Stock replacement Tremec Clutch Adapter Housing (also popularly known as Bellhousing) for 98-02 LS1 Camaro & Firebird, and 04-06 LS1 & LS2 GTO. This is the same bellhousing that GM (12453263 or 92142856) charges an insane amount for! #162A on the T56 Transmission Disassembled View, seen here, here and here.
NOTE: Although hundreds of GTO owners have installed this stock replacement bellhousing without issue, some 04-06 GTO owners have reported having a starter that is incompatible with this bellhousing. We haven't been able to pinpoint a rhyme or reason to the difference (it doesn't seem to be engine or model-year related) yet, but if your starter won't install with this bellhousing, you'll need to source a 98-02 Camaro/Firebird V8 starter, which your local auto parts store will likely have in-stock.
