was running at low speed, one radiator section provided enough coolant; a thermostatic switch turned off the starboard radiator section until more power was called for and extra engine cooling was required. Other structural changes included flush riveting for the fuselage, introduced in mid-1943. A streamlined round rear-view mirror, with a bullet-shaped fairing replaced the rectangular version. On new production Mk IXs the small, teardrop shaped identification light behind the radio mast was removed. While early Mk IXs converted from Mk Vcs had the original (smaller) elevator horn mass-balances, most had the enlarged version with the straightened leading edge. A new rudder of greater area, which could be identified by a "pointed" tip, was fitted to many Mk VIIs, Mk VIIIs, and mid to late production Mk IXs and Mk XVIs. All Mk VII and Mk VIII Spitfires had the following changes: The internal structure was strengthened and revised. On the wings the ailerons were reduced in span by outboard of the outer hinges. There had been some instances of earlier models breaking up in the air in steep high speed dives, it was thought, because of aileron flutter. The main fuselage fuel tanks were increased in capacity; 47 gal (213.7 L) for the upper tank and 49 gal (222.7 L) for the lower. In combination with the wing tanks this gave a total internal capacity of 122 gal (554.6 L), a near 50% increase over the 85 gal (386.4 L) carried by earlier Spitfire marks. In addition a 13 gal (64 L) fuel tank was fitted in each wing leading edge between the wingroot and the inner gun-bay. The main undercarriage legs, for the first time in the Spitfire's life, were fitted with forward-facing torque links. In addition, the leg doors were slightly concave, allowing the undercarriage to sit lower in the wheel wells when retracted: this meant the upper wing skinning was free of the small bulge which had hitherto been necessary to clear the wheels. The wheels themselves were a new strengthened Dunlop AH10019 "four spoke" pattern, replacing the "five spoke" pattern used since the first Mk Is. This revised undercarriage was also fitted on some mid to late Mk IXs and all Mk XVIs. A retractable tailwheel (Dunlop 2184) was fitted, covered by small doors when in flight. This also applied to the PR Mk Xs and most PR Mk XIs. Mk VII (type 351) Like