threaded inserts or simply thread it for something like 3/8
one of the few times i would smack you if i could...
NEVER mix IMPERIAL with METRIC!
use an M8...the next size up is actually M7, but that is such a rare size to find...its not worth it.
neither is a helicoil, unless you do constant tear downs, in which case, the kit can be worth it. you do every bolt, and never fear stripping em out again.
a mechanic "may" helicoil it for you, but the price can be highly variable...from a beer or two, to almost the cost of the helicoil kit itself.
and, while they are really easy, i have seen so many people stuff up helicoils simply because they have never done one before...
get a new cylinder as last option..actually, thats cheaper than the helicoil option!
oh, yeah... hack into the cylinder behind the stripped hole, with some luck you can squeeze a nut in behind... you can get lil skinny "locknuts" that are only half thickness...