vibration and cooling resulted in a redesign of the rear hull. This increased the design weight to nearly 50 tons and accordingly the suspension (which had been designed for 45 tons) was strengthened. Trackpads had to be fitted to protect roads from damage and the ground clearance increased. The design was accepted in the early 1960s. Britain and Israel had collaborated on the development in its latter stages with a view to Israel purchasing and domestically producing the vehicle. Two prototypes were delivered as part of a four-year trial. However, it was eventually decided not to sell the marque to the Israelis (since, at that period of time in the late 1960s, the UK was more friendly towards the Arab states and Jordan than to Israel), which prompted them to follow their own development programme. In 1957, NATO had specified that its forces should use multi-fuel engines. The early Leyland engine delivered around to the sprocket, which meant a top road speed of around and cross-country performance was limited. This was further hampered by the Horstmann coil spring suspension, which made it a challenge to drive cross country and provide the crew with a comfortable ride. Due to the cylinder linings being pressure fitted, coolant leaks within the cylinder block were common, resulting in white smoke billowing from the exhaust. In the late 1970s, engine design changed with the introduction of Belzona which was used to improve the lining seals. Engine output also increased, with later engines delivering some to the sprocket. This meant better performance and an increased speed. However, cross-country performance remained limited. Design The design of the Chieftain included a heavily sloped hull and turret which greatly increased the effective thickness of the frontal armour – on the glacis (from an actual thickness of ) and on the turret (from ). It had a mantletless turret in order to take full advantage of reclining the vehicle up to ten degrees in a hull-down position. For security reasons early prototypes had a canvas screen covering the mantlet and a sheet metal box mounted over the sloping glacis plate to disguise the configuration of the vehicle. The driver lay semi-recumbent in the hull when his hatch was closed down which helped to reduce the profile of the forward glacis plate. The commander, gunner and loader were situated in the turret. To the left side of the turret was a