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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.