They are just .125" rings of steel braized together, and the head gasket they use is a stock cummins head gasket with the valve contours milled out on a router table. I made a kit for mine in about an hour and milled my own grooves in another. That was the easy part... then we proceeded to mill off the intake, and build a custom intake / 2nd CP3 mount and port the exhaust and intake runners. Runs much better now and get way better milage. I think we're near the 800 hp mark on 37 inch tires and #2.