OhioM3W is this the mounting you have for factory fit rear mudguard?

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If so and you have for some reason an 8mm hole in the swing arm mounted bracket it bolts to you will need to use an M6 rivnut requiring an 8.9mm clearance hole. Earlier after market factory mudguard kits as I fitted to my 2016 model used four M5 fittings directly to swing arm and I used M5 rivnuts in preference to factory recommended drilling and tapping.

It occurs to me you may be referring to fitting of the rear wheel tray (not mudguard) which as can be seen employs M5 captive fixings into the ash frame. If one of these has let go it may be the hole remaining is now too large as evidenced by the 8mm hole you mention. An oversized M6 fixing may be needed unless you can somehow fill the hole with artificial wood and re-fix OE M5 insert.

Richard

2018 Roadster 3.7
1966 Land Rover S2a 88
2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone