The new U-5TS smoothbore tank gun was fitted into the Ob'yekt 140 turret at the end of 1960. The new tank received the designation "Ob'yekt 166". In 1960, both Ob'yekt 165 and Ob'yekt 166 prototypes passed their trials. The Uralvagonzavod was preparing to start serial production of the new tank, though the General Armoured Directorate (GBTU) was paying much more attention to Morozov's Ob'yekt 430, which was in development since early 1952. Morozov was supported by general Ustinov, who was in charge of Soviet military industry at the time. He did not see it as necessary to produce the new tank from Uralvagonzavod but soon the situation changed dramatically with the appearance of a new American main battle tank, the M60. In 1961, Soviet military intelligence discovered that Britain was working on a new main battle tank armed with a 120 mm tank gun. Because of this, Marshal Vasily Chuikov, Commander-in-Chief of the Soviet Army's Ground Forces, demanded an explanation of the "Kartsev's tanks" case. At a conference of GBTU and the Soviet ground forces committee, it became apparent that Morozov's Ob'yekt 430 tank was only 10% better than the serial T-55. Because of this, Morozov's project was deemed a complete failure. Though the representatives of Kharkiv Morozov Machine Building Design Bureau disclosed their work on the improved Ob'yekt 432 (which would ultimately become the T-64), Chuikov demanded that production of the Ob'yekt 166 medium tank be started immediately.