B21 said:
Also 'cos I'm pissed off bought some high tensile mother fuckers.. in 12.9 which is about as tough as they come..
https://www.westfieldfasteners.co.uk/BO_ScrewBolt_Unbrako_SHCap_M6.html
Do not use the grade 12.9 or 10.8 bolts for any application where there can be vibration. They suffer from something called hydrogen embrittlement, so while they are indeed stronger, they are also significantly less ductile and will just shear off instead.
I was checking a wind turbine base that was having the bolts completely shear off and it ended up being due to the vibration and overspeccing the bolts.
Limit to g8.8 if using regular bolts, or look at the manufacturer spec to see what grade they ended up using.