All posts above good.
I would also suggest remedies.
1.
It is relatively easy to hand-adjust reamers a few microns smaller via judicios use of fixtures and abrasives.
Start with loose and work your way down.
Each reamer is a "special" after You touch it.
Test via pins, calibrated.
Buy a set of 5-10-20 gage pins by 1 micron-2 microns. 100-200$.
Lowering a 5.990 mm reamer by 3 microns will NOT make it banana shaped etc. as You are making a very tiny adjustment.
Any abrasives would likely work.
Or 6.000 mm to 5.995 mm, say.
Since you are NOT trying or expecting to get 1, or 2, microns actual size but a "bit less", with some small variance.
E. 2 V blocks with rotational driver, some honing stones and oil, ground ref. blocks for holding, 2-10 min handwork will do it.
It (the reamer) wont be "perfectly" round, or straight, or even etc. but it WILL make exactly as-good holes a graduated bit smaller.
You won´t know how much smaller at first .. until You have experience.
But sub-micron handwork tolerances with abrasives are std in mirrors and lenses by hand for 100 years.
I have *no idea* of how much interference you need.
For 6.000 mm, steel/steel, normal, I would aim for 6.010 - 6.020 mm depending on load and use.
The *real* problem is customer design, drawing, specs.
They should have been able to spec it properly.
NOW, You are in a position to make money by properly communicating options, alternatives, costs.
This is difficult - but it is where YOU can make the most money.
