Here's a custom dual-diameter bit for drilling precise rear-counterbored pegholes to fit popular sealed tuning machines. This tool quickly pays for itself in a busy guitar shop—it does the job in one operation, so you'll get the work done in half the time. It's 7.8mm-diameter (.307") for the threaded peghead bushing, and 9.9mm (.390") for the gear housing. The brad point helps center the bit accurately for a clean, professional job. High speed steel.
Why counterbore the pegholes? The collar on a sealed tuner's housing has a larger diameter than the threaded peghead bushing. Under string tension, the installed bushing (and the peghead washer) can lean slightly in a constant-diameter peghole. This can damage the lacquer finish around the bushing washer. A peghole that's properly sized for the bushing, and rear-counterbored for the housing collar, prevents the problem.
This unique bit works for Grover, Schaller and Gotoh sealed tuners that require 10mm (13/32") pegholes.