I will likely pass on the oversized plug idea, because I do not want to increase my chances of pushing shards into my case. I will stick with tapping out the stripped out 12mm hole with a 1/2x20 tap, using a heavy grease to trap shards during the process. What I need to know is if I should drill the stripped hole before tapping? Mike has some comments below that seem to address this: Thanks for being long winded Mike; I like knowing why. I was interested in the form tap because of my worry about pushing cuttings into my engine case. My hole is only "blind" because I don't want to disassemble my engine... I hope the cut tap with heavy grease will work to trap cuttings. But any more guidance from you and the other machinists is appreciated! I don't have a tap/drill guide, but I was guided by a very good mechanic to use a second person to spot the other axis. I can check the right/left axis while turning the tap and the other person can stand to one side and check the up/down axis for me.