BT-7 Model 1935 Fast Tank
The concern over the vulnerability of riveted tank construction highlighted by the 1934 and 1935 fighting in the Far East led to the decision to redesign the BT-5 with welded construction throughout.
The BT-5 had been continually plagued with power-train problems as well. It was powered by the M-5 engine which was a Soviet copy of the American Liberty engine, manufactured for both tanks and aircraft at Zavod Nr. 24 (Frunze).
In 1935, A. Morozov in Kharkov completed work on a new clutch and braking system respectively.
These were mated to the new M-17T engine, a copy of the German BMW engine used in the T-28 medium tank.
The resulting vehicle with a redesigned hull was called the BT-7 Model 1935.
It closely resembled the BT-5 as it used the same turret, but the hull front had been completely redesigned and was more rounded, and the muffler at the rear had been entirely enclosed.
A less noticeable change was the design of a new track with smaller pitch which was less easily shed on fast turns than the earlier type.
Not only was the new hull completely welded, but it also offered the advantage of greater fuel capacity; additional exterior fuel panniers could be fitted if needed.
The BT was popularly called the Belka or Betushka in Russian service (Betka is the slang for the BT acronym, and Betushka is the diminutive form), and the tank was fairly popular.
The BT-7 Model 1935 remained in production until 1937.
Sources : Milsom, »Russian BT series«, AFV Weapons #37, and Zaloga/Grandsen, »Soviet Fast Tanks«, Military Modelling Jan.1982