what size helicoil is needed?

Hi guys this area 51 again ive got 2 head stud bolt holes stripped off ive been told about getting helicoil done but what size would i need? i have found a cheap equivalent on ebay but theres various size kits im lost with all this now lol.

 

also would this cheapo kit do as this scoot is taking me for more than its worth ha

 

More than likely M6.

 

Cheers Marsh ill get that set ordered i just hope it goes right never done this before