Incorrect rocker geometry/pushrod length that has the stud biased too far to in the rocker slot,thus as the rocker approaches full lift it runs out of slot in the rocker arm and consequently binds the rocker arm on the stud.
That or as mentioned valve spring coil bind,or some other mechanical interference (guide to retainer) issue.
A pic would help us understand the failure better.
Did it break on the underside of the rocker,or elsewhere?
Did it also bend the pushrods on those locations???
What springs are being used here?
What sorta seat/open pressures??
Fill us in and you'll get better feedback.
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