I was a little concerned when you said in a previous post that you replaced a bent valve but didn't seem to know what caused it?..I'm wondering if your valvesprings are not up to the task or there is something screwed up in your valvetrain..Did you check piston to valve clearance?

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1968 mustang..408 windsor.Vic jr heads and intake.Pro Systems 950HP holley, 1 3/4 hooker super comps,Custom CI solid roller camshaft,Probe shaft rockers, 4 speed with 4:11 detroit locker.