Core Bit Conversion Adaptor 5/8 Inch (Male) to 1-1/4 inch (Female) works great for using smaller diameter core bits threaded at 5/8"-11 on a core drill machine threaded at 1-1/4"-7. This adapter is usually required when using under 2" in diameter wet core bits or all the size of the dry core bits that have the 5/8"-11 female threaded using a Core Drill.