The nuts are M8. My recommendation would be to use an M8 flange nut (similar to that used on your strut tower), which would most likely eliminate the need for a washer. You'll probably need a shorter bolt on one of the hangers too as the OEM bolt is too long for most aftermarket exhaust. The length of the bolt, of course will depend on your exhaust. I would just get several in different lengths for best fit.
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