Production of M18 Hellcats ran from July 1943 until October 1944, with 2,507 built. Several changes were made during production. The first 684 M18s experienced problems with their transmission gear ratios which meant they could not climb steep slopes, and were returned to the factory for modification. Most of these early vehicles remained at the Buick factory after modification. Ten were later converted to T41/T41E1 command vehicle and prime mover prototypes, and 640 were converted to M39 Armored Utility Vehicles. The rest of the M18s built featured an improved transmission. The 76 mm gun M1 fitted to most Hellcats kicked up large amounts of dust when fired. This was enough to impede the vision of the crew, who had to wait until the muzzle blast cleared to fire accurately again. To solve this problem, the 76 mm gun M1 was replaced with the muzzle brake-equipped 76 mm gun M1A2 as soon as it became available; in the interim, the M1A1C gun, threaded for a muzzle brake but not so equipped, was used. Beginning in June 1944, roughly the last 700 Hellcats received the M1A2 gun. Hellcats with serial numbers 1350 and below had the naturally-aspirated R975-C1 engine, which produced 350 horsepower. The ventilation grate for the transmission and oil coolers behind the driver's hatch on these early Hellcats had a bulged shape, protruding above the line of the upper hull. M18s with serial numbers 1351 and above had the internally-modified supercharged R975-C4 engine, which produced 400 horsepower. At roughly the same time as the change in engine type, the shape of the ventilation grate was changed to be flush with the upper hull.