Built a steam boiler setup for doing stripping runs. Soldered the ferrules after hole saw cutting the openings. Wanted to share a couple ideas that helped.
A four inch hole saw is a challenge to hand drill through the keg top. Two inch round aluminium fits into the top of the kegs with two notches in the opening lip and retaining ring. This allowed the hole saw to be centred over original keg opening. It will not fit in Euro style kegs as sown that have threaded openings but could be turned down or threaded. It cane used repeatedly and is easy to make.