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1/16 is "standard" and all I've had occassion to use so far but some people have mentioned that you run into scales where the hole is worn larger or they drilled them larger to clean them up. ...and...

Hobby shop. couple bucks for 3/16, 2 rods 3 feet each. same for 3 feet of 3/32. I'm probably set for life.
They didn't have the washers though (actually they had a bunch in the "free" parts box but...