It wasn't friction stopping my servo push rods--it was torsion. With inch-thick end caps, the rudder linkage has to line up perfectly--and stay lined up perfectly. Or I'll have to drill out a wider tunnel, which I'm not willing to do just yet.

I've got a stack of dead r/c boards, and a cache of good motors that went with them. I'm thinking of sealing up a few to experiment with "wet" servos.

I guess leaks must be the most frequent problem with wet servos. Plus how do you keep rust at bay? And since I'm sealing in hot glue, I wonder how to keep this improvised casing from melting onto the shaft and paralyzing the sub when it's halfway across the pond.