breaking-cap

from The Century Dictionary.

• noun A cap placed over an opening in the cylinder of a steam-engine, so that if water collects in the cylinder the cap will be broken and thus open an outlet for it and prevent injury to the engine.
• noun A cap or cup-shaped casting, used in machines for rolling iron or steel, such that if a piece is fed to the rolls too cool, or too hard, the casting will yield first and save the more costly roll.