To increase header size on popular 15deg & 18deg GM and aftermarket cylinder heads, it is common to use the Reher-Morrison header flange pattern. Dynatech aluminum adapter plates are CNC machined to 5/8" thick, and feature 1 1/8" windows to cut down on the amount of port-matching. Also we clearance the top for valve covers. Outer bolt pattern features threaded 3/8" steel inserts with "Reher-Morrison" header pattern. Includes plates and hardware for both heads.