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DuraPro then modify the fire rings with a special jig I made specifically for the job. It supports the bore of the fire rings but allows some stretching of the valve relief whilst still holding the fire rings flat. The fire ring Dia and cut out for the valves has to be changed to suit the big valves with high lift cams. Its a bit fiddly to get right but once its done it works well. This arrangement has been good for over 12 months now. I tried Nismo head gaskets but the relief for the valve is not big enough and it had to be stretched too much and I was concerned the base material for the gasket was bruised and wouldnt seal.
Re tensioning is required once it has settled down after heating and I usually pull the gasket down an extra 5 Ft pounds.