Trials with the new engine V-2K with 650hp already began in March 1942 in accordance with the design of a completely reworked transmission.
In July/August 1942 two KV-1S preseries vehicles were tested.
These had a combat weight of 42.3 tonnes and a top speed on the road of astonishing 43.3 km/h (the KV-1 barely reached the factory requirement of 35 km/h. 15-20 km/h was more realistic).
The KV-1s was still armed with the 76.2mm ZIS-5 gun albeit of improved design with parameters now equalling the F-34.
In July altogether 52 KV-1S had been manufactured.
In late August 1942, the first series-production KV-1S reached the Red Army.
In September 1942, the mass-production for the KV-IS was launched and followed by instructions to again increase the ammunition complement from 90 to 114, which increased weight to 43.4 tonnes.
A total of 180 KV-IS was built in that month, the highest-ever monthly output of this model.