Fuze No. 42 Mark IV

This fuze is fitted with a time delay capsule and was primarily used on illuminating flares and cluster munitions.

A slightly modified version was used on the Upkeep Mine, better known as the bouncing bombs used by No. 617 Squadron during their famously daring dam-busting raid. If the main hydrostatic fuze failed to detonate, the time delayed No. 42 fuze would set off the main charge.