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   "incompatible," There's a similar problem with some
   gold coating on the ox regs. Plan: replace these
   assemblies with a new design that doesn't
   have that problem.

 2.AC valves: problem is that those valves work off
   microswitches which indicate when the valve is in the
   commanded position, & shut off the motor before the
   valve is driven into a shock absorber. The
   gear actuators were all nylon, & could get stripped
   if this "hard cycle" occurs (as a result, say, of a
   uswitch failure). Many of those actuators were
   replaced with metal ones pre-STS-1 (e.g. all the
   manifold valves) - particularly those which were
   critical. They had planned to eventually change out
   all of them.
   On STS-1, coming out of an interconnect they
   reopened the RCS TK ISOL valves, and got a
   bp tb -> either the fuel or oxidizer valve
   didn't open? Ground has access to additional
   (redundant) uswitches, & saw that the valve
   had opened ... but either a uswitch had failed,
   or wiring failed, & caused a hard -cycle.
   That valve was cycled twice more during servicing, &
   gears are stripped enough that valve's failed open.