Boeing and the FAA conducted additional tests and research to further investigate why the rivets failed and what the possible repercussions of such a failure would be, including metallurgical examination of high-time bellcranks, material properties testing on old and new bellcranks, review of bellcrank failure rate data obtained from 767 operators, and examination of maintenance procedures to determine whether changes in procedures and/or intervals were warranted. The Safety Board monitored the FAA's and Boeing's tests and research into the bellcrank shear rivet failures.
Source: wiktionary