I wanted to post the link to the Ebay seller where we have been getting our diamond drill bits from to drill all of the tanks we have drilled this past year. These bits do not last very long - about 4 to 5 holes in decent time, more if you are extremely patient. I have not tried the bits that he now offers that have a centering bit in the middle of them. I think that I will try these next time I drill - they are the ones coated in yellow. The sellers name is Salty Depot on Ebay if the link goes bad over time. http://stores.ebay.com/saltydepot_D...W0QQcolZ2QQdirZ1QQfsubZ10899382QQftidZ2QQtZkm