M3A1 on proving track at American Locomotive Company 1943
The M3A1 medium tank emerged as an improved version of the M3 series, introducing key changes in the manufacturing process, particularly the use of cast armor. This innovation allowed for the production of a streamlined upper hull, reducing certain risks and offering a more advanced design while keeping the tank’s weight and protection comparable to previous rolled steel armor designs.
Key Features and Changes in the M3A1
Cast Upper Hull:
- One of the most significant advancements in the M3A1 was the use of a cast upper hull, which replaced the riveted hull found in earlier M3 tanks. This shift eliminated the hazard posed by rivets fracturing under impact, reducing the risk of rivet shards injuring the crew inside the tank.
- The cast hull provided a smooth, streamlined shape, which offered the same level of protection as riveted or rolled armor without increasing the tank’s overall weight.
Design Modifications:
- Hatch Adjustments: In the M3A1, the right-side roof hatch was repositioned to an angled location toward the rear of the tank where the hull sloped down. On early models, the hatch was hinged at the front, which required it to swing upward and forward, a cumbersome design that was later corrected by moving the hinges to the rear.
- Pistol Port: The pistol port on the rear wall of the fighting compartment was eliminated with the introduction of the cast hull.
- Side Doors: Early M3A1s featured side doors, but these were later removed from production models, retaining only a pistol port on the right side.
- Ventilators: As with the original M3 tanks, three ventilators were installed on the M3A1. However, because of the shifted roof hatch position, the right-side ventilator was moved forward, just ahead of the hatchway.
Production and Use
- Production of the M3A1 began at the American Locomotive Company in February 1942, continuing until August 1942. A total of 300 M3A1 tanks were produced.
Diesel Engine Testing: The Guiberson T-1400-2
In July 1941, a test program was initiated to evaluate the performance of the Guiberson T-1400-2 diesel engine in the M3 series tanks. The M3A1 was included in this testing, as the Ordnance Committee had initially specified that the M3, M3A1, and M3A2 tanks could be powered either by the Wright R975 radial engine or the Guiberson diesel engine.
Performance:
- The Guiberson diesel engine offered several advantages over the radial engine, including greater torque at lower engine speeds and double the range due to its lower fuel consumption.
- Despite these benefits, the Guiberson diesel suffered from reliability issues, requiring frequent repairs, which led to the discontinuation of further testing. By June 1942, the Adjutant General ordered that the Guiberson engine be phased out as soon as alternative engines became available.
Limited Production with Diesel Engines:
- Although the American Locomotive Company initially planned to use the Guiberson diesel in their production run of the M3A1, only 28 M3A1 tanks were ultimately completed using this engine before its use was halted.
The M3A1 represented an evolution in tank design, introducing cast armor to improve both structural integrity and crew safety. Though it saw only limited production, it laid the groundwork for future U.S. tank innovations. The tests with the Guiberson diesel engine, while promising in terms of fuel efficiency and performance, were ultimately hindered by reliability problems, leading the Army to discontinue its use.