Way back in the mid-60s, GM, Ford and others used a nickel alloy for the hotter-running exhaust valves, welding them to a mild steel stem for penny-saving. My '65 Corvair had such exhausts that were thus magnetic only on the stems. Back then engines seldom turned even 6000 rpms so most owners didn't notice the weld was a potential weak spot. Except on hard working air cooled VWs or 400 Ford motor homes climbing a long hill in summer towing a trailer....
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