Sheet metal screws were used. Truth be told, one of the areas most susceptible to play is at the swivel connectors where they screw into the stubs at the header flange. The stubs and the swivels are threaded, but the swivels are made of aluminum and the stubs are steel. To help address galling and also probably due to repeated use, the threads are not very tight, allowing movement. In some cases I drilled into both the stubs and swivels and ran screws in to try to stabilize things, which seemed to help. You would not think a little play in the threads would make much difference, but at the other end of a lever, it adds up. combine that with a very restricted installation location for the tubes, and it doesn't take much to cause some interference.