Any opinions on what may have caused this?

pittsburghracer said

Man I hate roller lifter that have the tie-bar on the outer side of the roller lifter. Is there any chance they hit the side of your block, I see the pushrods are close to the V on the lifter bar with W2 offsets. Are they clearing. Any chance of tight valve guides??? What’s the open spring pressure?

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Yes, they appear to have plenty of clearance, I see no evidence to indicate otherwise. The heads were inspected back in the spring and everything looked good. Unfortunately I don't know what the open spring pressure is off the top of my head. I think I have the specs, I'll have to find them. I really think this was just a bad push rod that just eventually failed. I haven't found any other damage.