Having suffered numerous blown gaskets on no.2/3 manifold, I made up a pure copper version, but this too has failed after just two hundred miles. Although I had the manifold face ground true, I notice that after a while, when hot, there is a gap of approx 1.5mm at the top when all is torqued up. It would seem the sheer weight of the 4:1 system is pulling the top edge of the manifold away from the head. Thinking laterally I am wondering if I get the manifold face ground at an angle, i.e making the plate say 2mm thinner on the underside than it is on the top, this will cure the problem. Has anyone been done this route?