The Stovebolt Site Forums The Shop Area Driveline Saginaw 4 Speed

Hello. I am new to the forum and have a 1951 3100. I have a 1955 235 that I am going to rebuild and also want to install a more user friendly transmission. I currently have a stock 4 speed. I was thinking about a Saginaw 4 speed with the Patrick's conversion. I will really only be using the truck around town and at speeds no higher than 65mph. Any advice or recommendations would be greatly appreciated.
Thanks!

User friendly Transmission? The SM420 is a bull dog of a transmission. A saginaw 4 sp is a definite step down in my book. An incredibly weak trans in my opinion. I'd seriously think about retaining this SM420 trans and installing some taller rear gears instead.

Thanks for the quick response. I think the previous owner installed taller rear gears. My issue with the current stock 4 speed (floor shift) is that 1st gear is not really useable and it is pretty sloppy. Would rebuilding it (with different gears?) fix some of that? Thanks again for your opinions.

FZ3,

I have a 1978 El Camino SS that came from the factory with a 350 engine and Sagnaw 4-speed. There has never been a problem with the Sagnaw and they do come in 3 varieties of gear ratios. The Sagnaw is not as robust as a Muncie, but for normal street driving the Sagnaw is fine. The only maintenance mine has ever required is replacement of the rear seal. While I was at it I also replaced the rear bushing.

Awesome. Thanks for the info!

FZ3 , when you say its "pretty sloppy" what are you describing ?

There seems to be a lot of play in the shifter and it is sometimes hard to get into gear (especially 3rd to 4th). I am not sure if this is normal. I would prefer to keep what I have but am just not sure if there is a way to change the gears so that I have 4 useable gears.

Your SM 420 truck four speed trans is just showing signs of old age

being that it's ( 67 ) years old .

There are NO .......magical or imaginary .......changing of the internal gears

that, I'm aware of , the SM 420 first gear is usually used when a heavy load is on board .

Most people drive this transmission ( unloaded ) like a three speed starting off in second gear .

There are many reasons for problems / difficulty when shifting into 3rd gear .

1.) a worn synchronizer cone .
2.) a worn 3rd. gear bushing
3.) a worn clutch hub & or clutch sleeve .
4.) worn out mainshaft bearings.......front & or rear .
5.) a worn thrust washer on the mainshaft .

Only after opening up the trans & thoroughly going thru it can the

problem / problems be diagnosed .

Of course after opening it up.........it opens up a new can of worms .

Sounds like you need a real OLD pro to work on your transmission .

The wobbly gear shifter lever problem is again just signs of old age .

see this stovebolt post :

https://www.stovebolt.com/ubbthread...d-up-4-speed-gear-shift.html#Post1194887


If you think by jumping thru all the hoop's needed to change over to a saginaw

four speed trans is a good idea..........go for it !

I would fix whatever is wrong with the SM 420 .

The SM 420 is a ' beast ' of a truck four speed trans , no transmission was ever

built to last forever & that includes the saginaw 4 speed & the SM 420 .

However......they can be rebuilt / repaired .



George
'54 3600

The SM420 was used from 1947-1967 as I recall. We had a Bearing kit installed in April of last year and it works fine now. it was 145.00 from A1 transmission parts in Denver.
Their number is 303 623 4923. There are other vendors as you will see on this site. Hope this helps Bill

Earlier I said the saginaw was an incrediably weak 4sp. I didn't say this without personal experience/prejudice. In 1980 I auto crossed my 1975 Cosworth Vega. It had about 130 horsepower, a saginaw 4sp and a 3.73 posi rear gear. At the end of the day it was not shifting into 2nd anymore. The next weekend I had a transmission specialist go through it and he replaced 2nd gear, the synchro ,the clutch hub and slider. Two weeks after that back out to the auto cross for some more mild to medium action, second gear and sychro again toasted. It was discussed after that , i was going to have to make a transmission change to a muncie or super T10 if I were to continue autocrossing. The trans had less than 15K on it.

Yak said he runs one behind a 350, if it has lived long he must be driving it very sanely. In a truck with any kind of likewise duty, installing a saginaw in the place of the sm420 would be like going into battle with a jeep instead of a sherman tank. I recently went through 2 SM420s top to bottom that were removed out of 1/2 ton trucks. Each received bearing kits, new lock rings, thrust washers, gaskets and minimal small parts. I run one in my my 53 after removing the standard muncie 3 speed. The same sm420 were used all the way up to military duece and a halfs.
The final gear ratio in both the saginaw and sm420 is the same 1to1 so stepping down to the saginaw makes no sense to me in any way shape or form.

Steve

Thanks to everyone for all of the advice! Since I am planning to restore my truck to original condition (no restomod stuff), I think I will have my original 4 speed and shifter rebuilt (and install a new clutch while everything is apart). Thanks again for all of the responses.

FZ3,

I don’t know about your pickup, but when new my now restored ’36 Chevy had a 3-speed manual transmission as standard equipment and a 4-speed was an option. The 4-speed was the same as the one Chevy put in 1 and 1 ½ ton trucks. It seems to me that a pickup truck that will be restored and used for pleasure driving only would benefit from a transmission that is better suited to street driving than to a large truck hauling heavy loads. In the ’36 the 3 and 4-speed transmissions are interchangeable. So when I restored my ’36 it seemed like a 3-speed was a better choice since it was lighter, more compact, shifts much more easily and has gear ratios that are suitable for cruising over to the park for a picnic with only a cooler and a couple of lawn chairs as the “load” in the bed.

As others point out 1st gear in the 4-speed “beast” you presently have is not useful in street driving, making it a 3-speed in practical application. When I converted my 1971 Dodge van that I bought new and still have from the factory 3-speed manual transmission to a New Process Model 445 “Camper Special” 4-speed transmission I got gear ratios much more suitable for towing my travel trailer but there were trade offs. The 1st gear in that 4-speed is not useful in street driving and I find myself using only 2nd and 3rd gears when putting around town without the trailer and 2nd, 3rd and 4th when driving without the trailer on the highway. Second, truck 4-speed transmissions with their big, robust gears and synchronizers don’t flick easily through the gears like lighter duty transmissions. In the case of my van the trade offs are worth it because of the much improved towing but would not be otherwise.

Anyway I thought I’d share those thoughts with you.

The sag is not a terrible trans. I have had one for quite a few years behind a 261 motor. its really all about gear multiplication. The vega version of the sag has a 3:11 first gear. So if you wanted to do a little highway driving, you could use the sag with say a 3.08 rear end and have a 9.57 effective first gear ratio. I have always thought that an effective first gear ratio of 10.00 or in that neighborhood is pretty good to get a lower powered car or truck off and running from a stop. Anything lower, and you have issues with lugging the motor and stressing the crank so 9.57 is not terrible and you have decent highway gears. So if the rear was say higher (numerically), 4;11 rear say, you end up with a 12.78 effective first gear ratio which may be a little too steep. Like the others have said, in fourth gear, they are both 1:1.

So what gear is actually in the rear of the truck, and what is the first gear ratio of the stock transmission?