The true dimensional effect of the thickness of a coating layer (plating or painting) on a fastener is much larger than most people think.
Take a M6-6g bolt (1mm pitch) and a M6-6H nut (1mm pitch) fitting situation for example.
If a bolt and nut fit nicely before being coated/plated, will they still fit well if a 5-micron zinc layer is put on both of them?
Seetable below to check ISO 965-1 on tolerance and deviation of 6g/6H first.
|Grade 6 Tolerance
|Minor diameter of internal thread
|Major diameter of external thread
According to the table, when the bolt and nut are made at maximum material condition, the gap between the threads will be 26 microns.
It sounds much larger than a 5-micron-thick zinc layer; however, the layers might cause a poor fit after plating.
There are two factors to make a supposition.
Firstly, consider a threadless/unthreaded cylindrical rod and a threadless/unthreaded round hole mating situation.
When a rod is "wrapped" by a 5 micron layer, the diameter actually increases by 10 microns. The same principle applies to hole features and plating shrinks the hole size by 10 microns.
In other words, a 5-micron layer on them will take out 20 microns of gap between the rod and hole.
Secondly, there is an additional factor for plating thread features.
A 5 micron plating layer on 60 degree thread actually leads to a 10 micron effect in radial direction (Or sometimes we say pitch diameter).
This is because of trigonometric effect (see below image for reference) on the thread feature: 5/cos60 = 10.
With superposition of the two factors above, 5-micron layers will take out 40 micron clearance between inner thread and outer thread.
Going back to the initial question of thread fit, a brief answer that Sun Through can provide here is:
it depends on the tolerance left by the maker when fitting, but they will interfere if the fasteners are made in maximum material condition.