panhead rocker block

Has anyone ever had the the rocker block studs shear on a panhead. At first I thought it was a stuck exhaust valve, but upon disassembly discovered all four rocker block studs were broken off. I cannot find any obvious reason for the failure. Has anyone ever witnessed this same condition and/or have any ideas why it happened? If there is a reason for the failure that is "fixable", I would like to complete repairs while it is apart and not have to disassemble the head for may miles to come.

Shearing the studs themselves is not only unusual, it is HAS to be an indicator or some sort of abuse over their long life time.
After all, the aluminum threads in the head itself would be considered weaker than a hardened stud.

And the only abuse I can think of is over-torquing. Just replace them.

Thanks for the input boys. Carl, I think we have met. If you are the guy from SoDak, we met at the AMCA run in Steamboat Springs 2000. I was riding the red and white flathead and spoke to you of mutual aquaintances in NoDak, where I'm from, originally. Anyway, when reassembling last night (with new studs), we discovered that when torqued down there was a slight binding of the shaft. Closer inspection revealed a shiney spot on the top half of the rocker block "bearing" surface, indicating possible binding during operation(?). A light pass on the honing machine resulted in no shaft binding upon assembly. My thought is that a slight bind in the rocker shaft was accentuated during operation because of the difference in the heat expansion coefficient between the brass rocker block and the steel shaft. The shaft binding (although not completely frozen) created excessive pressure on the valve train and the weakest stud failed first at its base and the other three followed by shearing at the nuts.