It can be done,but you have to be very careful. I have done this dozens of times with no comebacks. Your biggest concern is no chips in the cylinder. The way I do it is; first I coat the thread cutting tool with a stickiest grease I have. I only go in about two revolutions at a time. Then I back out, clean the tap, recoat with grease and repeat the steps again. After I have cut all the threads this way I take my air nozzle and blow out the cylinder. (Forgot to say that I have the piston about 1/2" down TDC with the valves closed). Before I install the Heli-Coil I coat it with Sleeve-loc. I allow the Sleeve-loc to dry for 24 hours before breaking off the tab and installing a plug. This is what works for me, don't know if anyone else has a better way. I would like to learn something new, as this does take a bit of time. I'll be watching for more replies.