I dont know the answer for the size, but a helicoil is the best way to fix it.
I guess I must have missed you with my new member torque wrench recommendation: get one, a Harbor Freight 3/8" drive torque wrench is better than nothing.
The thing about motorcycles is just about every fastener is a steel bolt into a cast aluminum part. With steel into steel (like most car parts) you can tighten a bolt or nut till it feels snug, then give it a good twist till it doesnt want to give anymore - you can feel when its tight enough
but you cant do that with aluminum parts - they dont have the same feel, they will keep giving and giving and they will strip right out when you over torque them.
All the torque specs are in the yamaha factory service manual for your bike, and the standard ones like spark plugs and oil drain plugs are in the owners manual. You can download both for free in pdf format (google them, easy to find).
A ten dollar torque wrench will save you $hundreds in grief. If you get a better wrench, all the better still.
Sorry I didnt catch you on this sooner, its common mistake on motorcycles - you are not the first.
Last edited by KCW; 11-26-2018 at 06:41 PM.