Modern Vespa

Every resource I can find tells me this is an M10 with 1.5 thread pitch, but when I try to thread one it feels too big and I worry about damaging the aluminum swing arm.

Am I being too shy or has anyone seen a different size? Maybe I just need to ream it out with an M10 1.5 tap? is it possibly SAE?

This scooter has variations in the kickstart quadrant, headlight, and wiring. Did Vespa have a different exhaust bolt measurement prior to the VBB since that's exactly what I'm seeing on other parts of the scooter?

Just going by memory, but maybe some motors use M8 there. Try the bolt first with no muffler to make sure you have the right size. That swing arm muff bolt hole does get messed up pretty easy and that's why they went to the thru bolt hole on the newer three port motors. If the M8 is obviously too small, then yes tap it out for the M10.

m10x1.5; the original heads were 14mm. take a tap and chase the threads.

Thanks guys - yes, my frustration was that the M8 is definitely too small and the M10 seems too big ( Goldilocks syndrome, ritght).

The bolt has been out for nearly ten months and the original is missing, hence the hesitation. I'll proceed with chasing the threads with a tap and go from there.

That hole is easy to have strip.
I chased mine - then replaced it with a stud/loctite.
Then use a nut that comes on and off.
Means the cases no longer see repeated removal and replacement of a bolt.
Its a nice upgrade/ have been very happy with it/ zero problems once I went that way.