I inserted a 1/2" crimping die into a chuck drill only to find that it won't crimp the size listed on the die. So it's 5/8". The 1/2" copper tubing I was trying to insert would probably work if I used a hammer on the end of the tube to insert, but the practical part of this probably only applies to soft copper tubing. Good to know, but disappointed that they have advertised that hard pipes can also be assembled with tools.