What year sm420 are these? Pics

[Benjitis1]

Got 2 SM420s im lookin at. Have pics and numbers for both. The numbers on the tranny on the left are C296 7GM 3837334, and looks like it has ribs on the side of the case. Numbers for tranny on the right are C169 4GM 591665. Trying to decipher what the years are for each. Middle pic is the tranny on the left in first pic. Im trying to get an sm420 that doesnt have the oil slinger groove, dont need oil spilling onto clutch, or anywhere else.Also a pic of gears behind PTO cover? They look okay? If one of these will work, i'll go look at them and inspect closer( rings, gears, etc), but they are 2 hours away, so id like an idea before i spend $$$ on fuel.

 

[vtcruiser60]

From the Novak website:

Identification

The SM420 transmission is 10.7 " long and features a cast iron top cover that is retained by eight bolts and a main case of cast iron. The case itself has a prominent bulge on the passenger side in order to house the reverse idler gear. Later versions had two horizontal ribs cast into the passenger side of the case.

The common GM SM420 has a 1-1/8" diameter, ten spline input shaft that has a stick-out of 6-1/2" from the front face of the transmission. The pilot tip is ~19/32" in diameter. The front bearing retainer flange measures 4-11/16" and the bearing retainer tube is 1-3/8" in diameter. The front bolt pattern is the consistent GM Muncie / Saginaw four-bolt pattern as found on GM bellhousings and transmissions from the 1940's through the 1990's.

The SM420 most commonly has a large, 10-spline output mainshaft for both 2wd and 4wd versions. Some fairly rare 2wd models (likely the 2-ton versions) featured a 35-spline and a male threaded output. These versions also have a rare, 22-spline first gear journal, instead of the usual 11-spline "skip-tooth" journal.

The last year of the SM420 (1967) had a backup light switch installed in the top cover to meet that year's federal mandates.

* 1947-1953:
1. No ribs on the case
2. Grooved worm on the input shaft instead of neoprene seal
3. Synchro rings all differ from each other. 1st-2nd synchro ring is bronze
4. Weep hole at 6 o'clock on the bearing retainer snout
5. Six digit casting number on the case, usually like "59xxxx" e.g., 591665.
* 1954-1963:
1. Same as previous but uses a neoprene input oil seal
2. No weep hole at 6 o'clock on the bearing retainer snout
* 1964-1967: Same as previous except
1. 1st-2nd synchro ring is aluminum and 3rd-4th synchro rings are identical to each other
2. Seven digit casting number usually like "37xxxxx", "38xxxxx" or "39xxxxx"
* 1968-1981: Military Con-Diesel version. Features the strongest case and optimized internals. Usually equipped with unique top cover, but can be converted to standard top-shift control.

 

[vtcruiser60]

The ribbing on the unit to the left means it is a 1954+ vintage sm420. The serial number starting with a 38...confirms that it is a post 1964 era tranny. If it has a back up light switch in the top cover you are in good shape.

As for determining whether it is a good tranny or not, someone else will have to weigh in.

 

[Poser]

The unit with the ribs will have the seal in the input bearing retainer for sure, as it did from the factory.

The other can be verified by removing the four bolts on the retainer.

 

[vtcruiser60]

Steve...how big a problem was the oil seepage from the early sm420 series trannys? I know Novak sells an updated retainer to correct this, but it is awfully spendy.

 

[Poser]

Steve...how big a problem was the oil seepage from the early sm420 series trannys? I know Novak sells an updated retainer to correct this, but it is awfully spendy.

If you are getting an oil transfer issue, ie the gear oil from the transfer case is making its way into the transmission and over filling it, then it will weep out of the input shaft when sitting stationary over a period of time. If oil transfer is not an issue and the transmission oil level is correct, it is not a factor.


However, the reason I finally decided to change mine out was because the clutch disc became saturated with gear oil after this:

I would like to find a known-good late 420 (1965-1967) and replace the one in my truck currently. I have a miss-matched top cover and occasionally, the transmission will stick in third gear.

 

[Pin_Head]

The one with the ribbed case is a 67, the last year of production in GM products, or a gamma goad tranny. Can't tell about the other one from the picture, but it should have a date code casting above the 59xxxx part number.

 

[Benjitis1]

So the C296 stamped on the left tranny means a 1967? The date code for the tranny on the right is C169. Under C169 is 4GM. Under that is 591665. The guy just emailed me the info of a 3rd tranny he has, the date code is A288. With the 1GM stamped under that, then 591953. ANy idea on years for this? He "thinks" this one came out of a 60's something, van and was in good working condition, just chatters a bit, which sounds like normal for this tranny.

 

[shelfboy1]

i read somewhere that the 420 was made from 41 to 47 and the 465 was after 47 .it looks like that info was wrong think it was military vehicles .could some one confirm when the 420 was made .i have several gm 4 speeds i thought were 420s out of 42 to 44 military trucks and they dont look like the one in the photos here.hope this isnt a hijack thanks

 

[Pin_Head]

The date codes I have seen are a letter for the month (A-L), two digits for the day and then 2 digits for the year. D2163 would be April 21, 1963. There is also a date code on the top cover casting. Maybe they changed the date code somewhere along the line. It is kind of hard to rationalize your codes with the known years of production.

 

[Pin_Head]

i read somewhere that the 420 was made from 41 to 47 and the 465 was after 47 .could some one confirm when the 420 was made .

The 420 was used from approx 1947-1967, and longer in military gamma goats. After that, the 465 was used. The predecessor to the 420 was a non-synchro "crash box", that resembles the 420. It was used from the mid 30s up to about 1947.