Good report Al.  It is interesting that there are streaks from only the back side.  Is it possible that you have cleaned the front side but due to accessibility, didn't clean the back side?

No; never did any cleaning of the plate.

 

I received my close tolerance bolts and swapped out the AN-6A bolts.  The close tolerance bolts were definitely snugger in the holes, but all could be installed by hand without needing to be tapped or driven in with a hammer.

That’s true; certainly not a press fit – but I don’t think it matters.  With the 20 bolts torqued to about 20 in-lbs – beyond the torque needed to turn the lock nuts – (I think that’s about the right number for these bolts; I’d have to double check) the clamping force between the flange and the plate is so high as to eliminate much in the way of shear forces on the bolts.  If you’re concerned, a thin film of JB weld on the bolts before putting them in would eliminate any clearance – and make it impossible to ever remove them.

 

Al