I lost my XB - S rear caliper to carrier bolt somewhere on the way to work today. Thankfully we have stocks of metric and imperial fasteners so I grabbed a few 5/16 and 3/8 and went out on my break to fix it but it seems the thread is a M8 - but a very baggy M8? The 5/16 UNC wasnt going to go in without a fight and I didnt want to force it.

What fastener should be fitted?

If M8 'fits', The bolt you lost probably wobbled around making it baggy before it dropped out altogether. You might need to get a helicoil in there.

Cheers
Stot

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1999 Buell M2
1977 Royal Enfield Bullet 350

sounds about right

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Buell Ulysses XB12X 06/08

It fell out in 20mins on a 14 mile run - doubt much wear happened in that time? I had whipped the caliper off pre mot to grease sliders and check pads.

But yeah, there may be historical wear...can I tap a M8 out to 5/16 UNC or is that just going to leave another baggy mess. Not a fan of the helicoil, maybe I'll weld up and re-machine.

5/16 major diameter is only 7.94mm so you would be threading into already loose threads, no meat to it.

If you can fit a 3/8 bolt through the hole in the calliper you could probably use that, you could even open the hole in the calliper without issue if you don't mind that kind of thing. I did this to my front calliper as the bolt was stripped and glued in when I got it.

You would need to weld, redrill and tap or Helicoil it to do it properly really.

Cheers
Stot

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1999 Buell M2
1977 Royal Enfield Bullet 350

Just go get the proper bolt for it!
No need for screwing things up even more.

Twinmotorcycles do have them , but maybe Maz to!?

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The sound of a V-twin engine braking is music to my ears!!

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Buell Ulysses XB12X 06/08

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The sound of a V-twin engine braking is music to my ears!!

Which one are you missing?

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The sound of a V-twin engine braking is music to my ears!!