Probably a grade 8 (10.9 metric). Find yourself a carbide tip drill bit. Here we have masonry bits with carbide tips that will work. I have to use them on broken muffler bolts. I had only one to fail due poor bonding.
One comment about HSS bits there are different grades and many home improvement stores sell the el cheapo grade versions. I buy mine from my nut, bolt and screw distributor. Those home improvement ones I brought would not even cut a fraction of an inch and now the ones I am using will an inch or so into grade 5 several times before dulling. Also don't over speed the bits as they will lose their hardness due to over heating.
Last edited by AVB; 28/12/16 09:43 AM.
