When I was making up my collector flange for the exhaust system, we designed a template that acted as both the design for the gasket and the flange itself. I searched high and low, but was unable to find anything available off the shelf other than direct from GM, this is a unique flange/bolt arrangement that appears to be only used on the GEN V engine. GM wants $57.00 for a gasket and almost $600 for each downpipe that connects to the header, so here’s a simple solution.
This is the header system that the template will fit, click on the image for a higher resolution view.
Collector template -download this PDF file for fabrication.
This template is scanned full size on a regular sheet of paper so when you print it out, please print actual size so the image doesn’t get distorted. The actual diagonal distance between bolt holes is about 3 7/8″. The pipe diameter is 2 1/2″, but we elected to design the flange so the exhaust tubing entered into the hole rather than be welded on the outside, for thickness we used 5/16″ thick steel. It’s your choice, but adjust the diameter of the exhaust hole based upon the size of the exhaust tubing your’e using.
For the gasket we used aluminum and heat tempered it so it was pliable, worked great and saved a ton of money.