The End Caps ID is 1.5". This slides in and bolts in and takes the ID to 1.2". This takes the sound from 92.5 db to 91.1 at 4800 rpms. You may get a refund on shipping costs when this is bought with the End Cap or other items. depending on how it all fits in the box

1.2" ID Quiet Insert for 2020 -21 end cap