^You need the 34.9mm clamp not 31.8. 31.8 is the seatpost diameter so you need to account for the extra material in the frame.
https://www.bikeparts.co.uk/products/m-part-seat-clamp-with-rack-mount-34-9-mm-black
@Irishane started
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^You need the 34.9mm clamp not 31.8. 31.8 is the seatpost diameter so you need to account for the extra material in the frame.
https://www.bikeparts.co.uk/products/m-part-seat-clamp-with-rack-mount-34-9-mm-black