The Stovebolt Site Forums The Shop Area Engine Shop 3-speed to 4-speed Swap-out, new Pics!

To finalize my purchase on an SM420...

I went to the salvage yard today and found some early 50's chevy, gmc etc.

On those 4-speeds that appear to be closed driveline, there is a ujoint midway between rear diff and tranny.

On my 3-speed its one piece.

Do I need a whole new drive shaft? Will the 3-speed one I have now work?

The big question is:

How will the 4-speed tranny plug into my 3-speed drive shaft, or will it? How does that connection work?

HELP!!!!!

Thanks all,

Dallas

I am not sure but I think the orig, 4 speed on the 50"s were on 3/4 ton, you have a half ton right?

Dallas,

A 50's (at least up to 1954) 4-speed driveshaft is the same length as a 3-speed driveshaft. the 4-speed was an option on the 1/2 ton trucks. On a 1/2 ton truck you need the enclosed driveshaft end-piece on the transmission (the bell fitting) - this end piece replaces the seal housing (or, is it a bearing housing?) on the open-shaft 4-speeds transmission.

I am using a 1954 4-speed from a 1.5-ton truck with the enclosed driveshaft adapter on my 1954 1/2 ton pickup.

Tim

The 4 speed transmission is most likely from a GMC that was a long box that extra piece does it have a worm gear on one end for the speedo and then a u-joint and the other piece slips onto a shaft? The whole piece that mounts to the torque tube is 18" or so. If you can send picture can tell you for sure or I can send you one.

Ok guys. I have consulted the official "GMC Maintenance Manual".

I am at work and dont have a scanner, or I'd push the images to you. . sorry.

However, here is the scoop:

In the manual, in the "Propeller Shafts" section, it shows two scenario's.

1. 101-22 (short bed)
2. 102-22 (long bed)

Even though my vin reads 102-22, it appears I may have the setup of 101-22. I have no idea why that is, may be a year change somewhere in there.

The difference between the two is a secondary shaft used in a tubular housing between the main drive shaft and the transmission. I think this is the unit that is about 18" in length, not sure. The front and rear shafts are connected through a univ. joint. Each connection is in a ball type housing.

TTTtttBB---BB-------ddDDD (102-22)
TTTtttBBBB------ddDD (101-22)

If this makes sense: The big t's are the big part of the tranny, then smaller t's are the smaller part of tranny, the BB's are the bell type housing for the joints. The d's for differential.
And the ---are the shafts.

On my setup, the backend of my transmission is 1/2 of the ball housing with the female end loose within.

(TTTtttB)unbolts here(B---BB--------ddDD)


When I unbolt the housing, and take out the tranny I am left with the other 1/2 of the housing and a male end hanging loosely.

Am I going to need this "adapter" as spoken of above? Never heard of it? What do I need to look for at the salvage yard as far as 4-speeds go? Should I take my 3-speed out there and compare the female end of the housing?

I hate to sound so oblivious here, but this is quite confusing to the virgin restorer.

Ideas on the logistics of this swap?

Thanks fellas...

Dallas

Dallas, if you saw a short "midshaft" to a carrier and then on down to the rear end in an enclosed shaft, that is a 1/2 ton GMC longbed. The short shaft made up the difference in wheelbase.

Right church, wrong pew.

You need a 1/2 ton 4 speed complete setup for your truck to convert it. Try Jeff at Bowtie Bits for help as they sell rebuilt SM420s.

Before I call Jeff, do you know what is all involved in the "complete setup" to make this conversion?

I am hoping its not the entire drive line.

Also, Any reason my tag reads 102-22, and I dont have that extra center shaft? Mine just goes tranny to drive shaft, there is no middle shaft.

Thanks for all the communication, all help is good help!

-dallas

Possible explanations are:
Cab swap/replacement, ID plate swap/replacement, chassis swap/replacement.

My guess is the ID plate was part of a brain transplant. If you can post a photo of the ID plate, I might be able to tell looking at it if it is original or has been changed.

Dallas on the end of the transmission should be one half of a ball housing. If you slide the half ball assembly off. You should be left with one complete ball asembly now unbolt the ball asembly. The short end of this u-joint should slip over the the transmission shaft. If the shafts splines line up you will have to press speedo gear off other u-joint and on to this one. To attach to trans mission there is a bolt that goes through centre of ujoint and there should be a washer with a hole. bolt ball assembly to transmission. Hope this helps. Is your transmission a 10 spline.

Dallas,

As mentioned above, get the u-joint assembly and speedometer gears (the drive gear on the u-joint yoke and the driven gear that screws into the enclosed driveshaft bell-housing).

If the transmission is from a truck larger than the half-ton, it is possible the u-joint rear yoke will not work with your driveshaft. I had to obtain a 4-speed u-joint that was specifically made for a 1/2 ton enclosed driveshaft (it is harder to find that a 3-speed u-joint). In addition, the speedometer drive and driven gears are made to match the ratio of the rear end (and tire size). It is almost impossible to find the correct combination of speedometer gears for a 1/2 ton 4-speed drivetrain (unless it comes with the set-up from a 1/2 ton truck). Nonetheless, if you can't find the correct speedometer gears, use whatever gears you can find - you will just have to correct (in your head?) your speedometer readings.

Good luck, Tim

To answer molley45,

The 3-speed that came off of there was:

From bell housing face to cup houseing mount (end to end minus shafts sticking out) is appox. 14 inches.

The shaft thats sticks into the engine side has 10 splines, the female propeller shaft and male propeller shafts on the differential side of the tranny have 17 splines.

. . .back to interpreting the input so far.

Thanks guys,

Dallas.

I wanted to post these pictures for you all to have a gander at, to put a picture with what I am talking about. Keep in mind, I am in the restoration process, so my frame/truck look ruff. . .hey, I just started the project in October!!!!!

Here is my old 3-speed:

[img]http://www.dallasmaddox.com/truck/tranny1[/img]

Here is my drive line with transmission detached:

[img]http://www.dallasmaddox.com/truck/driveline[/img]

Here is looking down the back-end of my transmission, notice the female end I have been speaking about:

[img]http://www.dallasmaddox.com/truck/tranny3[/img]

I know some people were calling me regarding the setup I had going on.

As you can see, I do indeed have a longbed, and the extra ball-type joint as seen on my driveline is proof of that.

Rob, here is my birth-plate (who knows why my mom painted this truck purple back in the 70's. . .boggles me):

[img]http://www.dallasmaddox.com/truck/nameplate[/img]

I have been told I can take the u-joint and female end off of my 3-speed to adapt the incoming 4-speed up, no problem. That, and the spider gear with the speedo connection.

Well, thoughts?

ps, thanks for the phone call Valentine, based on the pictures, what do you think?

THANK YOU all very much,

-dallas

I did the same thing you are trying to do, but with a 55 1st series. Mine has an open, one piece drive shaft. I don't really understand what all of your settup is, but as a newbee, such as myself, I figure you may benefit from one piece of information that was vital to my change over.... I went to a junkyard where they keep all the drive shafts in an old school bus. I had measured what I needed for drive shaft length when I got the 4 speed in the truck. I noticed differences in the u joints on different shafts. The junk yard elmer told me that at NAPA you can get U joints to convert one size U joint yoke to another, and therefore use any driveshaft of the right length. And, it worked for me.