Spanner Size

Mine's a nice snug fit with a 14mm socket.

 

I always try and use a six sided socket or spanner if possible. Multi pointed versions of the "correct" size can sometimes be a sloppy fit.

 

Definitely standard metric size. Are you using good quality tools? Has the bolt head been damaged by a previous owner?

 

Nothing wrong with the bolt head, it looks good as new (the rounded corners are normal). And your socket is top notch but it might be worth trying another socket or ring spanner?

 

Measure it, but I'd guess a 14mm single hexagon socket siting on the flats would sort it.
A bi- hex socket is rarely ideal, but you haven't specified which snap-on socket you have there.

 

Try a 9/16 AF ring spanner it works for me

 

I hate to suggest it but do you have a small adjustable? Failing which be prepared to sacrifice one of your feeler gauge blades to take up the slack when using the 14mm spanner.

 

I don't recall any problems with that bolt on my old 2012 SE Bonneville.
Pretty sure I used a standard 14mm socket or spanner.

 

TRIPLE X
Try a UNF spanner. not sure of size.
Joe