The Stovebolt Site Forums The Shop Area Driveline 3 on column to 4 on floor

Now, I'm new to AD trucks and still looking for one with a SM420 but it seems a 4 speed is much rarer on survivor trucks. So, how hard or easy is it to remove a column shift and pop in a 4 spd? Would a rear end gear swap be warranted as well? Do both share the same steering column?Talking about OEM here.
Thanks
Brian

Same column ,same torque tube ,I don’t know what the difference is in mounts for the tranny but the Sm420 hangs off the bell housing and I think the 3 spd has a cross member .The transmission cover is different ,has a bump and the hole for the shifter. The steering seal on the column is different 4 spd is round with the hole for the column the 3 spd has another hole for shifter rod.The Two transmissions have the same final gear so will drive the same 1st gear is basically a parade gear as you can walk faster on a 4 spd and is also 1st gear is non synchronized so you have to stop to go into 2nd or start in 2nd gear.

The only real difference between 3 on the tree steering columns is the attachment points for the shift box in front of the firewall and for the shift lever knuckle just below the steering wheel. There are two different PN's for the jacket, one for 4 speeds without the attachment points and one for 3 speeds with the attachment points. You could cover those holes or weld them closed and grind smooth before repainting. But as far as the transmission itself, the 4 speed will bolt in place of the 3-speed. You'd need to be sure it has the u-joint and ball housing socket on the rear. You could find a donor 4 speed on most big bolts. Should be lots of those in farm country.

[on edit] Kevinski has some good points. Floor seal is different (I think both are available from the usual vendors.) Later 1/2 ton's didn't have a transmission mount crossmember (my 51 does) which brings up another question on the length of the trannys. It's possible that the 3 speed is longer, which would require a longer torque tube for a 4-speed. Can't tell from the parts listing whether that's the case. Nope: Only 1 differential housing and torque tube listed for 1/2 ton's with manual transmissions. A truck with a Hydramatic Transmission would be different.

Hi Brian,
Fortunately you have some interesting options if you are willing to hunt for and change your rear end. You could use some of the 4 speed transmissions from the 1960s and later. You could use a 3 speed fully synchronized transmission popular from the mid-1960s forward...but any of these is going to have a 1 to 1 final gear ratio so it won't be a benefit at higher speeds. In order to get a reduced final ratio you could do one of four things: use a rear end with a different gear ratio, use an overdrive three speed transmission, use a four speed converted to use the same overdrive unit that was used in the 1960s and later or you can use a more modern transmission (like the T5) which if you get the right one will bolt up to your bell housing...like I did.

I have a 3.07 rear end and a T5 with the gear ratio chain that begins with 4.03 and ends with .86. So the first three speeds feel exactly like the "stock" AD pickup. Meaning I can put 1000 pounds of "stuff" (maybe like railroad ties or loose gravel or a cow) in my truck and drive around without any problem...no burning of the clutch, etc. Then when I shift into 4th, I get a 25% overdrive type benefit because my rear end is 25% less than the 4.11 rear. If I'm carrying a lot of weight, I can just stay in 4th. And when I shift into 5th I gain another 14% benefit and at 60 mph my engine is only spinning at around 2100 rpm.

Those are a few options. Others may come along with more.

Brian - One other essential set of component that can get overlooked when swapping a torque tube 3 speed with a torque tube 4 speed is the front yokes of the u-joint assembly that connects the transmission output shafts to the drive shaft inside the ball collar.

The 3 and 4 speed yoke sets all use the same u-joint, but the forward yokes are actually different, having different casting and part numbers. Below is an extract from a Chevy Master Parts Book with the two different front yokes marked for 1948-54 AD 1/2 tons w/3 and 4 speeds. The rear yoke of the assembly in both cases is the same. IIRC, there is also a difference in the speedometer driven/drive gear sets from 3 and 4 speeds that coincides with the differnt front yokes.

I used the 1/2 ton as a starting point, since you didn't indicate what year and model AD you are working on. Adding those details, either in your post or in your signature line, would help us help you by pointing you in the correct direction for the best answers.

PS - If you're hunting for an SM420, feel free to post an add in our Truck Parts Wanted forum as it's a great place to get help finding what you need.

“IIRC, there is also a difference in the speedometer driven/drive gear sets from 3 and 4 speeds that coincides with the different front yokes.”

I’m not sure if this is relevant but I needed the different gear sets and a different front yoke to change my 3 speed to 4 speed. I haven’t switched yet though.

Glen

The only time you'd need to change out the speedometer gears is if you source a 4-speed from a big truck which would have a higher numerical rear end ratio than a 1/2 ton. Stock ratio for 1/2 tons is 4.11 and a bigger truck would have a 4.56 or higher gear ratio.

Here and on the following page is information of stock speedometer gear sets based on truck and tire size and rear gear ratio.

Thanks for all the info gang, it'll give me something to ponder.
Brian

All of the above pertains to the 3100 I believe, if you are looking at a 3600 result may be different as the rear drive shaft on the 3600 is a 2 piece, torque tube from the transmission and open from the middle bearing to the rear axle. I'm guessing 3100's are a lot more common but there are 3600's out there.

I believe the transmissions are the same between 3100 and 3600. On the 3600 the enclosed intermediate driveshaft fits into the ball socket on the transmission the same as the torque tube on a 3100.