get a heli coil and put lock tight on the heli coil and install it the lock tite should prevent it from slipping and the spark plug should go in and out like new.

http://alcoafastener.thomasnet.com/c...keenserts-reg-

or use this if u can find it in the size you need. but you will never get it out again

you drive it down and it will lock it but it will never come out again. i dont know if they make it in the size u need but its called a keen sert.