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M931A2 6x6 TRACTOR

M931A2 6x6 tractor, as photographed at the Las Vegas National Guard Armory in the mid-1990s. Photo is scanned from a print.

The M931A2 is the tractor version of the AM General-built M939-series of 5-ton capacity, six-wheel-drive trucks first introduced in 1983. The M939-series of trucks are general-purpose military vehicles, designed primarily for off-road tactical use, with a top speed of 65mph and featuring an automatic transmission. The M939-series trucks were intended to supplement, not replace, the older M809-series 5-ton tactical trucks in the US Army & Marine Corps inventories. Some of the more notable improvements & upgrades the M939-series trucks featured over their older M809-series brethren were: Automatic transmissions; Improved power steering systems; Complete air brake systems (wedge brakes); Improved cooling systems; Improved electrical systems; Three crew member cab; Tilt hood; Hydraulically-powered front winch (not all models equipped with winch). The M931 (without winch) & M932 (with winch) are the tractor variants of the M939-series, equipped with fifth wheels to tow semi-trailers and loads of up to 37,000 pounds cross-country and 55,000 pounds on highways

The M939-series trucks initially came equipped with 10.00x20 non-directional cross-country (NDCC) bias-ply tires (the classic military "knobbies"), but in the 1980s most were upgraded to A1 status, which included Super Single tubeless radial tires (such as this tractor pictured above) and an upgraded diesel engine. However, the original non-directional bias tires were found to be particularly dangerous in panic-braking situations on wet pavement and other low-traction situations, so the Army & Marine Corps were compelled to add various Anti-Lock Brake packages in later years; I particularly remember several fatal accidents around Camp Lejeune, North Carolina, in the late 1990s involving M939-series trucks with Super Single radial tires, and one significant finding was that fitment of Anti-Lock braking packages would significantly reduce these fatal occurrences. The US Army commissioned a study that found from January 1987 through June 1998 the M939-series trucks made up only 9% of the total Army Military Vehicle (AMV) fleet, but were responsible for 34% of the AMV fleets' accidents resulting in fatalities. Comparison of US DOT accident statistics to M939 accident statistics showed that over a 10-year period "the fatality rate of occupants of the M939 averaged about 30 times higher than the fatality rate for occupants of comparably sized commercial trucks. Also noted in the GAO report were the results of a TACOM tire study conducted from 1995 - 1997. That study concluded that the M939s were being used on road more than originally planned. The original intent was to drive the M939s on highway and secondary roads 60% of the time and 40% off road. Current data indicates the M939 trucks are being driven on roads 80-90% of the time and only 10-20% off road." As a result, the Army passed an order that these trucks could only be driven at 40mph on highways in an attempt to limit accidents, injuries, and fatalities. [All statistics and quotes from]

Further, the study found that "the accident scenario for all M939 trucks occurs during panic stop situations and is worsened on wet pavement. In panic stop situations the trucks wheels lock up causing engine stall. This causes loss of power steering resulting in uncontrolled skidding creating accident and roll-over situations. Extensive testing of ABS for this truck has shown that ABS will eliminate 100% of the engine stalls and wheel lock-up regardless of the skill level of the drivers. The accident scenario for M939 basic vehicles with NDCC tires (the non-directional "knobbies") occurs during panic stop situations on wet pavement. The front wheels lock up, the NDCC bias tires react like ice skates and stopping distance is increased by 245-320 feet over trucks with radial tires." [All statistics and quotes from]

The A2 versions featured various improvements on the A1 series trucks, the most notable being the Cummins 6CTA 8.3L inline-6 turbocharged diesel engine. The 6CTA was lighter than the venerable NHC-250 (non-turbo) engine it replaced but produced more horsepower and torque in stock form. The other big addition with the A2 series was the standard addition of Central Tire Inflation System (CTIS), evidenced by the triangular-shaped sheet metal on each outer wheel that covers the CTIS plumbing. The "Super Single" radial tires on 2-piece steel wheels are a staple of the A2 and most A1 series trucks; the tire size is 14.00x20, which is a 49-inch tall tire on a 20-inch diameter wheel. The 2-piece wheels feature an O-ring sandwiched between them, enabling tubeless radial tires to be installed on them. All M939-family trucks use the venerable Budd 10-lug wheel stud arrangement, which means stud-piloted wheels; also, the lug nuts will always tighten to the front of the truck, and loosen to the rear (yes, the driver's side wheel studs & lug nuts are left-hand threaded). The A2 series also uses the Allison MT-654 automatic transmission.

In the 1990s the US Army began fielding the Stewart & Stevenson-built FMTV 2.5-ton and 5-ton trucks, which will eventually replace the M939-series trucks in the AMV, while the US Marine Corps adopted the Oshkosh-built MTVR 7-ton trucks to replace their fleet of M939-series trucks.

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Silver State Specialties was created in 1999 to offer quality resin accessories, replacement parts and conversion kits, suitable for many 1/25-scale plastic model kits.  SSS specializes in accessories and parts for plastic model pickup, SUV, 4x4, and heavy truck & trailer kits.  1/32-scale and 1/35-scale parts and accessories will be available in the future for model truck kits, such as the 1/32 Monogram Snap-Tite and ERTL snap-together kits, as well as 1/35-scale Revell, Italeri, AFV Club, Trumpeter, Airfix, Heller, Tamiya, and other military model truck kits.  Some of these smaller scale model kits are quite well detailed, and a wide variety of photo-etched, resin, and other accessories are widely available for most of these military truck model kits. What's even more exciting is the line of 1/24th & 1/25th-scale military model kits and conversions we are in the early stages of developing, to nicely compliment your ERTL, AMT, Italeri, Revell, and other brands of truck models.

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SSS also has master patterns partially completed for many 1/16-scale model truck accessories and conversion kits, mainly for the Monogram-Revell 1/16-scale model truck kits.  Some projected accessories for these 1/16-scale kits are front drive axles, different wheels and tires, air cleaner accessories, pusher and tag axles, oil filters and engine accessories, and possibly different engines.  Cab & hood conversion kits remain a possibility for these monsters as well, but are still years away from being offered as there are more pressing projects to push forward with first.  But someday they should be available!

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