1951 Plymouth Cranbrook Rear Axle Swap

[LarryF]

I am looking for anyone who has removed the entire rear axle and drum assembly and replaced it with something that bolts in place and has disk brakes with the same tire bolt pattern and also connects to the OEM drive shaft. I want something that will allow highway speeds. 

[Sniper]

There is nothing that meets all your parameters.

[Plymouthy Adams]

the driveshaft is the killer for the all in one swap......the components for the driveshaft outside of cut, weld and balance will cost a bit more than most folks like to shell out....

[LarryF]

  On 4/4/2025 at 7:38 PM, Plymouthy Adams said:

the driveshaft is the killer for the all in one swap......the components for the driveshaft outside of cut, weld and balance will cost a bit more than most folks like to shell out....

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thanks guys. I am not the best mechanic and have some health issues. I want to get power steering and possibly an automatic transmission also to make it easier to enjoy. 

[Plymouthy Adams]

sounds like the best bet here is a retrofit with clip....

[bacelaw]

If highway speeds are a main concern, just replacing the rear gears can make a significant difference.  Do you know what the current rear gear ratio is?

[Dartgame]

1990's to early 2000's cherokee, not grand cherokee. This is not a drop in. Has the right width and 3.55 : 1 ratio in most of them and cheap in bone yards. you'll need to weld in new spring perches and a new driveshaft to convert the whole mess.

[Loren]

Simplify your life, get one of these.

Bolts right in, uses the same speedometer gear as your old transmission and it was a factory retrofit.

Won't hurt the value of your car.

Turns a 55 mph car into a 70 mph car and won't hurt the charm of your car. 

[LarryF]

What transmission is that? 

[Loren]

Plymouth Automatic Overdrive R10-G1

It is basically the Plymouth 3 speed transmission with a Borg-Warner Overdrive unit attached.

The Overdrive adds an automatic forth gear which lowers engine rpm 30%, thus the higher speed.

If the driver seeks to pass, it downshifts automatically and when the driver lifts the gas pedal it upshifts automatically.

One of the features is “Freewheeling” which allows the car to coast without putting the transmission in neutral.

The Overdrive was conceived as a gas saver. Freewheeling has the added feature of better road holding on slick roads. If you detect a skid you let off the throttle and the car has a much better chance to right itself.

Overdrive was offered ( finally ) in May 1952 and can be retrofitted back to 1940 models.

The one in the photo has just been gone through with the one year only ( 1940 ) gear set which is the only optional gear ratio for the Plymouth transmission.

Since you aren’t changing the rear axle, that is still an option if you upgrade the engine with say a 230 crank and rods.

The original Overdrive cars came with 4.1 to 1 final drive ratios which mean they were still 55 mph cars.

The standard transmission cars had 3.9 or 3.73 final drive gears, which makes them 65 or 70 mph cars with the addition of the Overdrive.

The next optional gear ratio is 3.54 to 1, which was used in the V8 automatic transmission cars up to 1956.

Since the Overdrive transmission is a factory produced modification that was available from the dealer the purists ( like me ) appreciate them greatly.

Installed correctly they will give the same long service of the standard transmission and use much of the same parts.

 

I think it’s lot of work to change rear axles from other cars or trucks. Changing to an Overdrive is much easier, plus I see nothing wrong with the original axle.

The Plymouth axle is a 3rd member type so to change ratios you simply change the 3rd member. Which is much easier to service than the integral type.

To check on your current ratio look for the ratio stamped into the housing under the fill plug.

[QEC]

The overdrive would be a simple swap if they were easily found and not the same cost of the entire car...🤔 Of course

that does not solve the need/desire for an automatic trans which is a plenty big swap by itself. And then, the rear disc need/want.

 

The Explore and Cherokee axles are a good swap and cheap enough at the local pik-n-pull.

 

Of course, all of this is academic if the OP lacks time, space, tooling, skillset or cash........

[Sniper]

Swapping in an automatic isn't going to happen easily or for cheap.  Wilcap makes an adapter to do this,  Starts at $1250.

 

Then you still need the transmission and driveshaft, plus some sort of shifter setup.  Figure out how to mount the trans since it's a single mount vice the original's two point mount.  So now your engine is a single point and so is the trans and it'll swing in the breeze.   Your engine/trans combo needs to be, at least, a three point setup.

 

You can keep the original axle if you want, but no parking brake setup if you swap the trans.  So you need to figure out what to do there.  There used to be a pinion mounted parking brake setup for Ford 9" axles that might be adaptable.  Of you can sort something out, maybe a bike disc brake setup as a starting point?  If you swap the rear axle as well and go with the suggested Jeep units then you will have to sort out how to actuate those parking brakes.

 

All doable, just not bolt in, or cheap, or simple.

 

 

Edited April 5 by Sniper

[kencombs]

  On 4/5/2025 at 9:44 PM, Sniper said:

Swapping in an automatic isn't going to happen easily or for cheap.  Wilcap makes an adapter to do this,  Starts at $1250.

 

Then you still need the transmission and driveshaft, plus some sort of shifter setup.  Figure out how to mount the trans since it's a single mount vice the original's two point mount.  So now your engine is a single point and so is the trans and it'll swing in the breeze.   Your engine/trans combo needs to be, at least, a three point setup.

 

You can keep the original axle if you want, but no parking brake setup if you swap the axle.  So you need to figure out what to do there.  There used to be a pinion mounted parking brake setup for Ford 9" ales that might be adaptable.  Of you can sort something out, maybe a bike disc brake setup as a starting point?  If you swap the rear axle as well and go with the suggested Jeep units then you will have to sort out how to actuate those parking brakes.

 

All doable, just not bolt in, or cheap, or simple.

 

 

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Lots of truth there! 

  especially the last paragraph. 

  I'm electing to use my original '56 pickup rear end.   I have most of the pieces for the disk brake on the pinion you mentioned and will need them as I'm installed a 4spd OD, A833. .   For those keeping the original trans it seems that it would be  fairly easy to adapt a disk to the spot where the  drum/band are located. Lots of disk sources, motorcycle and even go cart parts are available, and used are fairly cheap.   In my case I need to devise a support to mount the caliper and adapt the cable arrangement.   Since I have no illusions that it is really and 'emergency' brake, just a parking brake, it should meet my expectations.

[Loren]

The Plymouth club president Russell Nardi, who makes the T-5 adaptor has an axle mounted disc parking brake available now.

He has blitzed facebook with ads for it, so I am surprised no one has mentioned it before.

While I don’t like the T-5 ( if you have to have one, his adapter is very good ) I do like his E brake set up.

 

I used to have big long arguments with my Dad over “Needs vs Wants”. It was more about philosophy than engineering.

He’d say I want this or that on my car and I’d say, “You don’t need that, there’s a better place for your money over here.”

He had his dreams but I had the car that ran. I don’t think I won the argument though, because he’d get so angry I had to let him have the last word.

 

In the end one should do what they want. This is a hobby after all and it’s supposed to be fun.

Being a cheapskate I hate to see people spend time and money on stuff that won’t do what they want or need.