The Stovebolt Site Forums The Shop Area Driveline 51 3100 3spd to 4 spd

I'm a new guy here and looking for ideas. I'm looking at a 51 1/2 ton with a 3 speed. I want to know how much has to change to put an original 4 speed in it. I haven't bought the truck yet but most of the others I've found in my price range also have the 3 speed. I live in the mountains with steep roads. I feel the 4 speed would be much easier to get rolling and when I was a young'en my dad had a 51 with a 4 speed so that's what I want. Do I have to change the steering column to get rid of the shift linkage or can it be removed and appear as original? I'm not dead set in this truck but leaning heavy towards it.If I need to hold out for a 4 speed truck I will but we don't have a big assortment of AD trucks to pick from aroud here. Thanks in advance for all comments.

The 4 speed is driven like a 3 speed. 1st is a very low gear. Dad used it at a walking speed to pull stumps and big rocks. It is not synchronized and is not what you want to get rolling every time. It has a different torque tube length and a different "Ball joint" from the 3 speed. Yes you can remove the linkage and the column will look the same. It will have some mount holes that can be seen. Not a big deal. If the truck you are looking at is at a good price, and you don't mind the work, the torque tube can be be replaced if you can find one, with careful basic skills. Not a deal killer in my mind. Find a 4 speed, and the floor cover, Find a torque tube and the "ball" and you can change it all out. Sell the 3 speed, and torque tube and you might come close to even.

Hi Thad,
I take it you're probably in the Northwest part of Arkansas (which is truly a beautiful place). The overall first gear ratio with the stock 4 speed as I'm sure you recall was very low. That low 4 speed ratio wasn't so terrible when you and your dad were younger and gas was cheap, however...well, let's start here since you're looking for ideas: How do you plan to use the truck? Often or infrequently? Daily driver or work truck? City or farm? Will you be hauling rocks or mostly people?

Jon

You would not need a different length torque-tube rear end. The 1948-54 truck three speed and 4-speed transmissions are the same length. You will need any year SM420 4-speed with the torque-tube fitting; or, you can put a torque-tube fitting on the end of any year SM-420 transmission.

Wow! I thought that the tube would be different. I learned something. Is the ball assembly the same as well? Thanks!

Yeap the ball is the same also. One advantage he might have is 2nd is slightly lower in ratio than the low in the 3 spd. 3.58 to 1 for the 4 speed. Plus all three top gears (2nd, 3rd,and 4th) are synchronized.
Problem is finding a 4 speed 1/2 ton universal.
I had one made up from an open 3/4 ton universal and the rear part of an enclosed 3 speed universal.

John

Cool! The issues the OP faces are even less than I had thought.

First off yes I live in NWA and feel blessed to live here. As far as driving the truck it would be occasionally. I would give it a restore and a flat/stake bed. Other than the bed it would be close to original. As far as gas being cheaper when I drove my dads truck I have no idea how much gas was then. He bought the gas and I burned it. Sometimes I just sat on the seat and pretended to drive. That was over 40 years ago but I remember it like it was yesterday. LOL Thank you to all who commented on this. Y'all are a great bunch. Thad

Please forgive me for this but is the torque tube the enclosed drive shaft tube? Could I use the torque tube fitting from a 3 speed on a 4 speed? Sorry for the dumb questions. I have quite a bit of mechanical experience but not on these old trucks yet.

They are not dumb questions.

The enclosed drive-shaft is also called a torque-tube.

The torque tube fitting is integral to the 3-speeed transmission case. The torque-tube fitting is a bolt-on part of the 4-speed transmission.

Actually it is an enclosed driveshaft on these..... No torque is transmitted through the enclosed driveshaft. The leaf springs transmit the "push" or "pull" when in motion. The universal has a slip joint.
Pre'49 Fords, Mercurys and Lincolns had torque tubes. "push or pull" was transmitted by the front of the torque tube pushing or pulling on the rear of the transmission. The universal was fixed on these with no slip joint.

John

Agree with Whitelightning, no engine torque transmitted.

I think it was so called because it absorbed/controlled the torque of the rear axle housing wind-up. Seem to remember reading that here on the forum.

The term used by GM in Shop Manuals (and other GM documents) was torque tube drive (as apposed to a Hotch-kiss drive). That might be technically/mechanically incorrect but it is/was a commonly used (misused?) term to describe equipment in these trucks.

I always heard the advantage with a torque tube or enclosed drive shaft was the grass in the fields did wind up around the driveshaft......LOL

The enclosed and torque tube you don't have to worry about pinion angle changing under acceleration.

Disadvantage is more un-sprung weigh.....

John