The hole is large enough for a #8 countersunk screw, but the head does not fit completely flush. The magnetism is transferred to the (steel) screw head so things still stick, maybe just not as well. Screw #6 is flush. Also note that this alloy is somewhat brittle. So if you tighten them on uneven ground, they will break in half. However, they are very useful and very numerous. Subjectively, their strength seems to be the same as other neodymium magnets of comparable size; in other words, they are amazingly strong.
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