1951’ B3B rear end swap

[Pawhuska]

Hey fellow B3B owners, or (knowledgeable) enthusiasts. I have a stock 51’ B3B 1/2 ton TWD truck. Due to this model originally built to be a work truck; (Job Rated), it has a 4:10 or 4:11 rear end. It can climb a tree, but I’m hesitant to drive faster than 35 mph, due to possibly pushing the RPM’S more than 2200 as per the manufacturer’s recommendations.
 

My question is: are there any higher ratio rear ends, possibly from 40’s or 50’s Dodge cars or trucks that can be swapped for my current rear end that has the same mounting applications? Something I can do without modifications. 

(I’m not a welder or have a machine shop). I appreciate any input you may have. Thanks.

[Jim Shepard]

My understanding is that there are some 3.73 pumpkins from MOPAR cars of the same era. Should be a direct bolt-in. Don’t know which makes/models. One side benefit is that you get to keep the hand brake…

Edited November 23 by Jim Shepard

[QEC]

...where is that Hollander manual...?

[Young Ed]

22 minutes ago, QEC said:

...where is that Hollander manual...?

The cool thing about the Hollander manuals is they never stopped printing them. Dad bought them maybe 10y back

[Tony_Urwin]

Using a 3.73 or 3.54 car differential is a nice way to keep your truck looking stock, if you can find one. You might want to consider swapping the entire rear axle. There can be issues with axle splines, I think.

[ggdad1951]

On 11/22/2024 at 4:47 PM, Pawhuska said:

Hey fellow B3B owners, or (knowledgeable) enthusiasts. I have a stock 51’ B3B 1/2 ton TWD truck. Due to this model originally built to be a work truck; (Job Rated), it has a 4:10 or 4:11 rear end. It can climb a tree, but I’m hesitant to drive faster than 35 mph, due to possibly pushing the RPM’S more than 2200 as per the manufacturer’s recommendations.
 

My question is: are there any higher ratio rear ends, possibly from 40’s or 50’s Dodge cars or trucks that can be swapped for my current rear end that has the same mounting applications? Something I can do without modifications. 

(I’m not a welder or have a machine shop). I appreciate any input you may have. Thanks.

 

I have a B3D with the same gears...might be taller tires, but I get 60mph under redline RPM easy....put a tach in and you'll know for sure where you sit.  Bigger thing at 60MPH is the steering and braking.

[Merle Coggins]

If your engine is in good condition it'll easily spin up to 3500 RPM without issues. Even with 4.10 gears you should be able to run around 55MPH without issues. It certainly sounds loud, mostly due to the fan noise. That and the fact that you are used to modern engines spinning at around 2000 RPM at highway speeds. 

I found a 3.73 differential assembly out of a '50 Plymouth that was a direct swap into my axle. It allows me to get up to 75 MPH when needed, although I don't like pushing that hard if I don't need to. It's hard to get a clear pic while driving, but here you can see my tach at 65 MPH (indicated), running at around 3100 RPM. When I push it up to 75 (indicated) the tach will show just under 3500. 

 

 

[Pawhuska]

I’m interested in finding options to swap out my truck’s rear end. 
I’d like to find a 373 to replace my 4:10. If any of you gents know of a car or truck that was made with that rear end that I could swap out without any modifications, please advise. Thanks.

[Merle Coggins]

Look at Chrysler, DeSoto, Dodge, and Plymouth cars from the late 40's thru early 50's. The 3.73 differential gears was an option in all of the Mopar vehicles, but those would be your best bets to find something. You'll have to do some digging to find the stamping on the top of the differential housing to know what gears are inside. 

 

I got lucky to find the 3.73 diff that I currently have in my truck. There was a forum member here, many years ago, that put up a post regarding an axle that he bought. He was looking for a 4.10 diff to use in his '53 Suburban, which he used to pull a boat, or camper trailer, and it had an OD transmission. He purchased an axle from a '50 Plymouth thinking it would have 4.10 gears. It turned out it had the 3.73 gears. At the time I happened to have an extra truck axle with 4.10 gears, so I sent him a message about doing a diff assembly swap. I offered to ship my 4.10 diff to him and it he would ship his 3.73 diff back to me. After some picture trading we decided the only difference was the drive flange, as the driveshafts are different between the cars and trucks. I pulled the drive flange off of the pinion, crated it up, and shipped it to him on my dime. It was exactly what he needed so he put his 3.73 diff back in the crate and shipped it to me. I replaced the pinion seal and reinstalled the drive flange. It was a direct replacement into my axle. 

 

I've heard that sometime in the early 50's ('52 or '53) there was a change in the spline count for the axle shafts and side gears. However, some here have reported having good luck in swapping the side gears in the differential to correct that mismatch.

 

 

[Young Ed]

33 minutes ago, Merle Coggins said:

Look at Chrysler, DeSoto, Dodge, and Plymouth cars from the late 40's thru early 50's. The 3.73 differential gears was an option in all of the Mopar vehicles, but those would be your best bets to find something. You'll have to do some digging to find the stamping on the top of the differential housing to know what gears are inside. 

 

I got lucky to find the 3.73 diff that I currently have in my truck. There was a forum member here, many years ago, that put up a post regarding an axle that he bought. He was looking for a 4.10 diff to use in his '53 Suburban, which he used to pull a boat, or camper trailer, and it had an OD transmission. He purchased an axle from a '50 Plymouth thinking it would have 4.10 gears. It turned out it had the 3.73 gears. At the time I happened to have an extra truck axle with 4.10 gears, so I sent him a message about doing a diff assembly swap. I offered to ship my 4.10 diff to him and it he would ship his 3.73 diff back to me. After some picture trading we decided the only difference was the drive flange, as the driveshafts are different between the cars and trucks. I pulled the drive flange off of the pinion, crated it up, and shipped it to him on my dime. It was exactly what he needed so he put his 3.73 diff back in the crate and shipped it to me. I replaced the pinion seal and reinstalled the drive flange. It was a direct replacement into my axle. 

 

I've heard that sometime in the early 50's ('52 or '53) there was a change in the spline count for the axle shafts and side gears. However, some here have reported having good luck in swapping the side gears in the differential to correct that mismatch.

 

 

My 48 has one of those 3.73s with the side gear swaps. Wasn't a big deal except we learned it the hard way when my axles wouldn't slide into the housing all the way. 

[Pawhuska]

Thanks buddy