The SM420 is a heavy duty truck transmission, produced between 1947 and 1967 for GMC and Chevrolet pickups, and through the early 1980s for military applications. It is praised in by the off-road community for its 7.05 : 1 "Granny Low" first gear, as well as its superior strength and trail worthy ruggedness. The top loaded transmission was used in a number of heavy applications, including buses, industrial equipment, and military trucks with up to 2 ton ratings. In GM applications, the transmission laid the foundation for the similarly popular SM465 for the 1968 model year. The "SM" in SM420 stands for "Synchro-mesh", although the transmission is only synchronized in gears two through four, which are helical cut. First and reverse gears are spur/straight cut and non-synchronized. The most common applications use a 10 spline input and output, though some 2wd applications featured a rare 35 spline output shaft. For four wheel drive applications, the transmission was only compatible as a divorced transfer case setup. A backup light switch was added to the 1967 model in order to meet the new Federal mandate requiring backup lights to be activated when the transmission is put into reverse.
Muncie SM420 Transmission Ratios
| 1st | 2nd | 3rd | 4th | Reverse |
| 7.05 : 1 | 3.57 : 1 | 1.70 : 1 | 1.00 : 1 | 6.78 : 1 |
Muncie SM420 Transmission Specs
| Type | Heavy duty 4 speed truck transmission | |
| Applications | • 1947 - 1967 GMC/Chevrolet pickups, buses, military vehicles, and industrial applications • Industrial and military applications through the early 1980s |
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| Case Material | Cast iron w/ cast iron top cover | |
| Case Length | ~ 10.70" | |
| Input Spline | 10 spline, 1 1/8" diameter input shaft (for GM applications), 6.50" stick-out | |
| Output Spline | 10 spline output shaft is the most common | |
| Weight | ~ 150 lbs | |
| Fluid Type* | Straight 50W or 80W-90 gear oil Recommend Amsoil Severe gear 80W-90 full synthetic gear oil |
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| Fluid Capacity | ~ 5.5 quarts | |
| Identification | Top loaded transmission w/ driver side PTO plate. Reverse is to the driver side and down, standard H pattern 1-4. |  |
*Required fluid type and fluid capacity may vary with application and model year - refer to vehicle's owner manual.