Pump unions in the 4 main sizes, 2" being the most common.

NONE of the dimensions relate to ANY measurement on the union.

Sizing help, union outside diameters :

1" union = 2-2/8" - 57mm

1-1/2" union = 2-13/16" - 71mm

2" union = 3-3/8" - 81mm

2-1/2" union = 4-1/4" - 105mm