I had the same type of problem on my knuckle when I sheared the bolts on the steering arm. It is hard to do it with a hand held drill motor. Even with slow speed and cutting oil, I kept dulling bits. I finally set it up on my drill press and they cut like butter. Watch the chips coming from the bits, they should be no darker then a ôstraw colorö. If they start to get darker then apply more fluid. As Roger said good bits, slow speed, and good cutting fluid. I use a product called ôRapid Tapö. I get it at an industrial supply place that sells machine tools and products.