That was cool as heck. I have a tap and die set but have never used it. My Lishen 270 amp cells came with M6 and I was debating drilling and tapping to replace with M8. Not sure if I'll do it but now I wouldn't be too scared to try!
Nice job cinergi. Your helicoil looked like it only has about 4 threads.
When I was testing with mine I couldn't get more than 4 helicoil threads in there without drilling deeper. It is better than original.
Well timed & thought out, nice & simple instructions on how to rethread cells. This may also be applicable to those people who have received those Lishen Cells with crooked holes but I fear that may require going up in size from 6mm to 8mm. So from 1/4" to 5/16".