1955 1st series 4 speed tranny

[akeane01]

hi,

I have a 1955 1st series C3100 truck with a 3 speed. I would like to convert that to a 4 speed on the floor instead. Does anyone know what transmissions will fit the truck? I think there was a 4speed option on the original truck..

thanks

[raycow]

Any 1955-67 SM420 transmission will bolt to your stock bellhousing and fit your existing clutch. Some of these from larger trucks had a parking brake mounted on the rear. You don't need this, but it won't cause any major problems as long as you remove the unneeded parts. Avoid the 54-earlier transmissions intended for a torque tube as these will need modification to work with open drive.

Whichever transmission you get, you will need to replace the driveshaft, and of course cut a hole for the shifter.

Ray

[akeane01]

ok, thanks

I was asking since I saw that blue 55 truck someone found recently in the barn with the 4speed so it perked my interest..

Let me ask this.. Was a 4 speed an option on the 55 1st series C3100 truck?

alan

[raycow]

Originally Posted by akeane01

Was a 4 speed an option on the 55 1st series C3100 truck?

Yes, it was an option on almost any truck that came with a 3 speed as base equipment.

Ray

[akeane01]

ray,

I have been reading posts and yours as well in other areas.

Can you straighten me out on the 4 speed transmission. I think i would like that better than the 3speed onthe column but not sure.

I see that the SM420 works from 55-67 and will bolt up ok. I read that the transmission is noisy and had low gearing. You put in a T5 with a spacer that was needed if I got that all correct.

I am not sure where to get the driveshafts but it seems like it will need to be modified.

I am unsure on direction at this point and ask what you think is the best option for my 55 C3100?

alan

[raycow]

Originally Posted by akeane01

I am unsure on direction at this point and ask what you think is the best option for my 55 C3100?

It all depends on your intended use of the truck.

If you plan to use your truck for towing or other hard pulling, the 4 speed is a worthwhile upgrade because of its extra low 1st gear. Otherwise, you don't really gain much for general driving. When I had my 4 speed I never used 1st gear, so it was effectively a 3 speed.

If you want more comfortable highway driving, consider some kind of overdrive transmission. The T5 is one possibility, but you could also use a 3 speed with overdrive, which was a factory option and would allow a slightly easier installation.

If all you want is to get rid of the column shift, you could install a floor shift conversion on your present 3 speed, assuming you don't have any other problems with it.

My truck originally had a SM420 and a 3.90 rear, which was very unpleasant at highway speed. Any useful rear axle swap would have involved welding on the axle housing, which I didn't particularly want to do. Instead, I went with the T5 because it has an overdrive 5th gear. However, if my truck had come with a 3 speed instead of a 4 speed, I would have replaced it with a 3 speed + overdrive. Btw, I did not use a spacer with the T5, but shortened its input shaft and bearing retainer instead.

For most transmission swaps you will likely need to modify or replace your driveshaft. I got lucky and found a salvage yard driveshaft that was the right length, so all I had to do was swap the yoke. If you can't find the exact length you need, just get one that is too long and have a driveshaft shop shorten it.

Ray

[akeane01]

well, I dont plan on towing or hard pulling at this time anyway.

hmmm.. I never knew there was a 3speed with overdrive? I looked in my 1954truck manual(thats what came ordering the 55 manual) and all i see is the regular 3speed and the heavy duty 3speed. There are slightly different gear ratio's between the two.

can you tell me what tranny this overdrive version is?

[raycow]

Overdrive didn't start until the 55 second series. It was the SM318 with a 4 bolt side cover and the forward part looks just like the 3 speed you have now. The overdrive unit is in the tail housing and the overall length from bellhousing face to rear seal is about 21". That should be the same as your present transmission.

Ray

[akeane01]

ok, thanks

is there a link to figure out the serial numbers on the SM420 transmission?

[raycow]

Originally Posted by akeane01

is there a link to figure out the serial numbers on the SM420 transmission?

I wasn't aware that they had serial numbers. Could you please post an example?

Ray

[OrrieG]

FWIW right now on Boise Craigs list there are two 50's three speeds with overdrives listed for sale, I remember one is a 58. Just search for "chevy" in the auto parts section.

[akeane01]

Ok, thanks on the tranny info..

Ray,

this is what I have found on figuring out the year of the SM420 with casting number. Maybe saying serial number is not valid. Maybe you already know this stuff..

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

1964-1967: SM420
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"


SM420 Trivia
You can determine the date of manufacture of your sm420 by decoding the casting mark on the top of the
drivers side. 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. In the example below, H3061 means August 30, 1961.

alan

[raycow]

Alan, thank you for sharing that info. I knew about the casting numbers and input shaft seal, but not about the synchro rings. That's why I like this board so much. Almost every time I visit here I learn something new.

Ray

[alden]

Originally Posted by raycow

It all depends on your intended use of the truck.

If you plan to use your truck for towing or other hard pulling, the 4 speed is a worthwhile upgrade because of its extra low 1st gear. Otherwise, you don't really gain much for general driving. When I had my 4 speed I never used 1st gear, so it was effectively a 3 speed.

If you want more comfortable highway driving, consider some kind of overdrive transmission. The T5 is one possibility, but you could also use a 3 speed with overdrive, which was a factory option and would allow a slightly easier installation.

If all you want is to get rid of the column shift, you could install a floor shift conversion on your present 3 speed, assuming you don't have any other problems with it.

My truck originally had a SM420 and a 3.90 rear, which was very unpleasant at highway speed. Any useful rear axle swap would have involved welding on the axle housing, which I didn't particularly want to do. Instead, I went with the T5 because it has an overdrive 5th gear. However, if my truck had come with a 3 speed instead of a 4 speed, I would have replaced it with a 3 speed + overdrive. Btw, I did not use a spacer with the T5, but shortened its input shaft and bearing retainer instead.

For most transmission swaps you will likely need to modify or replace your driveshaft. I got lucky and found a salvage yard driveshaft that was the right length, so all I had to do was swap the yoke. If you can't find the exact length you need, just get one that is too long and have a driveshaft shop shorten it.

Ray

Was there a specific T5 you used? I've heard the s10 ones work, but they are apparently different between the 4 cyl/2.8/4.3. Also, which driveshaft did you find that matched? What gears do you have in the rearend of your truck?

Thanks!
Alden

[raycow]

Originally Posted by alden

Was there a specific T5 you used? I've heard the s10 ones work, but they are apparently different between the 4 cyl/2.8/4.3. Also, which driveshaft did you find that matched? What gears do you have in the rearend of your truck?

My T5 is definitely came out of an S10, but I don't know what year or engine because the ID tag was missing. It has a mechanical speedo drive, which makes it 91 or older. I counted the gear teeth and it has a 3.50 1st gear and a 0.69 overdrive. The rear end is original to the truck and has 3.90 gears.

The driveshaft came out of some kind of Ford product, judging from the yoke splines. I took it because it was the right length and the U-joints matched the ones in my truck. T5s take the very common 27 spline GM yoke, so it was no problem to find a yoke that fit.

Ray

[alden]

Originally Posted by raycow

My T5 is definitely came out of an S10, but I don't know what year or engine because the ID tag was missing. It has a mechanical speedo drive, which makes it 91 or older. I counted the gear teeth and it has a 3.50 1st gear and a 0.69 overdrive. The rear end is original to the truck and has 3.90 gears.

The driveshaft came out of some kind of Ford product, judging from the yoke splines. I took it because it was the right length and the U-joints matched the ones in my truck. T5s take the very common 27 spline GM yoke, so it was no problem to find a yoke that fit.

Ray

Thanks a bunch for the answers. Did you have to make any modifications to the T5 to mate up to the 235?

[raycow]

Originally Posted by alden

Did you have to make any modifications to the T5 to mate up to the 235?

Yes, I had to shorten the input shaft pilot about 3/8" and the front bearing retainer enough to fully expose the splines. You can get a commercial spacer plate that eliminates the need to do this (about $130), but my time is a lot cheaper than my money, so I did it the old-fashioned rodders' way.

http://hamiltonintakes.com/products/...5-swap-adapter

Ray