Cessna parent company Textron Aviation has issued a revision to Service Bulletin SEL-71-13, originally released in August 2020. The bulletin was created following the discovery that NAS1149F0763P washers may not have been installed on the bolts connecting the engine to the engine mount. It applies to all Cessna 172S models.
The bulletin requires inspection at the next 100-hour or annual and that the washers be properly installed following instructions in the bulletin.