T6 Medium Tank prototype
Most of the information on this page is courtesy of Joe DeMarco. Note: some of the information on this page was compiled using a technique informally referred to as "counting heads." It is based on the ongoing study of period documents and photographs, as well as surviving Shermans. Due to the limited nature of available reference sources, some of the information presented here must be considered as "educated guesswork."


T5 tank    T5 tank

In the 1930s, as the United States struggled through the Great Depression, the country was overwhelmingly isolationist. These conditions were not conducive to the development of a costly offensive weapon like a Main Battle Tank. The Ordnance Department had designed a few Light Tanks to support the infantry, and similar "Combat Cars" to support the [still] existing horse cavalry. Late 1930s efforts to develop a Medium Tank resulted in the T5 / M2 series (above, left, T5 Phase III in 1938), which were essentially enlarged versions of the M2 series Light Tanks. It was soon realized that the T5's 37mm main gun would be inadequate on the modern battlefield, but the engineers appear to have lacked the knowledge, resources and technical ability to come up with a design that could mount a larger caliber weapon in a revolving turret. As an expedient, in 1939, the T5 Phase III was modified to mount a 75mm pack howitzer in the right front of the hull (above right), in the manner of the French Char B1 bis. The Fall of France in June of 1940 changed the political landscape, and created a sense of urgency that the United States begin mass production of a Medium Tank as soon as possible. A design was needed immediately that could meet the Army's requirements for mobility, protection and firepower, and this resulted in the M3 Medium Tank. From the beginning, the M3 was thought of as an interim design. Its major shortcoming was, of course, the limited traverse of the sponson mounted 75mm main gun. In August 1940, even before the M3's design was finalized, the Ordnance Committee, Technical Staff emphasized that the next step in development would be "modification of the Medium Tank, M3" by "relocating the 75mm gun in the turret." It was intended to replace the M3 in production as quickly as was practical.


M3 medium

The M3 pilot was completed at Rock Island Arsenal in March 1941, and photographed at Aberdeen Proving Ground shortly thereafter. The riveted construction harkened back to World War I. The sponson mounted 75mm gun along with the turret mounted 37mm, evinced such descriptions as "land battleship" and "rolling fortress." The tank stood over 10 feet tall, causing one tanker to remark, "It looked like a damn cathedral coming down the road." Although originally intended to be a limited run model, the geopolitical situation forced its mass production, so that ultimately, 6258 M3 series Medium Tanks were manufactured from June 1941 through December 1942. 4373 were supplied to the Allies as Lend Lease.


M3 medium

M3 medium

As the country geared up to become "the Arsenal of Democracy," the automobile, locomotive and steel manufacturers quickly adapted to "war work." They began to introduce more efficient production methods, along with many technological advances. The photos above show the earliest conception of the M3 upper hull, produced as an experimental casting. In September 1940, this was thought to be the largest armor casting ever made. The steel industry was able to demonstrate that such castings could be produced to Government ballistic specifications. Furthermore, casting reduced the amount of assembly time, and eliminated the rivets of the original fabricated hull design. It was stated that this method offered a savings of about $3000 per tank. On June 19 1941, the Ordnance Committee "authorized the use of cast upper hulls for medium tanks" as an alternate method of construction. On that same day, the Committee approved creation of a full size wooden model, to be followed by the Medium Tank, T6 pilot.


T6 mock up

Above shows the wooden model begun at the APG Carpentry Shop in May 1941, and photographed in August. Since the T6 would employ the same suspension, power train and engine as the M3 Medium, the model's lower hull was "borrowed" from the M3 mock-up. Thus, the design effort was freed up to concentrate on the new turret and upper hull. While a welded hull would have been easier to fabricate from wood, the designers at APG opted to employ the latest technology, and "carved out" a large casting. As the technology now allowed for a turret of sufficient size to accommodate a 75mm gun, it was determined that these would be made exclusively by casting. The patterns for the turret and hull were sent to the General Steel Company for the initial castings for the T6. The commander's cupola (1) was taken from an M3 Lee. The gun mount was designed for the longer M3 75mm, which was not yet available, so additional length (2) was added to the installed M2 gun.


T6

An unspecified M3 Medium served as donor for the T6's lower hull, power train and numerous other components. In the photo above dated July 2 1941, a bit of the M3's upper hull (1) can be seen. The lower hull was completely stripped out. Work appears to have begun on test fitting of the firewall (2) and one of the vertical fuel tanks (3).


T6

The M3 had a crew of seven, which was reduced to five on the T6. In the above photo dated August 26 1941, the driver's seat (1) can be seen to be installed on a platform raised off the belly plate. The seat for the assistant driver (or bow gunner) (2) was installed on a swivel affixed to the side of the hull. The swivel mount was replaced by a pedestal mount on production Shermans. The assistant driver was not provided with an overhead hatch on the T6. An escape hatch was installed in the belly plate directly behind his seat (inset). Item 3 is the identification plate for the "Mack Medium M3 Powertrain." Item 4 is the battery box. Item 5 shows the normal installation of the bogie units. For some reason, the middle bogie on the right side was bolted on through some reinforcing bars (6).


T6
Click on the photo for larger size

The T6 was rolled out for presentation on September 3, 1941. Richard Hunnicutt, author of the seminal work on the Sherman wrote, "Although the cupola was part of the wooden mock-up, the writer has no evidence that it was ever installed on the pilot tank." In fact, the cupola from the M3 Medium (1) was installed on "presentation day," but in a conference later that same day, it was decided to replace it with the commander's split hatch which became standard on production Shermans. The conferees also determined that the hull side doors (2) were to be eliminated, and that a rotating periscope would be retrofitted in the turret roof in front of the loader's position. Note that the tank can be seen to be "riding high" on the M3 type bogies. The interior stowage arrangements had not been finalized, so that the T6 was not fully equipped on September 3rd.


T6

Various dignitaries posed with the T6 on "presentation day." General Jacob Devers was head of the Armored Force. Colonel William Hardigg was Director of the Proving Center, which oversaw the T6 Project. General J.B. Rose was Commandant of Aberdeen Proving Ground at the time. Note that the protectoscope (1) taken from the M3 Medium, was installed a little forward of center on the right side of the turret, but toward the rear on the left. The grab bars (2) were eliminated with the side doors. The round object (3) which served to protect the antenna mount, was replaced with a ventilator on production M4A1s.


T6

The T6 was photographed again on September 16th. In this front view one can see that the M3 cupola was removed, and that double counterweights were added to balance the short barrelled M2 Gun. The sight rotor (1) was linked to the main gun. The assistant driver was also provided with a sight rotor (2) linked to the bow machine gun. The flexible weapons are shown elevated, and the designers quickly realized that all of the various rotors were vulnerable to bullet splash damage that could disable the guns. In adapting the M3 differential housing to the T6, it was necessary to weld in a bolt strip extension to the right section. Until a revised casting became available, early manufacturers of the Sherman were shipped M3 differentials (inset) that they had to alter in house.


T6

In this view, note the rear contours of the T6 upper hull. This area of the casting was revised significantly before the M4A1 entered production (inset). The T6's tool stowage was similar to the M3's. M3 Medium production commenced in June 1941. Less than 300 units had been accepted by mid September. As the M3 entered service, users began to report that the "pepper pot" exhausts (1) were creating an intense heat condition on the engine deck. In some cases, it was noted that the tools' leather straps were melting. The Ordnance Department tried to remedy this without disrupting M3 production by means of a "Quick Fix," but ultimately the exhaust system had to be redesigned. The new set up did not become available to production until May 1942. About 70 of the first M4A1s were accepted with the pepperpot exhaust, but it was ordered that they be updated before entering service.


T6    T6
Click on the photos for larger size

The overhead photo on the left provides another view of the rear contours of the T6. It was realized that the M3's air intake (1) was vulnerable, and an armored cover was designed for use on production M4s and M4A1s. The photo on the right shows the intake cover in the open position on the very first production M4A1. This change forced a rearrangement of the tool stowage. Comparing the two photos, one can see that the T6's cast-in bullet splash was interrupted, whereas it was continuous back to the fuel cap covers on production M4A1s. Note that the rear hull ventilator (2), standard on all Shermans, is absent on the T6. The locking pins that secured the armored fuel cap covers were in the M3 "short and straight" configuration, whereas they were longer and angled on production Shermans.


T6    T6

The overhead photos compare the T6 (left) with the third M4A1 (Serial Number 7) made by Pressed Steel Car. Without a hatch, the assistant driver's vision was restricted to straight ahead, even under non combat conditions. In the T6's Daily Logs for October 1 1941, the designers did away with the bow machine gun sight rotor, and provided the assistant driver with a hatch configured as a mirror image of the driver's. The designers also determined to provide the main gun with a rotor shield (1), and the bow machine gun with a ball mount (2) before series production began. The headlamps (3) were relocated to the hull. The lifting handle (4) on the T6's driver's hatch can be seen to be mounted towards the rear and on an angle. This "early" position carried over to some of the first production Shermans. PSC Serial Number 7 had had its handles (5) relocated to the standard position by the time it was photographed in December 1942.


T6

The photo above provides an interior view of the driver's hatch and the direct vision device. As mentioned previously, the General Steel Company cast the T6's upper hull, and their distinctive logo, a G inside a shield, appears on the hatch. It can be seen to have a casting part number of D-50884. When it was decided to provide the assistant driver with a hatch, the part number remained the same, but a "B" suffix was added to the driver's hatch, and an "A" to the assistant's. Although the direct vision appears to provide protection, the designers noted that small gaps permitted the entry of bullet splash. Ultimately, direct vision was ordered eliminated on June 24 1942 for cast hulls and August 13 for welded hulls. It would take some months before the redesigned drivers' hoods could enter production, and several thousand Shermans were built with direct vision.


T6

For voice communication, an SCR 508 (or subsequent improvement) was mounted in the turret bustle of standard US Shermans. The T6 was built to be a command tank, and the photo above shows the installation of the additional SCR 506 long range radio. The radio can be seen to be partially blocking the side door, but, of course this was not an issue, since the doors were to be eliminated. It is to be noted that production Shermans were built as standard tanks. Command tank radio installations were retrofitted by the Signal Corps as conditions required.


T6

The photo above illustrates the antenna problem encountered with the command tank's radio. Reports from the field were coming in to the effect that greater ventilation was needed for crews inside the M3. In November 1941, it was determined that the Sherman would be equipped with 3 ventilators - 2 in the hull and 1 in the turret. The protuberance circled was originally made as an antenna bracket, but served as a location for one of the three ventilators on the M4A1.


T6

The novice Army designers had a certain obsession with firepower. The M2 and M3 Medium tanks were described in press releases as "rolling fortresses," since they were bristling with guns. An unfortunate consequence of this was that the T6 was equipped with a pair of fixed .30 cal machine guns, as shown above. The designers appear to have been reluctant to concede that these were wasteful and superfluous, but the fixed guns were finally eliminated in early 1942. Only Lima Locomotive, Pressed Steel Car and Fisher Body produced a few of their first Shermans with the fixed machine guns installed. On subsequent units, hulls already produced with the MG holes, had them filled in by welding.


T6

The challenge in the development of the US Medium Tank was to design a superstructure that could carry a turret large enough to accommodate three men, and mount a 75mm gun. For this purpose, it was thought that the turret ring would need to have an inside diameter of 69 inches. In retrospect, this was one thing the designers "got right" from the start. It was determined that the turret of the new Medium Tank would be cast armor, while the upper hull would be of either welded or cast armor construction. Above shows a left rear view of the T6 turret. The M3 type protectoscope (1) was replaced with a solid door, and the pistol port on the right side was eliminated from the design in late 1941. Originally, the T6 turret did not have lifting rings (2). These were added to the turret and gun shield during development, and became standard. The turret basket was encased in steel mesh. It wasn't until early 1943 that this was recognized as a safety issue, as the mesh "trapped" the turret crew, and isolated the drivers. In April, work was begun to redesign ("skeletonize") the turret basket. In August 1943, as part of a more comprehensive "Quick Fix" Modification, the mesh was instructed to be removed from existing Shermans.


T6

In this unusual view of the turret casting, one can see how the gun shield, sight rotor and commander's cupola were bolted on. It was immediately decided to add a periscope for the loader. A little later, a ventilator was added to the design. Their future positions are shown in black. A Lima Locomotive Memorandum dated January 23, 1942 stated that only the "first 30 tanks using Rotor Device." Later turret castings had been revised to incorporate a gunner's periscope. As best we can tell, the first 10 M4A1s produced by Pressed Steel Car were the only other Shermans that were accepted with sight rotor turrets. The turret's height was about 3 feet, and the armor basis was 3 inches on the front and 2 inches on the other vertical surfaces. Although it underwent numerous revisions, the basic shape of the T6 turret was used on 75mm and 105mm Shermans until the end of production in mid 1945.


T6

In this view through the commander's hatch opening, note the brackets (1) installed in the turret bustle. They could accommodate either the US or British standard tank radio. The commander's (2) and loader's seat (3) were affixed to the turret basket wall, and were height adjustable. A bit of the 75mm gun's breach and breach guard (4) can be seen. With only one hatch in the turret, the breach guard presented an obstacle to the loader as he attempted to escape in emergency situations. During the initial design work, a loader's hatch was considered, but rejected in the interest of simplicity. As a result of numerous combat reports, a loader's hatch was added as a standard item on Sherman turrets in late 1943. 12 ready rounds can be seen mounted to the wall of the turret basket. Ready rounds were very popular with tank crews, but were extremely vulnerable.


T6

The gunner's seat (1) was mounted to the turret basket floor directly in front of the commander's station. The gunner was provided with manual elevating and traversing controls (2), along with a power traverse mechanism (3) that was considered one of the best features of the tank. A gyrostabilizer was also provided that, in theory, permitted firing of the main gun while on the move. Unfortunately, the technology was not quite ready. The device was too complicated for average crews to adjust and maintain and, in practice, it was mostly unused or disconnected. The turret was designed to carry the 75mm gun as seen here. However, the gun mount was "so designed that alternate armament may be used." Initially, the British intended to employ their 6 pounder gun, but trials and actual combat with Lend Lease Grants, clearly demonstrated the superiority of the 75mm. Later tests showed that the turret could accommodate a 105mm Howitzer, but that it was too small to mount a 76mm gun with proper crew efficiency. The 69 inch turret ring made it possible for the Sherman to accept a larger turret without any major alterations to the existing hull. In January 1944, a larger turret for the 76mm gun was introduced in production.


T6

The "Military Characteristics" defined for the improved Medium Tank did not list a specific power plant or fuel type. Due to 1930s' budgetary constraints, no effort had been made to design a dedicated tank engine. Thus, the T5 series and the succeeding M2 and M3 Medium Tanks were powered by an "off the shelf" 9 cylinder radial aircraft engine that was widely used by the Army Air Corps, and had a proven record of reliability. Designing the US Medium Tank around a radial engine dictated that the tank would sit about a foot higher than would have been the case had a comparable in line or vee type engine been available.


Chrysler Multibank

General Motors GM    Ford GAA

As the US Military commenced its massive build up in the early 1940s, materials priority appears to have been given to the Navy and Air Corps. Recognizing that there would be a critical shortage of aircraft engines, the Army turned to the automotive industry for alternate power plant designs. Chrysler quickly presented an expedient which combined five 6 cylinder auto engines in a star configuration. Perhaps reflecting the desperate nature of the tank engine situation, the Army accepted the so called "Chrysler Multibank" for production on both the M3 (109 M3A4s) and the M4 (7499 M4A4s). It must be noted that the outlandish design actually gave good service. General Motors married two of its 6 cylinder diesel truck engines to produce an effective power plant that was introduced on the M3 Medium series in January 1942, and remained in production until May 1945 on the M4A2 Sherman. Finally, the Ford Motor Company presented an experimental 500 HP V-8 engine that soon became the preferred power plant of the US Army. It was used to the end of production on the M4A3 Sherman, as well as on its replacement, the M26 Pershing. Photo of the Chrysler Multibank courtesy of "BearGrease".


T6

In August 1940, the British Tank Mission was authorized to place an order for 1500 M3 Medium Tanks "modified to the British [Grant] turret." They did not care for the M3's design, but were desperate for tanks, since they had lost most of their armor a few months earlier during the Battle of France. Also in August, perhaps not trusting that the Americans could come up with an acceptable replacement, the British and Canadians set about to design their own version. In January 1941, it was determined that the "M4 Cruiser to be made by Montreal Loco Co." would be built on the lower hull of the M3 Medium, and like the T6, would employ a cast upper hull and turret. It was intended that 90% would mount the 6 pounder (57mm) firing solid shot, while the remainder would carry the 75mm for close support roles. Christened "Ram," the pilot was completed in June 1941, and is shown above undergoing tests at APG in August. The 6 pounder was not yet available, so a 2 pounder was installed on the pilot, as well as the first 50 production units.


T6

Both the Ram and the T6 achieved the goal of redesigning the M3 Medium so that the main gun was mounted in a revolving turret. If the redesign is considered a race, the Commonwealth team won, as they completed the Ram pilot 3 months before the T6, and 110 units had been manufactured before the first production Sherman was accepted on the last day of February 1942. However, the Ram was designed around the British 6 pounder gun. The turret ring was only 60 inches in diameter, and crew conditions inside the turret were described as "cramped." This would only have been exacerbated had it been decided to fit the Ram with a 75mm (or larger) gun. Ultimately, the T6 was considered by all parties to be the better design. While the Ram was not used as a "combat tank," it served admirably as a training vehicle for the rapidly expanding Canadian Armoured Force. When sufficient supplies of Shermans became available, Rams were converted to various "funny tank" roles such as Command and Observation Post vehicles, and most notably, "Kangaroo" armored personnel carriers. 1950 Rams were produced from December 1941 through July 1943. The photo above shows the assembly line at Montreal Locomotive on October 26, 1941.


T6

An unfortunate gap exists in the history of the Sherman's development. To date, it would appear that no photos have surfaced showing the welded hull pilot that was assembled at Rock island Arsenal. (An RIA teletype dated October 18, 1941 stated that the pilot was "now under construction here.") For the time being, we will have to make due with an image of a model photographed at Pullman Standard on November 27, 1941. The model differs from the T6 pilot in that it was made WITH the assistant driver's hatch, and WITHOUT the side doors. The turret appears to be identical to the T6's after the M3 cupola had been eliminated, but before the split hatch was installed. The original T6 gun shield was replaced with the improved M34 gun shield (1), although the M34 rotor shield was not installed on the model. A single ventilator (2) can be seen next to the driver's hatch. In production, small hatch welded hull Shermans would have 4 ventilators: 1 beside each of the drivers' hatches, one on the turret and one inside the turret splash on the right rear. We are hoping some photos or a report about the welded hull pilot will be found eventually. It is possible that it was shipped to Pullman or another firm for use as their production pilot.


T6

A 1-piece differential housing was designed at the same time as the T6. The prototype was shipped to APG in early 1942, and installed on the T6 in March. It was intended that the simplified 1-piece housing would quickly replace the 3-piece, but some of the power train manufacturers, including Chrysler, stated that they could not retool without disrupting production. Thus, the 3-piece differential housing continued in use alongside of the 1-piece, and was not completely replaced until the end of 1943. Note that the prototype had a slightly raised "lip" well forward of the bolt strip. In this photo, it would appear that a section of cast armor containing the fixed MG apertures was welded into the hull. Also, note the added MG dust cover fittings. The bow MG dust cover became standard on Shermans by the Fall of 1942.


T6

As Sherman production ramped up in the Summer of 1942, the "M4, #1, Shop Pilot" was used in various tests, including steel track trials. The mishap that was photographed on July 11 1942 permits a good view of some of the changes made to the T6. A standard M4 / M4A1 armored air intake cover (1) was added. The turret was retrofitted with a number of items, including the commander's split hatch (2), blade sight (3), lifting rings (4), ventilator (5), spot light fitting (6) and (hole for) the loader's periscope (7). The T6's original short M2 gun was replaced with the M3 gun. Most likely the entire original gun mount was changed out for the standard production M34 Combination Gun Mount. Note how the original T6 gun shield was replaced with the M34 gun shield (8), although the M34 rotor shield was not added.


T6

In this view one can see that the original pepperpot exhaust was replaced with the so called "high" exhaust pipes (1) and the externally mounted air cleaners (2). The "round" as opposed to "square" Vortox air cleaners are not seen in period photos until the end of 1942, so these may have been test parts. The pepperpot holes were blanked off (3). Production hull castings were extended down about 8 inches in the rear directly over the tracks. The open area (4) was blanked off, and an elongated oval hole was machined out on the upper hull to provide for grouser storage. One can see rivets on the belly plate of the T6's M3 Medium lower hull. The military characteristics of the M4 called for "fabrication by welding" of the lower hull. While the vast majority of Sherman lower hulls were welded, Pressed Steel Car continued to use riveted lower hulls until the Spring of 1943. We have found no explanation for this exception, but it may be that PSC was tasked with using up the remaining supply of M3 Medium riveted lower hulls once production was terminated. Company correspondence mentions that the interior protruding rivets of "our special lower hull" necessitated a slight divergence in the standard arrangement of the Sherman's internal stowage.


T6

In early 1943, APG was directed to design some Field Modification Kits that could provide the differential housing with greater protection. The welded construct shown above was rejected because it extended beyond the front of the tank. After its days as a test vehicle, the T6 Pilot was sent to the Ordnance Museum for "historic purposes." The photo above is dated February 1947, and is perhaps the last known image of the tank. Notice that the original T6 gun shield was reinstalled, with lifting rings retrofitted. The T6 may have been destroyed along with many other of the Museum's AFVs during a Korean War era scrap drive. It is hard to believe that the curator, Colonel George Jarrett, would have permitted this had he had a say. We continue to hold out a small hope that it is stashed away somewhere at APG.


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