I send you these materials on trials of fighting and armoured vehicles BT-7A, T-38, FAI and trucks GAZ-AA, ZiS-5, ZiS-6 regarding their performance during border and internal maneuvers executed by the NKVD Internal Forces in 1937-1938.
Experience in using these vehicles in conditions of middle Asia revealed several design defects that must be corrected when building these vehicles.
I ask you to inform me of your decision.
Division commander Kovalev
Of trials of BT-7A, T-38, FAI armoured car, GAZ-AA, ZiS-5, and ZiS-6 trucks.
Trials were performed in the following conditions:
- Total distance: 2500 km (without assistance, combined on tracks and wheels)
- Terrain: various
- Mountains with steep grades, mountain passes at an elevation of 2500 m to 4000 m, mostly off-road, up to 1000 km long.
- Rocky bed of a mountain river, 200 km.
- Off-road on a salt marsh, 250 km.
- Dirt roads with up to 50 cm of loess dust, 100 km.
- Loose sand, from flat sand roads to loose dunes with grades up to 40 degrees, up to 1000 km long.
- Climate conditions:
- Mountains with a normal temperature of +15, salt marshes with a temperature of +50, mostly through regions without water.
- Sand-salt marsh terrain with air temperature ranging from +10-15 degrees to -15 degrees.
- Mountain passes up to 4000 m high in winter conditions with snow banks with temperature ranging from -20 to -40.
The results of the trials showed the following behaviour of vehicles on various terrain:
1. BT-7A tank
Light gravel highway, average speed of 30 kph: Tanks worked normally. Since the tracks weren't broken in, locking pins were cut off and track pins fell out.
Mountain terrain with passes from 2500 m to 4000 m high: The effectiveness of the fuel mixture decreased, had to be enriched. The result was excess use of fuel, black smoke from the muffler, engine overheating. The engine power decreased by 30-35%. Due to a large power reserve, a 30% decrease in performance did not have a noticeable effect on the vehicle's performance.
The performance of the BT-7A in these conditions showed that they can, without significant changes, especially without an altitude corrector for the carburettor, cross mountain passes over 4000 meters high.
On steep slopes, fuel could drain out of the carburettor, causing the engine to desynchronize. The water temperature reached 100-110 degrees and oil temperature 80 degrees in August.
Suspension: the track pin and horizontal spring plug have significant drift after 500 kilometers of use.
Marching on a mountain approach, flat road on salt marsh, and loess dust: On rocky terrain, the existing design of drive wheel and track links results in frequent jamming due to saturation of the tracks with mud and rocks.
It is possible to cross salt marshes at high speeds, but only without turning and over new places each time.
The BT-7A tank is completely unusable in loess dust conditions and is not protected against it. The lack of protection from dust of transmission components is noticeable. Initially, the complete saturation of tracks, bearings, and disks did not allow for changing gears. It is not possible to clean the main friction clutch in this design without removing the gearbox. In the last stages of moving across loess dust, the main friction clutch disks showed grit up to 2 mm in size, which disabled the clutch. Further use of tanks involved only using the final drives without disengaging the main clutch.
Due to dust getting into the magneto, the tank could not be started. Other electric components were disabled due to dust, such as the starter, the generator, etc.
Engine: the loess dust gets in with air through the air filter, side pockets and cracks, gets into the engine, and forms and abrasive mass when mixing with the oil, resulting in fast wear of cylinders, rings, pins, floating pins, and bearings. As a result, the engine starts working unreliably. The oil system stopped working due to wear of the oil pump gears.
Note: two engines broke down over these trials due to tearing of the piston rods of the 1st and 2nd cylinder. The VVS M-17F manual describes this as a manufacturing defect.
Suspension: the final drive bearings wore down quickly.
The BT-7A tank is adequate for use in the mountains. The tank needs improvements for use in sandy or dusty conditions to protect the transmission, engine, and electric equipment from dust.
- Requires dust protection with a more powerful oil filter, better sealing of the carburettor and coil pipe housing.
- The fuel nozzles should be moved in order to avoid a chance of fire if gasoline leaks on the magneto contacts.
- The valve rocker leaks oil during use and cannot be repaired.
- The gear-based oil pump and its connector wear quickly.
- The nozzles cannot be regulated without removing the top armour plate.
- It is difficult to access the engine during use.
- The main friction clutch must be protected by a special case (from the engine to the gearbox)
- There must be an intermediate connector between the main clutch and gearbox that allows for work on the clutch without removing the gearbox.
- Strengthen the bearings of the main friction clutch and improve access to them and the ratchet disk.
- Improve access to the oil nozzle of the final drive bearings.
- Cover the control pull rods with a special casing.
- Improve the gearbox link (springs break, locks pop out, other breakdowns).
- Install strengthened ball bearings.
- Redesign the drive wheel in order to strengthen the disk and rim.
- Change the tracks to use finer links.
- The lights turned off due to poor quality of lightbulb sockets.
- Up to 80% of ammeter and voltmeter breakdowns were due to poor quality of materials or assembly.
- Starters broke down due to poor quality of relay contacts.
- Bronze bearings of the launch magneto wore quickly.
- The generator plug connectors are unreliable and hard to reach.
- The electric equipment layout is complicated and hard to reach during repairs.
- The ignition button contact (made from a latten brass spring) is low quality and breaks quickly.
- Electric wiring close to the turret creates a fire hazard.
It is necessary to:
- Protect the magneto, generator, and starter from dust and moisture. The started magneto should be installed on ball bearings.
- Remove the plug connectors from the generator and lead the wires up to the control panel near the generator.
- Allow access to the starter relay without removing the starter.
- Note: a Bendix gear starter is desirable. In addition, the tank should have another way of starting the engine aside from the starter.
- Wiring inside the tank should be done in accordance with fire safety. Wires in the turret should be covered in pipes until they reach their contact.
- Small connectors should not have friction contacts, and should be connected with either bolts or solder.
- The battery should be moved to the rear of the tank, as during operation water, oil, and dust gets on it, and leaking electrolyte corrodes the radiator.
It is also difficult to remove the batteries for charging, and in the winter impossible to install the engine heater without it. It is also impossible to charge the batteries without removing them.
In the same conditions, other vehicles were tested:
- T-38 tank: demonstrated excellent performance in all conditions, aside from:
- Fast wear of final drive gears.
- Insufficient engine power for the vehicle's weight, causing overheating, and low speed on land and water.
- It is necessary to add air filters to the engine.
- FAI armoured car: showed good results. It is necessary to add an air filter and improved radiator.
- GAZ-AA: showed good performance and endurance. Needs an oil air filter and an improved radiator.
- ZiS-5: performs poorly on sand. The reductor is very weak, mostly the gears and bearings of the main gear break. Needs a stronger radiator and a rigidly installed differential like on the GAZ truck.
- ZiS-6 (three-axle): needs a more powerful engine. The existing engine can only provide a speed of 12 kph on the described terrain. The demultiplexor is very weak, the gears of the first gear break often. Needs a stronger radiator.
Military Engineer 3rd Grade, Babushkin
Military Technician 2nd Grade, Kurochkin"