FV 4201 Chieftain The FV 4201 Chieftain was the main battle tank of the United Kingdom during the 1960s and 1970s . It was one of the most advanced tanks of its era , and at the time of its introduction in 1966 had the most powerful main gun and heaviest armour of any tank in the world . The Chieftain also introduced a supine ( lying backwards ) driver position , enabling a heavily sloped hull with reduced height . Overview . The Chieftain was a radical evolutionary development of the successful Centurion line of tanks that had emerged at the end of the Second World War . The British had learned during the war that their tanks often lacked sufficient protection and firepower compared to those fielded by the enemy , and that this had led to high casualty levels when faced with the superior German tanks in World War II . Leyland , who had been involved in Centurion , had built their own prototypes of a new tank design in 1956 , and these led to a War Office specification for a new tank . The design was accepted in the early 1960s . Chieftain was designed to be as well protected as possible and to be equipped with a powerful 120 mm rifled gun . The heavy armour came at the price of reduced mobility , chiefly due to engine power limitations , which was perhaps the Chieftain ' s main drawback . The engine selected took the multi - fuel route and as introduced gave less than the planned output ; improvements to the engine did not increase power to the desired value . Design . The Chieftain design 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 mantleless turret , in order to take full advantage of reclining the vehicle up to ten degrees in a hull - down position . The driver lay semi - recumbent in the hull when his hatch was closed down which helped to reduce overall height . The commander , gunner and loader were situated in the turret . To the left side of the turret was a large infra - red searchlight in an armoured housing . The suspension was of the Horstmann bogie type , with large side plates to protect the tracks and provide stand - off protection from hollow charge attack . The Leyland L60 engine is a two - stroke opposed piston design intended for multi - fuel use so that it could run on petrol or diesel or anything in between . In practice the engine did not deliver the expected power , and was unreliable , estimated to have a 90 % breakdown rate , but improvements were introduced to address this . Primary problems included , cylinder liner failure , fan drive problems and perpetual leaks due to vibration and badly routed pipework . However , as the engine power improved the tank itself became heavier . The tank was steered by conventional tillers hydraulically actuating onto external brake discs . The discs worked via the epicyclic gearbox providing " regenerative " steering . In reality the discs and pads became soaked in oil and diesel and the steering became difficult . The gearbox was operated motorcycle - style with a kick up / kick down " peg " on the left which actuated electro - hydraulic units in the gearbox ; the accelerator was cable operated by the right foot . In the turret the loader was on the left and the gunner on the right of the gun with the commander behind the gunner . The main armament was the 120 mm L11A5 rifled gun . This differed from most contemporary main tank armament as it used projectiles and charges which were loaded separately , as opposed to a single fixed round . The charges were encased in combustible bags . ( Other tank guns had to store the spent shell cartridges or eject them outside . ) The combustible charges were stored in 36 recesses surrounded by water jackets . In the event of a hit which penetrated the fighting compartment , the water jacket would rupture , soaking the charges and preventing a catastrophic ammunition explosion . The gun itself could fire a wide range of ammunition , but the most commonly loaded types were high explosive squash head ( HESH ) or armour - piercing discarding sabot ( APDS ) , or practice round equivalents for both types . When the Chieftain was first introduced , a 12 . 7mm ranging machine gun was mounted above the main gun . This fired ranging shots out to a maximum of , at which point the tracer in the ranging rounds burned out . Later , in the late 1970s and early 1980s , a Barr and Stroud laser rangefinder replaced the ranging machine gun . This allowed engagements at much longer ranges , and also could be linked to the fire control system , allowing more rapid engagements and changes of target . The gun was fully stabilised with a fully computerized integrated control system . The secondary armament consisted of a coaxial L8A1 7 . 62 mm machine gun , and another 7 . 62 mm machine gun mounted on the commander ' s cupola . Service . Like its European competitors , the Chieftain found a large export market in the Middle East , but unlike the earlier Centurion , it was not adopted by any other NATO or Commonwealth countries . The Chieftain proved itself capable in combat and able to be upgraded with enhancements both for overall improvement and to meet local requirements . The Chieftain tanks were continuously upgraded until the early 1990s when they were replaced by the Challenger series of tanks whose design was influenced by that of Chieftain . The final Chieftain version used by the British Army until 1995 , incorporated " Stillbrew " armour named after Colonel Still and John Brewer from the Military Vehicles and Engineering Establishment ( MVEE ) , the Improved Fire Control System ( IFCS ) and the Thermal Observation Gunnery Sight ( TOGS ) . The first Chieftain model was introduced in 1967 . Chieftains were supplied to at least six countries , including Iran , Kuwait , Oman and Jordan . An agreement for sale of Chieftains to Israel was cancelled by the British Government in 1969 . The largest foreign sale was to Iran , which took delivery of 707 Mk - 3P and Mk - 5P , 125 – 189 FV - 4030 - 1 , 41 ARV and 14 AVLB before the 1979 revolution . Further planned deliveries of the more capable 4030 series were cancelled at that point . The tank ' s main combat experience was in the Iran – Iraq War of 1980 - 88 . Mark 1 and Mark 2 models had coaxial . 50 cal . ranging machine guns prior to the introduction of the laser rangefinder .