3. After Magnafluxing highlights the length of the crack, use a pointed punch to make marks along the crack. Then drill through the iron with a 3⁄16-inch bit. Make sure you’ve found the absolute end of the crack since the rounded terminus left by the drill bit stops further growth of the crack. The only cracks cold stitching cannot repair are those with limited access. That’s why it is mostly used on surface cracks.