The problem i had was caused by normal Zetec ones which WILL fail at a bar+. Proved over and over until serious damage was done

I have found the RS one's so far to be spot-on @20psi tho. (my problem with the first set was specific to my car Sketch .)

TBH i would for the minimal cost have it machined, then slot in so NGK 9s from Stu.