Nylon washers are a must for a dependable antenna stud mount installation. 

ABS has a tendency to crack, or worse, to compresses under pressure.  If allowed to compress, the tension that holds your coupling nut together diminishes and the coupling nut can come loose ...  sometimes causing the loss of your antenna. 

Each package contains eight (8) pieces.  All have a 3/4" (19mm) diameter and are made specifically for 1/2" holes and 3/8" bolts (industry standard sizes). 

The kit contains three washers with a 0.031" (1mm) shoulder, three with a 0.125" (3mm) shoulder and two with a 0.195" (5mm)shoulder.