Broken rocker stud. wrong length pushrods?

[jbuchert]

My guess was not enough thread engagement. A lot of stress was applied to only a small portion or tip of the stud. That combined with the rocking motion and high spring rate it was only a matter of time.

[Gaz64]

I agree with the other boys comments, you definately have a binding issue, be it pushrod to head, pushrod to guideplate, coil bind, retainer to guide etc.

If one has done this, the fault will be with the other 15 as well.

If that pattern on your valve stems is a wear mark, I'd be concerned and fitting a set of hardened lash caps, roller rocker wheels are harder than valve stems.
You'll be resetting your lifter preload more often until you discover the valve stems peened over.

When you built the engine, did you use a solid lifter for rocker geometry check?
Did you rotate the engine while checking the clearances within the rocker itself, to stud, to pushrod, to retainer, retainer to guide/seal, pushrod to head, pushrod to guideplate?
Everyone of these clearances is critical, if one of these is too small or zero, the inevitable will occur.

You must run hardened pushrods with hardened guideplates.