The Effect of Plating on Threads

The Major Diameter increases by twice the plating thickness.  This is easy to understand, because when measuring an outside, or Major Diameter, the plating thickness on both opposing sides is being measured.  Of course the same rationale applies to the Minor Diameter. So a common electroplated zinc thickness of 0.005mm will increase the Major and Minor Diameters by 0.01mm.
A little less obvious, but more important, is the effect on the Pitch Diameter.  This grows by four times the plating thickness.  For Pitch Diameter, the plating build is measured on four surfaces. i.e. two thread flanks on each side of the thread.  Thus that same 0.005mm plating thickness increases the Pitch Diameter by a significant 0.02mm.  
When the same effect on the female (nut) thread is taken into account, the pitch diameter is effectively grown by 0.04mm for a 0.005mm plate thickness. 
Washers
Unfortunately the importance of a washer is often only recognized in hindsight, after a component failure.  
For may people, the sole criteria in washer selection are: inside diameter, outside diameter, thickness, and surface finish.   In many applications, efforts to hold down costs result in low-cost, reasonably dimensioned, but mismatched washers being fitted in applications where the bolt and its washer are under load.   Fore example, we often are asked to supply mild steel washers, and/or spring washers with SAE Grade 8 bolts.   There may be instances where the bolts are not full tightened, and these are the correct choice.  It is far more likely that tightening the Grade 8 bolt will render the spring washer completely useless, and worse the mild steel flat washer will “crush” leading to bolt relaxation and possible joint failure.
Boltmasters stock hardened washers for Grade 8, and Class 10.9 bolts.  To prevent these high tensile bolts and nuts loosening, Conelock Nuts and a range of adhesives are also available in stock.