Actually, you should use a grade 5 bolt (8.8 metric grade) instead of a grade 8 (10.9 metric). Although grade 8's can handle a stronger clamping load, they are notoriously brittle and can be sheared off very easily. Grade 5's are a lot better at handling shearing loads.
I had the very same problem with my SVT. The stock bolt is a grade 8 bolt. I simply removed what was left and replaced it with a grade 5. I have not had any problems since.
