We would suggest that you look atour guide to BSP thread sizes for more information about plumbing thread sizes. This brass drain-cock valve has a spindle gland, so unlike cheaper type "B" drain cocks it will not leak water out past the spindle when used (very important if used indoors).
1/2" drain cock with BSP male end, type A to BS 2879. Brass and DZR drain cocks. DZR drain cocks are preferred (and/or specified) in areas where the water removes zinc from conventional brass. The lockshield prevents easy operation of the drain cock without a key.