The 4.25 inch diamond core drill worked great for drilling holes for a 4 inch dryer vent through two 8 inch cement block walls. There was no other way to realistically get the job done. The SDS pilot arbor that I ordered with the core drill was not as good. The one tiny set screw was not strong enough to hold the pilot drill to the arbor. It kept coming loose. Also, because the pilot drill was so long and flexible, it was difficult to locate the hole exactly where I wanted it. Like others have suggested, I removed the pilot after the hole was started. To be honest, this 4.25 inch core drill might have been beyond the limit of my non-professional hammer drill. The drill got pretty hot. The first hole took less than 15 minutes to drill through the 8 inch block. The second hole took about three times as long, largely because my drill was overheating. But I am pleased that I got the job done. The shipment came very quickly, and I had the job done the next day. It was well worth the price for this one job alone.