Yoreh Deah Siman 18 Seif 2

 

ב

שִׁעוּר הַפְּגִימָה, כָּל שֶׁהוּא; וּבִלְבַד שֶׁתֶּאֱגֹר בָּהּ כָּל שֶׁהוּא, אֲפִלּוּ חוּט הַשַּׂעֲרָה.

 

Seif 2

 

What is the amount of defect which disqualifies a knife?  Even a slight amount.  As long as it accumulates even if it is a hair-size it disqualifies.