Cold War Yugoslav Prototypes

Vihor M-91

Socialist Federal Republic of Yugoslavia (1985-2000)
Main Battle Tank – At Least 3 Incomplete Prototypes Built

Throughout its existence, the Jugoslovenska Narodna Armija (JNA, English: Yugoslav People’s Army) strove to develop a domestic tank design in order to break its dependence on foreign suppliers. The initial projects involved either reusing already available components or simply improving an available design. None of these ever reached beyond the prototype stage. The first successful locally-produced tank, although a licensed copy, was the M-84, which entered service in the second half of the 1980s. Despite being a competent design, the Yugoslav Military High Command wanted an even better-performing tank, which would lead to the Vihor project.

The Vihor with the mock-up turret. Source: www.srpskioklop.paluba

The First Attempts to Build a Domestic Tank

Following the end of the Second World War, the JNA entered a short period of close cooperation with the Soviet Union. This cooperation is reflected in the procurement of large quantities of military equipment, including tanks, such as the T-34-85. While the JNA was still in its early development phase, political tensions between Yugoslavia and the Soviet Union, more precisely between Tito and Stalin, began to arise. Stalin wanted to impose a more direct Soviet control of Yugoslavia, as in the other satellite Eastern European states, something that Tito fiercely objected to. This led to Tito’s famous ‘no’ to Stalin, the so-called Tito-Stalin Split, in 1948, which basically isolated Yugoslavia from the Eastern Bloc. The situation became even more critical as Yugoslavia’s eastern borders were surrounded by the Soviet allies. The possibility of a Soviet invasion was a real threat to Yugoslavia at that time, as the examples of Hungary in 1956 and Czechoslovakia in 1968 showed.

The JNA, at this point, was in a quite precarious situation. The Army was in the process of reorganization and rearmament and was heavily dependent on Soviet military supplies. The problem also resided in the fact that the Western powers initially refused to deliver any military support to Communist countries. One way to resolve the dependence on foreign aid was to introduce domestic tank production. The production of domestically developed tanks was something that the JNA was obsessed with. This was, at that time, an almost impossible task. It required a well-developed industry, experienced engineering staff, and, probably most importantly, time, all of which Yugoslavia lacked at that moment. The industry and its infrastructure were almost destroyed beyond repair during the war. Many specialized workers were either killed or displaced across Europe and the fact that the Germans took almost all machine tooling and equipment with them did not help either.

Nevertheless, in 1948, work on such vehicles was initiated. The Petar Drapšin workshop was instructed to produce 5 prototype vehicles. The new tank was designated simply as Vozilo A (English: Vehicle A), also referred to sometimes as Tip A (English: Type A). In essence, it was to be based on the Soviet T-34-85 tank with improved overall characteristics. While it used the same gun and the suspension, the superstructure and turret design were greatly changed.

While the 5 prototypes were completed, they quickly showed a number of deficiencies. Most of these were due to inexperience, lack of adequate production capacity, and more importantly, the fact that there were no design plans. All five tanks were generally different in detail from each other. For example, some were heavier by a few hundred kilograms. When the JNA field-tested these vehicles, it was not possible to make an accurate assessment of their capabilities. They could not be considered as prototype vehicles for possible future production. In order to get any useful information, it was necessary to produce several more vehicles, which were deemed too expensive. This led to the cancellation of this project.

One of five Vozilo A prototype vehicles built. Source:

While the Vehicle A project was canceled, in the years that followed, the JNA would conduct a series of different projects aimed at either developing a new vehicle by using existing components from available tanks or improving the performance of those vehicles that were in service. This led to a series of different experimental designs, such as the self-propelled Vozilo B (English Vehicle B), M-320, M-628 ‘Galeb’ (English: Seagull), and M-636 ‘Kondor’ (English: Condor), etc. These mostly included components from different existing tank designs, such as the Soviet-designed T-34-85 or the US-designed M4 Sherman and M47 Patton tanks. With a better relationship with the Soviet Union in the 1960s, T-54s and T-55s began to arrive in increasing numbers. The JNA initiated a project to locally produce a copy of the T-55 under the name T-34D. In the end, besides a few prototypes, nothing came from these projects. The reason for this was the inability of the Yugoslav industry to produce these tanks. At the same time, it was deemed cheaper to simply buy the new equipment from aboard. Ultimately, work on these would be suspended during the 1960s.

The hybrid M-636 was one of many attempts to build and design domestic tanks using components from other tanks that were in service with the JNA. Source: www.srpskioklop.paluba

The First True Domestic Tank – the M-84

For more than a decade, there were no attempts to develop a domestic tank design. After a long and exhausting negotiation with the Soviets, the JNA finally managed to purchase a license for the production of the T-72 Main Battle Tank (MBT) in 1978. The first prototype (possibly two) was finished in 1979. As the first T-72 tanks began to be produced, the JNA military hierarchy wanted to go further by developing a new improved design. While it was to be heavily based on the T-72, the new project was to incorporate nearly 60% of newly developed parts and components (tracks, electronic installation, improved engine, protection, etc). This would lead to the creation of an initiative known as T-72MJ, later renamed to M-84, of which some 650 tanks would be built in a few different versions.

The M-84 tank entered production in 1984 and, by the time production ended in 1991, some 650 were built. Source: www.srpskioklop.paluba

The Vihor Project

When the M-84 entered service, it was deemed a good design. More importantly, it fulfilled the decade-long dream of the JNA’s Military High Command of producing a domestic tank. Still, it was theorized that even this tank would eventually become obsolete and that the tank technology regarding protection, armament, and speed would progress further.  Thus, as the M-84 production was underway, the Glavni Vojnotehnički Savet (English: Chief Military Technical Council) initiated a new tank project designated as ‘Zadatak Vihor’ (English: Task Whirlwind).

The new tank was to have improved firepower, mobility, and protection to rival that of other modern tank designs in the world. To speed up the development time, the most advanced components of the existing T-72 and M-84 tanks were to be reused. Despite this, it was to be quite different from these two tanks.

In order to gain a better grasp of the new tank technologies, a JNA military delegation would be sent to a couple of countries around the world. In early 1985, one of the first countries visited was France and Ateliers de construction d’Issy-les-Moulineaux (AMX) tank manufacturer. The JNA delegation was presented with the new development of the AMX armor plates. The French engineers were highly interested in the T-72’s performance. The JNA officials were especially interested in the AMX engine developments and talks were initiated on the possible purchase of the V8X 1,000 kW engines. While serious negotiations were undertaken, for unspecified reasons this was never realized.

Egypt and China were also visited. As the Egyptian tank industry was modest, not much was learned there. China was more promising and the JNA delegation had the chance to see the Type 59, but otherwise, no deals were made. In the US, the JNA delegation visited the TACOM military center near Detroit in mid-1985.

Lastly, the United Kingdom was visited in 1986. At that time, the United Kingdom’s arms industry was in an economical crisis and was more than willing to sell various military equipment. The JNA officials were not keen to purchase any technologies from the United Kingdom as most parts would not fit or were simply too expensive to acquire.

In any case, the first drawings and calculations of what would become the new tank were completed in 1985. As no major issue was found with the first drafts, the project got the green light, and work on the first prototype began in 1987. The completion of the prototype stage was to be achieved by the end of 1994 or 1995, with a production of some 15 trial vehicles. If all went without a problem, a yearly production order of 100 vehicles was to be given. The production run was to begin in 1996 and end in 2012. This vehicle was to replace the T-55. The first pre-prototype vehicle was completed in 1989 and given to the Yugoslav Army for testing. However, this would never be close to achieving.


The first prototypes received the OBV A-85 designation. The production vehicles were to be known as Vihor M-95. In various sources, this vehicle is also known as either Vihor M-90 or M-91. The practical naming convention of the vehicles in JNA service was closely related to the year of introduction. Given that it was estimated that this vehicle would enter production in 1995, the M-95 designation (not to be confused with the Croatian development project with the same name) may seem appropriate. To avoid any confusion, this article will refer to it simply as the Vihor.

Vihor Design

It is important to note that the Vihor was in the early experimental development state, so much of its overall performance is not known with total certainty. If the development process was fully completed, new changes may have been implemented or discarded.


The overall Vihor hull was rather simple in its design. It could be divided into three sections. The front part, where the driver was positioned, was protected with a simple but steep angled armored plate. In the center, the turret with its main armament was positioned. Lastly, to the rear, the fully enclosed engine compartment was located. Its construction was made by welding mostly flat armored plates, with the exception of the front part. The Vihor hull design was more or less a direct copy of the M-84. To the front, there was a hatch for the driver that opened to the right side. The engine compartment was covered with a much larger access hatch.

The Vihor used quite a similar chassis to the M-84 and T-72. Probably the biggest difference is the slightly steeper angle of the front armor plate. Source: www.srpskioklop.paluba
An M-84 for comparison. Source: www.srpskioklop.paluba
Another view of the Vihor chassis (the turret is from an M-84 tank). Note the driver’s and the engine hatches are open. Source: www.srpskioklop.paluba


The Vihor was to be powered by the B-46-TK-1 1,200 hp engine. This engine was an improved version of the engine used on the modified M-84A/AB, the 1,000 hp V-46TK engine. The power ratio in this vehicle was 27.2 hp per tonne. In comparison, the T-72 had a power ratio of 18 per tonne, while the Abrams (depending on the variant) ranged between 23 to 26 hp per tonne. It received two turbochargers with an exhaust air cooling system.

Two sub-versions of this engine were proposed, one using components imported from abroad and a second variant with domestically developed parts. The engine could effectively work at temperatures ranging from -30°C to +53°C. This was a potentially great chance for export around the world.

With a vehicle weight of only 44 tonnes, the maximum speed achieved was 75 km/h. This speed even slightly exceeded the expectations and calculations made prior to its testing. Acceleration from 0 to 32 km/h required seven seconds. The transmission was a GC-TRONIC hydromechanical transmission that had 5 forward + 1 reverse gear.

The engine compartment was also cleverly designed to be as small as possible. The engine, with its dimensions of (L-W-H) 153 x 103 x 95 cm, and the transmission assembly took up only 3.4 cubic meters. This greatly aided to reduce the vehicle’s overall dimensions and helped to save weight.

The Vihor was to be powered by a B-46-TK-1 1,200 hp engine, which was basically an improved version of the used on the modified M-84A/AB tanks. Source: www.srpskioklop.paluba


The suspension consisted of six road wheels, a rear-drive sprocket, a front idler, and three return rollers. These were suspended using torsion bar units. While more or less a copy from the M-84, there were some differences. Firstly, the Vihor’s roadwheel vertical travel was increased to 350 mm in comparison to 280 mm on the M-84. The road wheels were built using aluminum alloys. The 580 mm wide tracks were built using either steel or a combination of aluminum alloys. Rubber rims could be added to the tracks. The weight of one track assembly was 1,900 kg. When equipped with rubber rims, the weight of these tracks was increased to 2,300 kg.

The Vihor had a simple suspension design consisting of six road wheels, a rear-drive sprocket, a front idler, and three return rollers per side. Source: www.srpskioklop.paluba
Close-up view of one of six road wheels. Source: www.srpskioklop.paluba
While the Vihor was to be equipped with standard all-steel tracks, replacing them with rubber rim tracks was also an option. Source: www.srpskioklop.paluba


The original electrohydraulic traverse system was replaced with an electromechanical one. Thanks to this system, the turret’s horizontal rotation speed was 20°/s, so it swung 360° in 18 seconds. In contrast to the generally round-shaped turret used on the M-84 and T-72, the Vihor received a quite different design. While the front was quite similar, the rear of the turret was redesigned and extended. The extra free space was used to store the radio and other equipment. On top of the turret, there were two escape hatches for the turret crewmembers. The one on the left was for the gunner and the one on the right for the commander. Various equipment and storage boxes were to be externally mounted on the turret sides.

Inside the turret, the radio equipment was located to the rear. This was an encrypted, frequency hopping radio with 16 programmed channels and a frequency range of 30 to 87.9 MHz. The command vehicles were to be equipped with additional radio equipment.

Rear view of a wooden mock-up of the Vihor turret. It was greatly extended to the rear to provide additional working space inside the turret. Source: Wiki

Armament and Ammunition

For the main armament, the 125 mm 2A46M smoothbore gun was chosen. This was the basic armament of the M-84 tank and Soviet-built MBTs such as the T-64 and the T-72. Given its availability and general effectiveness, it was logical to reuse this gun for the Vihor project. The difference was that it would have received a number of improvements and modifications to further increase its effectiveness and durability. These included adding a muzzle reference system (MRS) for measuring gun barrel curvature, thermal insulation lining of the barrel, using better raw materials for the production and improved production techniques for its construction, and testing a new quick-change mechanism, among others. The gun was to be provided with horizontal and vertical stabilization during the acquisition of targets. In order to help the crew with targeting, the Vihor was to be provided with advanced electronic ballistic computers.

The Vihor fire control system was a complex unit consisting of many elements, such as the day/night sight. Another interesting device with which the Vigor was equipped was a display for the commander connected to the gunner’s sight. This permitted the commander to see the targets that the gunner was aiming at. The Vihor was also equipped with thermal imaging with a magnification of 8x to 10x, a laser range finder, third-generation night vision, a laser-warning receiver connected to the externally mounted smoke launchers, etc. The electronic ballistic computer could be used to enter all necessary information regarding the target.

The electro-mechanical autoloader was basically the same as the one used in the M-84. This autoloader was located under the turret, on the tank’s floor. It held 22 rounds in its rotating transporter. An additional 18 rounds were to be stored inside the crew compartment. With these and other various improvements (like adding a bidirectional movement autoloader), the rate of fire was estimated to be around 10 rounds per minute.

It was requested that the gun, with all its improvements needed, be capable of piercing 400 mm of RHA armor at ranges of 2 km using Armor-Piercing Fin-Stabilized Discarding Sabot (APFSDS) rounds. When using High-Explosive Anti-Tank (HEAT) rounds, it was supposed to be able to penetrate around 600 mm of RHA armor.

Besides the main armament, the secondary armament did not change from the M-84. It consisted of one coaxial 7.62 mm PKT and a turret-mounted 12.7 mm NSVT heavy machine gun. While sources do not mention ammunition load, this would most likely have remained the same as on the M-84. This meant 2,000 rounds for the PKT and 300 rounds for the NSVT heavy machine gun.

The main armament of the Vihor consisted of a 125 mm 2A46M smoothbore gun. Secondary weapons consisted of two machine guns. Source: Wiki

Armor and Protection

The Vihor would have had increased armor protection compared to other modern Yugoslav tanks. The front hull side was angled at 71° and the new armor construction was to provide protection the equivalent of 650 mm thick homogeneous steel plate armor according to M. C. Đorđević (Odbrana Magazine). Other sources, such as like www.srpskioklop.paluba, listed the frontal armor thickness to be equivalent to 500 mm of homogeneous steel armor. Against HEAT rounds, it offered 600 mm protection. The flat side armor plates were much weaker, with a thickness of just 70 mm.

The turret front armor thickness is unknown. What is known however is that it was angled at 40° and provided the same level of protection as the hull front armor. Similar to the improved M-84 versions, the Vihor also had a cast turret. In addition, its turret front had a cavity that was filled with quartz sand mixed with an adhesive.

Additional protection could be acquired by adding anti-HEAT screens or Explosive-Reactive Armor (ERA). In the case of the Explosive-Reactive Armor, it was a domestically developed KAO M-99 type. These, in the best case scenario, provided an 80% increase in protection against HEAT rounds. More realistically, these provided additional protection in the area of 30% to 50%. Against kinetic rounds, it offered a slight increase of protection of around 25%. The M-99 armor was immune to fire up to 23 mm caliber rounds, including artillery shrapnel or detonations of close positioned explosive-reactive units. This armor added a total weight of 750 kg, a further 250 kg if the sides were also protected. The development of this armor began in early 1990s, and it was not yet ready to be added on the prototype. It was actually never fully installed on any Vihor tank.

The Vihor was also to be equipped with the BDK smoke dischargers. These consisted of 24 discharge units, divided into two groups, and placed on either side of the turret. The maximum effective range of this system was 500 m. Besides standard smoke rounds, illumination, anti-infantry, or anti-missiles flares could be used.

The Vihor was also provided with Nuclear Biological Chemical (NBC) protection. It received an inner lining that protected the crew from neutron radiation. A detector for biological weapons was also added. Lastly, an automatic fire extinguisher system was installed inside the vehicle.

The last and probably one of its greatest assets was its small size. Generally speaking, all Soviet tank designs (which were copied by JNA) had smaller dimensions than Western designs, and the Vihor was no exception. Its total volume was around 12.6 m3.

The domestically developed KAO M-99 Explosive-Reactive Armor (ERA) single box unit. Source: www.srpskioklop.paluba
While the KAO M-99 would see service on later Yugoslav vehicles, like this experimental improved T-55, it was not fitted on the first Vihor prototype. Source: www.srpskioklop.paluba
Vihor (once again with the M-84 turret). While not tested with the KAO M-99 ERA, the installation of smoke dischargers was trialed. Source: www.srpskioklop.paluba
Miniature wooden mock-up of a Vihor fully equipped with side skirts and smoke dischargers. Source: Wiki


The Vihor had a crew of three, consisting of the commander, the gunner, and the driver. Their positions were unchanged in comparison to the M-84 tanks. The gunner and the commander were placed in the turret, while the driver was positioned in the lower hull.

Like on the M-84, the Vihor would have had a crew of three, with the commander and the gunner positioned in the turret, while the driver was located inside the Vihor’s front hull. Source:

The Fate of the Project

The single pre-prototype was equipped with an M-84 turret and used for extensive drive testing. Depending on the sources, this vehicle managed to successfully drive between 1,500 to several thousand kilometers. No major problems with the first design were noted. While the development of the Vihor was underway, the Yugoslav wars broke out. This marked the end of many military projects, including the Vihor. The first pre-prototype test vehicle was located in Belgrade, nowadays Serbia, prior to the war. Due to a lack of documentation and proper equipment, it was not possible to fully finish this prototype. It would eventually be stored in the VTI Kumodraž depot. In 1993, a new Vihor project was announced, which was to have a stronger engine and hydrodynamic suspension unit. This project led nowhere and was likely just a propaganda tool to boost morale. At that time, Yugoslavia was under sanctions and in a dire economic situation, so developing such a design would have been almost impossible.

The surviving hybrid Vihor is currently stored in Serbia. Source: www.srpskioklop.paluba

Two completed prototype hulls were located at the Đuro Đaković workshop, while the two incomplete turrets were left in Slovenia when the war started in Yugoslavia. The Croatians would use the two hulls together with the available documentation and tooling to start their own tank development project. This would lead to the creation of the Degman and M-84A4D projects, which are currently at the prototype stage.

Based on surviving parts and documentation, the Croatian military industry would go on to develop the Degman. Source: Wiki


The Vihor was the JNA’s final attempt to develop a modern domestic tank design. It would have possessed a series of advanced systems and, combined with good overall driving performance, held the promise of becoming an excellent design. Unfortunately, its final realization was stopped with the outbreak of the Yugoslav wars. How would it have performed in future testing and evaluation is difficult to know precisely. It was nevertheless an interesting design initiated when Yugoslavia was in a huge political and economical crisis, which ended in a war and the cancellation of this and many other projects.

Vihor M-91 Main Battle Tank by Jaroslaw Janas.

Vihor M-91 specifications

Dimensions (L-W-H)  9.74 x 3.65 x 2.21 m
Total Weight, Battle Ready 44 tonnes
Crew 3 (driver, commander, and gunner)
Propulsion 1,200 hp​​  B-46-TK-1
Speed/off-road  75 km/h, 50 km/h
Range 600 to 700 km
Armament 125 mm 2A46, One 7.62 and one 12.7 machine gun.
Armor Equivalent up to 500 to 650 of homogeneous armor
Number obuilt At least three incomplete prototypes




Cold War Yugoslav Prototypes Has Own Video


Socialist Federal Republic of Yugoslavia (1956-1966)
Support Tank – 1 Built

After the so-called Tito-Stalin split that took place in 1948, the new Yugoslav People’s Army (JNA- Jugoslovenska Narodna Armija) found itself in a very serious situation in terms of acquiring modern military equipment. The JNA had been heavily dependent on Soviet military delivery and aid in armament and weapons, especially armor vehicles.
In the 1950s, the situation with the acquisition of military technology significantly changed. This period is considered to be the ‘happier times’ of the JNA, because of a sudden inflow of new armored vehicles, mostly from the West.
The West was initially in a dilemma whether to help the new communist Yugoslavia or not but by the end of 1950, the side arguing in favor of providing military assistance to Yugoslavia had prevailed.
In the middle of 1951, a Yugoslav military delegation (led by General Koča Popović) visited the USA in order to achieve military cooperation between these two countries. These negotiations were successful and, shortly thereafter, several US military instructors arrived to help in training for the future equipment that would subsequently arrive.
On the 14th of November 1951, an agreement for military aid was concluded (Military Assistance Pact). It was signed by Josif Broz Tito (Leader of Yugoslavia) and George Allen (American ambassador in Belgrade). With this contract, Yugoslavia was included in MDAP (Mutual Defence Aid Program).
Thanks to MDAP, the JNA received, during 1951-1958, plenty of new military equipment:
M4A3E4 Sherman (also one M4A3E8 for unknown purposes) – 599
M47 Patton II – 319
M7/M7B2 Priest – 56
M18 Hellcat – 240
M36 and M36B1 Jackson – 399
M3A1 scout cars – 300
M8 – 265.

M4A3E4 in Service

During the negotiations, the American Army wanted to get rid of older vehicles that were on stock first, but after some stressful and long negotiations, it was decided that the delivery would contain only this newer version. The ‘E4’ was a modernized and improved version of the M4A3 tank. It was essentially an M4A3(75) Wet Stowage with VVSS, up-gunned with a 76mm gun.

Yugoslav M4A3E4. Photo: SOURCE
The Yugoslav M4’s would be mostly used in military parades and for crew training in the following years up to the second half of the sixties. The first appearance of this tank in public was at a military parade held in Zagreb (May 1953). Due to obsolescence and lack of spare parts, which limited their operational use, it was foreseen by the Drvar-2 military plan to replace them with the Soviet T-54/55. After replacement, the Shermans were to be modified for other purposes (bridge carriers and engineering vehicles).
Unlike the T-34-85 (whose parts were manufactured by domestic factories and workshops and thus there was a real possibility of keeping them in operational use for a long time), M4 tanks were not used during the civil war in Yugoslavia in the early nineties, but they saw action in many Yugoslav and foreign war movies like Užička Republika (1974) and perhaps the most famous Kelly’s Heroes (1970).

Many Prototypes

The JNA was obsessed with the production of its first domestic tanks. The construction of a new tank required a well-developed industry and also demanded a high number of educated and qualified personnel. Due to a bad economic situation, mostly due to the damage done to the industry and infrastructure during the war, production of a new tank was not realistic or possible in the near future.
An attempt was made to find a compromise if the production of new tanks was not possible in the near future, then a solution may be to attempt to improve existing tanks. Several such projects were created during the 50’s and 60’s.
The first project was the Vehicle A (5 built), then followed by the proposed Vehicle B (self-propelled gun armed with a 90 mm gun). A series of different prototypes followed next, the M-320, M-628, M-634 and the T-34D. All of these were based on the already existing tanks (T-34, M4, M47, and even T-54/55). Perhaps the strangest and unusual was the modification named SO-122, which was in principle an M4 armed with a large 122 mm gun and equipped with a Soviet engine, a combination from the East and West (ironically, the same as Yugoslavia of that time).

The Beginning of the SO-122

At the meeting of the Chief Military Expert Council held on the 4th of February 1956, it was decided to rearmed one Sherman (due to the obsolescence of M4 tanks) with the 122mm A-19 gun, and to test whether this modification was feasible and cost-effective at all. The idea was that, by installing a large caliber in the M4 tank chassis, it would extend its operational service life. At the same time, this would increase its firepower and allow it to deal with more modern tanks and to be used as a heavy fire support vehicle.
The project design was assigned to the Military Technics Institute KoV (Vojno Tehnicki Institut KoV), and the head of the project was the Uprava Artiljerije. Fortunately for them, there was an independent earlier project with the goal to modify a few M4’s by installing a Russian V-2/2R engine. This helped to a certain extent to accelerate the development time and reduce costs.

Sherman equipped with an engine (V2) taken from the T-34-85. Photo:SOURCE

The New Engine

In August 1950, in Belgrade, a ‘machine bureau’ (Mašinski Biro) which consisted of 60 engineers and technicians was founded. Its primary function and purpose was to analyze the parts of the T-34-85 Soviet tank (the engine, gearbox and transmission were considered the most important) and, if possible, to reproduce them. If this task was successfully fulfilled, the next goal was to rebuild and produce this tank (the ‘Famos’ factory was selected for this job).
By the end of 1957, both the engine and gearbox were successfully produced in some numbers. Some 71 new V-2 engines were built by the end of 1957, with 50 more planed by the end of 1959. The stronger V-2R (520 Hp) engine was built in limited numbers.
In 1956, Famos factory engineers tested the installation of this engine on one M4 tank. The results showed that installation of a new engine was feasible, but there was a problem with overheating. Next year, several modifications were made to the engine but the overheating problem was still present. The tests continued for the next few years and, although slow, interest in this project was still significant. Around three or four M4s were equipped with the V-2 (under the code name M-634) and one with a V-2R engine.

Because of the secrecy of Yugoslav armored prototype’s projects, this is the possibly the only photograph of the SO-122. Photo: SOURCE

The Prototype

When designing the SO-122, beside the modified engine compartment, only the turret had to be completely rebuilt from scratch. The rest of the original M4 was unchanged. The work on the large tank turret equipped with 100 mm or 122 mm cannons started in the early sixties. Several designs were built and tested and great attention was paid to the design of the turret due to the new rear modified engine compartment. One of the most important requirements was that the new turret should have a full 360° rotation arc.
In 1964, the 100 mm project was rejected because of the great price. Work on the 122 mm project (code-named SO-122) continued. The main gun was built using some parts taken from the T-54 gun (hydroelectric system and the breech lock) and had a muzzle brake installed. In order to avoid any greater damage to the gun during vehicle movement, a heavy travel lock for the main weapon was also added. The gun was built by Zenička Železara and the final assembly was done in the ‘Bratstvo’ workshop in Novi Travnik.
Beside the main weapon, there is no information about the secondary armament (machine guns). The hull-mounted machine gun would have probably remained. Based on the picture of this vehicle, the design of the turret suggests that a second machine gun was planned, but this is only speculation at best.
It was planned that all SO-122 would be equipped with the new domestic-built engine. Because the tests on the new V-2R engine were successful, a decision was made to use them instead for this modification.
The crew would most likely have consisted of a gunner, a loader, a driver, a radio operator and the commander. The dimensions of this vehicle were similar to the original M4 tank. Due to the length of the 122 mm gun, it was certainly much longer than the original but the exact details are unknown. The American SCR-528 radio was replaced with a British SET-19WF with an operational range of 10-15 km.
As the SO-122 project was conducted under a great deal of secrecy, only a small amount of information of its characteristics (armor, elevation etc.) and only a few photographs exist.

Operational Test Use

When the complete prototype was ready, it was sent to Banja Luka and Nikinci for first field tests and, by the 1st of February 1965, they were completed. The results of these tests showed that the mobility characteristics of the original M4 tank (even with an increase in weight up to 33.5t) were almost unchanged. The top speed was increased up to 50 km/h but, on the other hand, the acceleration was reduced. The fuel consumption was also reduced from some 400-450 litres to 207-211 litres for 100 km. These modifications also influenced the increased the operational range up to 300 km.
A big issue was the length of the main gun. More than 3 m long, this could cause some problems (possibly even damaging the gun) when moving on uneven ground and in operating in urban areas or thick forests.
It was not recorded whether there were any structural problems when shooting the main gun, despite its greater weight and recoil. There were some issues with the turret ventilation and problems with the fumes that appeared in the turret during firing.
These tests were considered relatively successful and that this modification was feasible and not overly complicated for production. A preliminary decision for the production of 100 SO-122 (or 96 depending on the sources) was made before these tests were even done. Unlike the prototype, it was planned that the production vehicles should use the original engine (gasoline Ford GAA V8), mostly in hope to start the production as soon as possible.

Final Fate

Despite the production order for about 100 new vehicles, no new vehicles were ever built, for several reasons:
– Lack of armor penetration of the 122 mm A-19 gun in comparison with other guns in use, including the 100 mm used on – the T-54/55
– Poor depression and elevation which limited the fire support at longer ranges
– Development of new and improved anti-tank rockets
– The 1966 decision to retire the obsolete M4 chassis
– Long development time
The death of the Joseph Stalin (in 1953) and the gradual reconciliation of Yugoslavia and the Soviet Union in the late fifties, led to future negotiations of purchasing of new military equipment including modern tanks. New deliveries of more modern Soviet tanks also influenced the final decision for this project.
As the number of modern tanks increased, there was really no point to continue working on the SO-122. After nearly 10 years of development and testing, in 1966, on the orders of the State Secretary for National Defense General Gošnjak, the SO-122 project was canceled. The works on similar projects using the T-34-85 were forbidden. Due the great secrecy of the project, the fate of the SO-122 prototype is unknown.

The 122 mm M1931/37 (A-19)

The M1931/37 gun was designed primarily for the purpose of destroying enemy artillery at long distance. The gun itself was not new as it was designed in late 1927 and tested in October 1931. Production started in 1935 but, from 1938 on, the ML-20 carriage was used in combination with this gun. It was heavily used during the war. Modified versions were also used as the main armament on several Soviet armored vehicles like the IS series and the ISU-122.
The ML-20 carriage (used for later built M31/37) had a modern split trail, small shield for crew protection, two steel wheels covered by solid rubber tires and a pair of two cylinder springs (horns) for a better gun barrel balance.
Characteristics of the M1931/37 Soviet field gun mount were:
– Caliber: 121.92 mm
– Barrel length: L/46.3 (5,650 mm)
– Max range: 20,160 m
– Weight: 7,250 kg
– Muzzle velocity: 800 m/sec
– Horizontal field of action: 58°
– Elevation: -2 to +65
– Rate of fire: 5 – 6 rounds per minute
– Ammunition weight: 25 kg
– Ammunition types: HE and APHE
The Yugoslav Partisan captured several ML-20 and A-19 guns from the Germans during the war. Germany had itself captured a number of these guns from the Russians (under the designation 12,2 cm Kanone 390/1(r)) and used them during the war in many different roles. Those guns that were recaptured by the Partisans were used as coastal guns guarding the shores of the Adriatic Sea. They were used in the fight against the Germans but, because of the lack of ammunition, parts, and adequate towing vehicles, their use was limited.
After the war, in 1947, some 66 new 122mm M1931/37 guns were bought from the Soviet Union at a price of US$11,472 per unit. By decision of the JNA (Yugoslav People’s Army) military officials, 32 guns were given to the third Army, four guns were assigned to the First and Second Army, and 14 were kept in reserve. The remaining 12 were given to a special unit under a code name 8888-I. It was actually a secret code name for an operation helping with the rearmament of the Albanian army.
The M1931/37 remained in use until the nineteen-nineties, up to the civil war in Yugoslavia. During 1992, all available 122 mm (A-19) guns were used by the Army of Republika Srpska (Bosnia and Herzegovina) under the new designation M-31/37B1 (equipped with modern wheels). They participated in heavy fighting in 1992 near Srebrenica. The last examples of the M-31/37B1 were destroyed in 1996/97.


Dimensions (L-W-h) 7 to 8 m x 2.62 m x 2.74 m
Total weight, battle ready 33.5 tonnes
Crew 5 (Gunner, loader, driver, radio operator and commander).
Propulsion V-2R , 520 hp diesel
Top speed 50 km/h
Range 300 km
Armament 122 mm (A-19) M1931/37
Armor Up to 76mm
Total Built 1

Links, Resources & Further Reading

Oklopne jedinice na Jugoslovenskom ratištu, Bojan B. Dumitrijević i Dragan Savić, Institut za savremenu istoriju, Beograd 2011.
Modernizacija i intervencija, Jugoslovenske oklopne jedinice 1945-2006, Institut za savremenu istoriju, Beograd 2010.
Allied Artillery of World War Two, Ian V. Hogg, The Crowood Press 2001.
Military Magazine ‘Arsenal’, Number 47 2011.
Twentieth-Century Artillery, Ian Hogg, Amber Books 2000.

Illustration of the SO-122 by Tank Encylopedia’s own David Bocquelet.