A development of the earlier Napier Rapier, the Napier Dagger was a 24-cylinder H-pattern (or H-Block) air-cooled engine designed by Frank Halford and built by Napier and first run in 1934.
The H-Block has a compact layout, as it essentially consists of two vertically opposed inline engines lying side-by-side and driving side-by-side crankshafts. Since the cylinders are opposed, the motion in one is balanced by the opposite motion in the one on the opposite side, leading to very smooth running.
The Dagger had fast rotation, running at up to 4,000 rpm, but it had conventional poppet valves.
There were problems with cooling, maintenance, manufacturing, and weight, which were not solved during the Dagger's lifetime and went unresolved well into its successors lifetime, (Napier Sabre). The Dagger powered the Hawker Hector army co-operation aircraft and the Handley Page Hereford (a variant of the Hampden) bomber. The operational usefulness of the Hector was restricted by engine cooling problems, which made it unsuitable for operations in the tropics, and the Hereford was found to be unsuitable for combat because its Dagger VIII engines were noisy and unreliable. The Dagger also found an application in the experimental Martin-Baker MB 2 fighter.