A plug for providing a predetermined flow rate of leakage gas from a sealed package for purposes of calibration of gas measurement instruments, the plug having an axial passage therethrough with a threaded interior wall; and a bolt for threading into the plug axial passage, the bolt having an axial passage therethrough, and an insert for insertion into the bolt axial passage, the insert having a predetermined size opening for permitting only a predictable flow of leakage gas through the plug.