width of the mouth of an ox-goad that can enter. R’ Yehudah says: [Even if] it cannot enter. [If the crack in the sereedah] was rounded, we do not view it as being straight; rather its measure is the width of an ox-goad that can enter.