The KV-1 S was designed by N. F. Shashmurin. Armor was reduced to make tank faster as a common complaint from the front was that the KV-1 wasn't manoueverable.
The armor was made thinner and the turret made smaller. A vision cupola was added to turret.
Transmission, cooling, and lubrication systems were improved.
The KV-1-S had a new gearbox and main clutch installed.