The Stovebolt Site Forums The Shop Area Engine Shop Need Trans 101 help for "420" Granny Low

I have a 55 3/4 ton in transition and completely apart at this point and I'm back in the groove of working daily on it.

At the end of the restoration I won't want the existing rear end. Why? Mostly because it's 8 lug and it will only go abou 5 MPH in first gear. I believe some here have refered to this tranny as a 420(4 speed on the floor) and others a Granny Low. First off Are these the same animals? The real question is..Can I make this first gear practical and usable by going to a different rear end? One with a differenet ratio.

Thanks,

Keith

1st. gear on that transmission is designed to get a heavy load moving. It is not a regular road gear. The SM420 transmission does not have a synchronized 1st. gear, so the vehicle must be at a complete stop before engaging it, or severe grinding will result.

If you're planning to replace the 8-lug rear end with a 5-lug axle, you'll also need to swap front hubs for ones with the same 5-lug bolt pattern. There are plenty of options available for higher rear end gear ratios, and you could use a 5-speed overdrive transmission, or a Muncie or Saginaw 4-speed with an appropriate rear end gear. You'll be basically turning your truck into a car by eliminating the load-pulling capacity of the 3/4 ton drivetrain and the weight capacity of the heavy-duty wheel bearings.
Jerry

Jerry’s pretty much got it covered Keith. I would just add that your Muncie SM420 has a 7.05:1 first gear, not much that a change in rear end is gonna do to help that. The later four speeds would have first gears up in the 4.xx:1 range.
Take a look at this site for some very good information on the transmissions that we see in these old trucks:
http://www.novak-adapt.com/knowledge/sm420.htm
http://www.novak-adapt.com/index.htm

Denny Graham
Sandwich, IL

Keith, The SM 420, the "Granny Low" as you know it and "Granny Gear Trans" are all the same trans altho those terms are also applied to others out there, like the SM 465 Trans.

My old '64 C-30, 6-292 also had a SM 420, but I would still use 1st on the road even when empty. I would just use it to get the truck rolling a bit, then shift. I'm from the school that believes in keeping very little load on the crank, plus I like to shift. You can change the rear like mentioned above, but the only reason I would possibly want to change a rear is because of top end speed. How are your RPM's in 4th gear when the truck is going about 60 MPH?

Some good information here..Thank you boyz.

Lincoln during the 25 years I drove this I skipped 1'st gear. Will be doing the 5 lug or possible 6 lug disc brake conversion. "Turning my truck into a car"-I like that thought.

Question: is a 1/2 ton likely the same rear end that I have?

Denny -thanks for reading my mind..I'm was wondering if this was something that could be fixed with the rear end. Any chance a later model tranny would bolt up to my 235?

Bill -been about 10 years sin this was a daily driver I dont remember the top speed and rev's..
I do remember that whatever speed I drove it ...It felt way faster..and maybe a little scarey.

Keith

The early 1/2 ton rear ends were 6-lug, and didn't offer much better gear selection, 3.90:1 instead of 4.56:1 or lower. There are plenty of suitable 5-lug later model rears that work well, clear up to 3.08:1 or higher. A Chevy Astro van rear has a pretty good ratio selection, and about the right width, etc. There should be lots of threads on the subject if you use the "search" option. If you're going to eliminate the 8-lug rear axle and front hubs, it makes sense to upgrade to a much newer axle and brake system, possibly 4-wheel discs, or at least a power-assisted disc front/drum rear setup with repair parts more available than the 50-year-old original brakes.
Jerry

Greetings,

This is slightly a bit off topic. I, too, have this type of transmission in a '68 C20 and am having a problem with it. Recently, I shifted the truck into reverse and the shifter became stuck in what appeared to be neutral. The shifter seems loose in that position but the tranny is stuck in 4th gear. I am hoping that the internal shift linkage is out of whack rather than the whole tranny. Any suggestions? Thanks!

Daniel

Daniel;
The top of your transmission has all the shift links in it. there is something loose or worn in there. You can get at it by removing a floor panel and knocking the top inspection cover (with the shifter) off. You'll need your manual or the service of a tranny shop to align everything before reassembly.

Keith;
I changed my diff ratio from the original 5.14 to 4.10 from a '72 3/4-ton, and it changed the whole character of the truck. I use low in my SM420 to get up a boat ramp or to climb a sidewalk, and to get a big load moving up a grade. Otherwise, I still use only the top 3 gears, but now I can run 55-60 mph all day, which was out of the question before. The engine is still the original 216.

Hotshoe-Interesting...We toss those numbers around a lot..But it's hard to translate what
a change of a few ticks will feel like. I'm told I have a 7.01 1st at this time.

Thanks for the human side..

Keith

You boys forgot the old Rock-Crusher Pre-SM420 4 speed...

The "Original" Granny First Transmission.



Granny gear has its place, and as Hotshoe mentioned, I primarily use it as a three speed (2nd, 3rd, 4th) with 1st reserved primarily for pulling up trees and changing the rotation of the planet.

I rather like it... It comes in handy.

You can downshift into first using the double-clutch three-step, but you need to get the truck down to about 5-10 mph (depending on the rear end and engine) to do it.

Dmcneil you should have the later trans. It is a SM465. They changed in 68. You can pull the shift lever out by pushing down on the coller & turning it CCW about an 8th of a turn. Take a big screwdriver & stick it down in the hole & move a rail till all the slots line up. You might see whats wrong or put the lever in & try it again. If you have to pull the cover the reverse gear has to be half way engauged to pull it or put it back on.