Let me start by saying that I have purchased dozens of this type of magnets over the years and am fully aware of their limitations. These are the first that I have purchased that have cracked so easily. I lightly tightened the mounting screws and two or three of them developed cracks. Hopefully they are glued to the cup and the pieces won't fall out. They are fairly strong but I highly doubt that they have 100 lbs of pulling force that they claim.
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