An excerpt from Wikipedia. The first Mk I production machines were ready fairly quickly, with deliveries starting in December 1937. These early aircraft featured fabric-covered wings, and a wooden, two-bladed, fixed-pitch propeller. Initially the tailwheel was designed to be retractable; early on it was discovered that the Hurricane needed a larger rudder area to improve the control characteristics during a spin. To this end the lower part of the rudder was extended and a distinctive ventral "keel" was added to the rear fuselage. The tailwheel was now fixed.Early Hurricanes lacked armour or self-sealing tanks. They used "ring and bead" gunsights, with the ring being mounted above the instrument panel and the bead mounted on a post above the engine cowling. The standard GM2 reflector gunsight was introduced in mid-1939, although many Hurricanes retained the "bead". Fuel capacity was 97 Imperial gal (441 l) in two fuel tanks, each of 34.5 gal (157 l) in the wing centre-section, between the spars; the fuel was pumped from these into a reserve gravity-feed tank which held an additional 28 gal (127 l) in the forward fuselage, just ahead of the cockpit. This was the main fuel feed to the engine. The 7 gal (32 l) oil tank was built into the forward, port centre section. Early "K" serialled Mk Is were powered by the 1,029 hp (768 kW) Rolls-Royce Merlin C engine; from the "L" serial numbers the later Merlin II of 1,030 hp (768 kW) was installed. The main coolant radiator was housed in a fairing under the rear wing centre-section; the oil cooler was also incorporated into the main radiator.