Peerless-AV Peerless Internal Joist MountOverview Specifications The ACC120 Internal Joist Mount attaches between two ceiling joists above a finished ceiling. Its expandable design allows for attachment to joists on 16" or 20" centers. The mount features a 1 1 2 11. 5 NPS threaded fitting which accepts most Peerless Extension Columns. Because it is adjustable, the installer can fine tune the point of attachment prior to installation. This flexibility will result in the optimum viewing