BS EN ISO 7091:2000

1   Scope This International Standard specifies the characteristics of normal-series, product-grade-C plain washers in the 100 HV hardness class and of nominal sizes (nominal thread diameters) ranging from 1,6 mm to 64 mm inclusive. Washers of hardness class 100 HV are suitable for

— hexagon bolts and screws of product grade C in property classes up to and including 6.8;

— hexagon nuts of product grade C in property classes up to and including 6;

— case-hardened thread rolling screws.

If dimensions other than those listed in this International Standard are required,
  they should be selected from those given in [ISO 887](/search?q=ISO%C2%A0887). 
When soft material pieces are clamped, or large clearance holes in the workpiece are
  used, the user should check the technical suitability of this type of washer.