try to do the washer trick on my car, went to all different hardware stores in my area, the hardest bolt i can find is grade 8.8. ( little bit harder than grade 8).
do you guys think grade 8.8 will be safe?

 

A grade 8.8 metric bolt is roughly equivalent to a grade 5 SAE bolt. You'll want a metric bolt that's at least grade 10.9 (roughly equivalent to grade 8 SAE bolt). I would not use a grade 8.8 bolt on suspension pieces.

http://www.americanfastener.co...l.asp
http://www.k-tbolt.com/bolt_chart.html

You should be able to find them here: http://www.mcmaster.com