Best way to flush engine after rocker arm failure?

I’m not clear on what you used for hardware to put the shafts on the new heads.
Do the heads or rockers come with rocker shaft bolts?

I do not know if they would even stretch with that torque in aluminum heads.

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Rest assured...... if the bolt/stud isn’t stretched the correct amount....... it’s coming loose.

Since most people running aluminum heads aren’t experiencing the problem you did...... it would appear their bolts have been stretched.