The Stovebolt Site Forums The Shop Area Driveline Covert 3spd to 4spd / Will it fit?

I own an all original 54' 3100 with 3spd (small trans) column shift, torque tube, 6v. I love the truck but always wished it was a 4spd floor shift with a taller set of gears in the back. I have recently come across a 53' complete rolling chassis with a 4spd and torque tube. For the minds within.... Can I simply swap the trans with crossmember and torque tube? The 53' 4spd has a rear cross member where as my 54' 3spd does not. The rear engine mounts at bell housing are different on the 53'. Can I use my 54' bell housing? It would sure be nice if I could build the 53' torque tube and 4spd and swap over to the 54'. Any shared knowledge would be greatly appreciated.

Top gear is still direct on a 420, so, don't count on going any faster. Get out your tape and measure for the rest.

Ed

saratoga54,

The 420 Ed referred to is an SM420, the optional 4-speed transmission from 1948-1954 - these years had a torque-tube rear end.

As Ed posted, the top gear on the 3-speed and the 4-speed are the same (1:1 ratio/drive).
The 3-speed and 4-speed manual enclosed driveshaft transmissions in those years are the same length.

Regarding the clutch housing and the need for a rear-transmission frame crossmember:
- the rear-engine frame crossmember changed in 1954
- the clutch housing also changed in 1954
- these changes allowed the elimination of a rear transmission cross member in 1954 (for 3-speed and 4-speed transmissions)

The 1953 rear end was 4.11
The 1954 rear end was 3.90 (lower revs at higher speeds)
Either/both rear-ends can be converted to 3.55 (check the Tech Tip - Engines section)

Not familiar with the 3 or 4 speed transmissions mentioned above. Would they function the same as the 3 or 4 speed transmissions from 1939, the 3 speed has synchronizers and the 4 speed doesn't? In which case the 4 speed had to be double clutched.

Rick

The 1948-1954 transmissions mentioned above all have synchronized gears.

Those 1948-54 transmission have different lengths than earlier transmissions.

The torque tube should be the same length,I used a 51 torque tube in my 54 the 51 had a 3 spd,and my 54 has a 4 spd.

The 3-speed and 4-speed torque-tubes are the same length (48-54), except for the shorter torque-tube with the 1954 Hydra-Matic transmission.

To answer your question directly - yes, it will be a bolt-in swap. You will however have to change the gear in the rearend if you want to lower your cruising speed RPMs.

Wait - now I'm confused. I have never seen a 47-54 model truck with a factory rear crossmember. Maybe I'm confused about what you all are referring to as the "rear crossmember". All of these trucks that I ever worked on had a midships style crossmember that the bellhousing sat on and then the transmission just hung off the bellhousing and had no more mounts aft of that position. My idea of of a 'rear crossmember' is one that mounts to the tailshaft housing of the transmission.

From '47-'53 the bellhousing crossmember mount pads were parallel to the ground, in '54 and first series '55 the crossmember was very similar except the mounts became angled - most likely to combat engine/trans movement. The bellhousing style also changed to accommodate this change.

The SM420 will bolt directly to either style bellhousing/crossmember combination and work.You should be able to remove the 3-speed torque tube trans from your '54 and install the the SM420 4-speed onto the same bellhousing using the same clutch, etc, etc. It should bolt right in.

A rear transmission frame crossmember and rear-transmission mount was standard equipment prior to 1954 (allowing the rear transmission mount that "mounts to the tailshaft housing of the transmission" where the torque-tube cover attached to the transmission).

The 1954 change in the geometry of frame cross-member under the rear of the engine allowed the elimination a rear transmission frame crossmember (on 3-speed and 4-speed, but not with a Hydra-Matic).

I guess most of my confusion is because most of these trucks that I have messed with have not been half tons. I do know for a fact though that my dad's '52 3100 which originally had a 4-speed and a torque tube setup in it did not have a tailshaft mount or crossmember. I know this because I personally removed the drivetrain from that truck and there was no crossmemeber in that location. Only at the bellhousing position. I had to cut out the bellhousing crossmember because I installed a later model 250 L6 with a TH350 trans.

I just located this picture - I swear that crossemember in the rear was not on the truck my dad had. Either I'm losing my mind or his truck had a bastard chassis that didn't belong under it.

http://i161.photobucket.com/albums/t204/ctm_motortrends/CHEVY49/2010436.jpg

Here's what the frame under my dad's '52 looked like - no rear crossmember.

http://i173.photobucket.com/albums/w71/mr48chev/1948%20Chev%20pickup/006.jpg

Many rear cross members were apparently removed when people removed/replaced/repaired transmissions.

By the way, this pertains to 1/2 ton trucks.

Mechaic58, look for bolt holes in the frame in the area of where the the torque tube ball is located. The rear cross member was held on by 3 bolts on each end. Two bolts went through the side of the frame, and one through the bottom.

Thanks Carl - it's a moot point now, he no longer has the truck. There's no telling what was going on there. That truck had a hard life before we got it, I'm sure it was subject to many a shadetree during its life.

Thanks to all for the response. Looks like I'll find a set of 3.55 gears and get started. One last question.. The Filling station has 3.55 gear set that they say will fit 47-53 gear housing. Why would you think it would not fit the 54 housing? Curious! Thanks again for the valued responses.

You will need to use a 1948-1953 3100 ring-gear carrier.
I have done this twice - no problems.

Mothertrucker sells the R&P set at the best price.
Classic Parts sells the installation kit - it is worth buying/using.
SWEET has a written a very good Tech Tip here (link to Tech Tips at the top of this page).