If you haven't already, remove the valve springs and pushrods from the 2 cylinders that failed. There's got to be some witness marks from some components making contact where they shouldn't. Looks at the retainer bottoms to see if they've been contacting the tops of the guides....there will be contact marks. Look at the tops of the guides for witness marks. Inspect the springs for coil bind or some other type of mechanical interference. Inspect the pushrods to see if there are witness marks on them. Look at the slots under the balls on the rocker arms. Look at the studs. Are the rockers making contact on the outer circumference of the retainers? There will be marks on the rockers and retainers.
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