You may not have to drill at all into the case, it may be very close to what the tap drill size is. When you go to tap the case, coat the tap in grease, it will do two things, lubricate the tap and keep the metal shavings from falling into the case. After you install the insert, you'll want to torque the drain bolt to the proper spec, it shouldn't be too high since I'd bet it uses a copper washer. I would suggest after you put new oil in to ride it around, not too hard, to get the oil flowing and also to bring it up to operating temp about 10 - 15 mins, stop, drain the oil while it's still hot and once again install the oil drain bolt torqued to spec and re-fill with your favority lube. This will allow any metal shavings that weren't caught by the grease to be flushed out of the case. That should do it, you'll be ready to rock-n-roll. Hope this helps.