Being an engineer and trained as a machinist, I would imagine anything less than .005". But that is just my "gut feeling". Sabre probably posted the correct answer
But I would also consider checking them in a different manner as the surface you have them sitting on probably isn't flat (unless you are using a surface plate). I would suggest getting a known straight edge, laying it on the head surface, and checking with feeler gauges. Again, don't just assume a ruler is straight either. Make sure you are using a straight edge that is straight within .001".
