1. GM SM420

GM SM 420 Transmissions  

In 1947 General Motors’ pickup trucks received a transmission that rural America was anxious to receive. The transmission was the Sm420. It was a stout 4 speed transmission that could handle heavy abuse, and most importantly it had an ultra-low granny gear ideal for the low speeds needed for a strenuous agricultural life.  Fast forward some 60 years later and the transmissions are still popular as ever.  Only now the farmer has been replaced by the avid off-roader and wheat fields have been replaced with Moab rock.  

The Sm420 was introduced with an impressive 7.05:1 first gear. A first gear that still claims fame today as the lowest first gear ever offered.  The cast iron case measures at 10 1/2" length, the transmission top cover is also cast iron and is attached via 8 bolts. The input shaft measures at 6-1/2" in length with 1-1/8" 10 spline. .59 pilot tip and standard GM bolt pattern.  SM420 tranny

Early units of the SM420 contained threads on the input shaft and did not have a front seal.  The threaded input shaft transmission is not really a factor for most conversions; however, the threads will interfere with conversions requiring a retainer change.  The only current application requiring a retainer swap would be the SM420 to factory land cruiser engine.  This swap would require an input shaft change to the newer style. 

The SM420 was offered in both 2wd and 4wd vehicles. The transmission was always used with a divorced transfer case so essentially all SM420 transmissions were 2wd drive. The rear output shaft uses a large 10 spline; however, occasionally we come across a rare 35 spline.  In most circumstances we have been able to accommodate customers with this rare transmission by using input applications from our SM465 adapters.  

 The transmission retainer has a diameter of 4.686. When the SM420 was replaced by the SM465, the retainer diameter was enlarged to 5.125". Advance Adapters offers a reducer ring under part#716041 if the SM420 is used with the larger bored SM465 bell housing.  The transmission can further be identified by a driver side PTO port.    

 The SM420 also has significant large round bulge on the passenger side of the transmission.  The protrusion was required to allow internal clearance for the reverse idler shaft.   It is this hump that does not allow us to offer any adapters shorter than 4". Some conversions still may require a small amount of grinding for front driveshaft clearance.  Later versions had two latitudinal ribs cast in the in the same area. Over the years we have seen a few very thin adapters hit the market. These ultra-thin adapters require excessive amount of grinding to the point that the walls of the case become thin enough to actually upset the overall integrality of the transmission.  

The SM420 is a fully synchronized transmission in second through fourth gear. All gears are helically cut with the exception first and reverse.  All gears on the counter shaft are removable except 1st. In 1967 the U.S government required all new vehicles to have a backup light. The backup light switch can be located on the top cover and since this was the last year of the transmission locating this version may prove difficult. All SM420 transmissions have casting numbers located on the driver side of the transmission. The exact year of the SM420 is not typically important for ordering adapters; however, the coding information provided below may assist you in the event you may need to purchase some internal components. The SM420 has several casting numbers on the driver side of the transmission. A six digit casting number on the case, usually like "58xxxx" or "59xxx" indicates this transmission was built during 1947-1953. A seven digit series was used during 1964-1967. These casting numbers were usually cast with a "3" like "37xxxxx", "38xxxxx" or "39xxxxx". You can also determine the date of manufacture of your SM420 by decoding the casting mark on the top of the driver’s side.  These were smaller numbers cast above the large "GM5" stamp. The code starts with a letter that stands for the month (A-L), followed by two digits for the date and two digits for the year.  For example below, H3061 means August 30, 1961.  

Forward Speeds .... 4  

Dates Produced .... 1947 - 1968   

Manufacturer .... General Motors    

Case Length .... 10.50  

Bellhousing Length ... 6.50   

Over All Length .... 17.00   

Case Material ... Cast Iron      

Input Shaft Spline .... 1-1/8 10 spline  

Length .... 6.50   

Pilot Diameter .... .590   

Length .... 1.00      

Output Shaft Spline .... 10     

1st Gear Ratio .... 7.05   

2nd Gear Ratio .... 3.57   

3rd Gear Ratio .... 1.70   

4th Gear Ratio .... 1.00   

Reverse Gear Ratio .... 6.78   

Transmission Weight .... 150   

Fluid Capacity .... 5.5 Qts.  

Power Take Off Capable .... yes 


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