Useful for capping off unused stabbings on tanks. These require a suitable seal and clamp, available on shop.
 
Measurements for the blanks OD are:
1/2" - 3/4" 25mm
1" - 1.5"    50.5mm
2"              64mm
2.5"          77.5mm
3"             91mm
4"             119mm
6"             166.87mm
8"             217.7mm