SM420...
SM420 Transmission Conversion

By Jim Van de Ven


Last updated: 3/25/99
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jim_vandeven@hotmail.com

Reasoning:
The SM420 has a first gear ratio of 7.05:1. When used with a 3spd transfer case and a 4.11 ring and pinion ratio, your final crawl ratio will be 67.5:1.

Availablility:
The SM420 was available from 1947-1967 in 1/2 to 2.5 ton chevy trucks. The only change made was in the last year when a reverse switch was added to illuminate reverse lights. Some of the parts for the SM420 have been discontinued by GM, so finding some of the parts, such as gears, is difficult.

Required Parts:
To adapt the SM420 to the 2F (or F) and 3spd t-case you need:
3spd bellhousing (will bolt up to 2F)
3spd flywheel and pressure plate (I'm not sure if the 4spd will work)
Chevy 11" clutch disc (from something like a 75 4wd 1/2 ton chevy pu)
Special pilot bearing. It is the same inner (I think) and outer diameter for the 3spd tranny, but the inner shaft is about 3/8"
    longer, so it sticks out further.  This is necessary because the input shaft of the SM420 is a bit shorter than the 3spd
    Cruiser tranny.
Sleeve around the input shaft of SM420, where the throwout bearing rides.
Ring around the front bearing retainer of the 420.   It is about half an inch thick and has a wall thickness of about 1/2".  It
    fits around the front bearing retainer and assures that the 420 sits snugly in the 3spd bellhousing.
On the back side all you need is the 3/4" adapter plate that you bolt to the back of the tranny, then bolt the t-case to that.
The gear that rides on the output shaft of the 3spd tranny will slide right onto the output shaft of the SM420.
 
Discussion:
One thing about the 3/4" plate style adapter is that it rotates the t-case down so that the front output shaft flange is able to clear the tranny (it is still a close fit though and some people have to do a bit of grinding on the tranny).  This means that your t-case will sit about 1.5" lower than stock, which I find very helpful with the driveshaft angles on my spring-over.

Another thing that will have to be delt with is the linkage for the transfer-case. I have seen a few people build their own shifters, one type with two sticks, and the other like the Toyota single shifters. If you have an early model truck (like I have) you can use the stock vacuum shift for the 4wd, and just lengthen the rod going to the low range lever.

If you have a Cruiser that originally came with a 4spd tranny, you will have to address the problem of having different driveshaft flanges between the 3spd t-case and the stock pinion flanges.
There are 3 basic ways that I know of to make the change.
1. Split the driveshafts at the slip-yoke and use the 3spd driveshaft section at the t-case end and the 4spd driveshaft section at the differential end.  Make sure to keep the U-joints in phase and to have the driveshaft balanced before you install it.
2. Change the pinion flanges from the 4spd to the 3spd, they will bolt right in.  Then use the 3spd driveshafts
3. Change the output shaft flanges on the transfer case.  You will only be able to do this on the rear.  If you were to do this on the front I believe you would run into problems with interference between
the flange and your SM420.

 I think that is basically all of it.  I got my adapters from Classic Cruisers in Salida, CO.  They were the most helpful and seemed to be  reasonably priced.
 


This conversion is at the owners risk and I assume no part in any ill fate that might occur because of my directions. Use at your own risk.

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