Reference for Chullin 247:14

אמר מר

One need not scrape off the plastering nor [see to it] that it lies on the ground, but one need only cut it down to less than four handbreadths high inside.<span class="x" onmousemove="('comment',' The dispute therefore between R. Meir and the Rabbis is only with regard to an oven which was not unclean, concerning the measures necessary in order to prevent it from ever becoming unclean.');">⁵</span>

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