SS Exhaust Block-Off Plate. When upgrading your stock through-the-prop MerCruiser exhaust system to a custom through transom set-up, it is necessary to block-off the Y pipe inlet to the stern drive. This is most easily and reliably done by using a precision laser cut TCM Stainless Steel Exhaust Block-Off Plate for a perfect, leak-free fit. Always inspect the o-ring seal during installation. Specify year of transom assembly.