The next step in piston construction following the insertion of the piston liners.
The liners are expanded into the piston when initially inserted (see Piston Liners), but this is just to hold them in place and is only temporary. The liners still need to be expanded to their original. To do this, a series of spinning lathes are used. Each lathe is connected to a rod with a specificly sized ball on the end. As the piston is pushed against the rod, the ball at the end is forced into the liner. This accomplishes two things. First, and most importantly, the liner is now the proper size and shape to allow proper air flow when the trumpet is played. The second thing it does is tightly fit the liner to the piston body. The piston is now ready for the liners to be brazed into place.