When thinking about the problem, I envisioned a balance beam scale as being one that has a beam that can weigh one item at a time and the only way to compare two groups would be to make two measurements.  In searching the web it appears that the scale that compares weights is a balance and a balance beam is one that has a weight or weights that slide back and forth.  So using a balance beam scale I think it would take 5 weightings to find the rogue piston.

 

Regards,

 

Peter

 

Peter Van Arsdale

St Louis

(314) 677-4814