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I need a M8 nut
Very likely. Check the pitch too just in case it's a weird forrin size, normal M8 ISO coarse pitch is 1.25mm. Span ten pitches with your caliper to get a decent reading in the absence of a pitch gauge. You should easily be able to eliminate 5/16-18 UNC and 5/16-24 UNF from your enquiries, which you haven't done by measuring only the diameter
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I just got one of these, highly recommended.
http://www.swmanufacturing.com/parts-catalog/thread-checkers/the-original-thread-checker/the-original-thread-checker-combo/
I need a M8 nut for this bolt right?
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