[Steam-engine] Corsshed Pump

[Andy glines]

While I was cleaning up the crosshead pump on the
Huber I took the acorn nut off of the top of it.  I
noticed that there are holes drilled in the nut.  I
also remember hearing a conversation about the nut
acting as a emergency pressure relief.  They fellow
mentioned that the flat disk under the nut needed to
be made from a certian metal (lead I tink) that would
fail, thus relieving the pressure, if the pressure got
to high.  He also mentioned that it was common to see
the lead disk replaced with one made from another
metal like steel.  My nut has a copper disk under it. 
What is supposed to be there?  

Andy Glines
Evansville, IN

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