those look like their head is too small so will just screw into the axle without securing the cranks.
Washers?
Or flanged head hex: http://www.stainlessautomotivefastenings.co.uk/index.php?main_page=product_info&cPath=119_113&products_id=4722
@gbj_tester started
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Washers?
Or flanged head hex:
http://www.stainlessautomotivefastenings.co.uk/index.php?main_page=product_info&cPath=119_113&products_id=4722