Always more that can be said on a topic, but this section of the FAQ should be of use to you!

The reliability of Soviet pre-war tanks was poor, but increased as the designs were improved.

Reliability of BT tanks started out as very unsatisfactory. Half of all BT tanks participating in an exercise in 1932 broke within 250-300 km. I don't have data on the T-26, but considering the army's desire to increase lifespan to 1000 km, I don't think that it was satisfactory either.

By 1935, it would appear that the situation improved. A letter complains that, in a run of 3000 km, only two road wheels went out of action due to a cause not related to their tires. A road march of that length seems to suggest that the goal of 1000 km was at least reached, if not surpassed.

In 1936, the expected lifespan of BT-7 tracks was 2000 km. However, the army complained that the track links only last 700-800 km, and begin to sag after 100 km of travel. The ABTU, however, was satisfied with a tank that could drive for 2000 km, 1300 of which were driven in tracked mode. Individual BT-7 tanks drove for as much as 2500 km during trials.

The tires were another weak link in the BT-7's reliability. In typical use, this was not an issue, but when the tank had to make long marches at high speed in a hot climate, the tires would begin to degrade quickly. Trials in 1939 showed that, depending on the design of the tire, it could start showing damage after only a few dozen kilometers. A new tire was designed that performed satisfactorily after 500 km of a forced high speed march. The Americans actually faced similar issues in Sicily, where the heat and harsh terrain resulted in synthetic rubber parts wearing out almost three times as fast as they were expected to.

During the invasion of Poland in 1939, the 25th Tank Corps' tanks traveled 700-800 km without sufficient time to perform maintenance, and only 30 BT-7 tanks broke down (some of which required only minor repairs).

In 1940, BT and T-26 tanks at Repair Base #7 show some pretty impressive lifespans: hundreds of engine hours. Soviet metrics evaluated the reliability of engines in hours, not in kilometers, and it's pretty hard to match one to the other, but that's a fairly impressive figure for the time.

However, by 1941, all of these old tanks were worn out, with spare parts being hard to come by. For example, in June of 1941, 25% of all tanks in the second repairs category (used but suitable for battle) were actually unsuitable for battle because they had no track links. Deficiencies in spare parts and the chaotic forced marches of the first few months of the Great Patriotic War meant that a great number of these tanks were lost for technical reasons and could not be evacuated or repaired.

The replacements for these tanks were the T-34 and KV tanks, but they had their own problems. The expected lifespan of the V-2 tank engine was only 100 hours. The lifespan could not be increased because of a very simple issue: there were no worn engines. Factory #75, the factory that produced the engines, begged the Red Army to fully use up several engines so that they could be analyzed and the parts that wore down the fastest improved, but nobody wanted to break their shiny new toys. Drivers would get very little time training in these new tanks, and they had to settle for training in T-26es or, even worse, T-27s. These drivers did not know how to properly take care of their tanks, nor were they put in a situation where they could take their time and learn. The result was predictable: many tanks lost for technical reasons. The KV was expected to last for 2000 km, but again, this was only possible with an experienced crew, which were not exactly common in 1941.

The lifespan of the T-34 continued to improve as the war went on. An improved gearbox that would last for 2000 km was designed in 1942, and tires that could last for 2000 km designed in 1943. T-34s in 1943 were also putting out a lot more engine hours before the engines had to be replaced: the warranty period was increased to 200 engine hours.

In 1945, the warranty period for the V-2 used in the T-34 was increased to 250 engine hours, and the one used in the IS-2 to 200 hours. However, when properly maintained, getting 300 hours out of a T-34 or an IS-2 was not unheard of. Particularly skilled drivers like Iosif Antonovich Perederiy, whose T-34-85 traveled for 2700 km and 370 engine hours, nearly doubled the expected figure.

P. Samsonov BT-7s in Poland

P. Samsonov HPZ BT-7 and T-35 woes

P. Samsonov BT-7M Tires

P. Samsonov Engine hours

P. Samsonov Soviet Tank Production, 1932

P. Samsonov Replaceable Tires

Maintenance of Armor in World War II

D. Shein Poryadok v tankovykh voyskakh

P. Samsonov Engine crisis

P. Samsonov Gently used

M. Baryatinskiy Tyazheliy tank KV v boyu

A. Ulanov, D. Shein Perviye T-34

P. Samsonov KV-1 Modernization

P. Samsonov T-34 Tires

P. Samsonov Improved gearbox

P. Samsonov Extended warranty

P. Samsonov Ogledow combat diary

P. Samsonov Heroic driver