R10 overdrive into a '48 DeSoto

[Art Bailey]

Hey All,

 

 I'm pretty sold on the idea of putting an overdrive in my '48 fluid drive DeSoto. Thanks to the Overdrive Spotter's Guide, I know that the R10 is 19" from front to the back edge of the brake band. My trans is something like 15", if I remember right (I probably don't). Shorter, in any case. It seems like I'm going to have to shorten my driveshaft a few inches if I go through with this plan. 

 

Questions: 

• Is a non-fluid drive bell housing a little shorter than the fluid drive bell housing? Or is it the same bell housing?
• Would deleting the fluid drive make it a drop-in conversion on the 237 block, no driveshaft shortening necessary? 
• I know hot rodders shorten drive shafts all the time...anyone out there have experience with this, and know the pitfalls to watch out for? 
• Or, is the R10 out of the question all together for a long block? I know they were for the 52-56 (?) Plymouths, which (as far as I know) were the short block engines.

 

Thanks!

Edited September 20, 2025 by Art Bailey

[Saskwatch]

I do not have a lot of experience with the R10 overdrive but I have installed one in our Canadian ,1948 Dodge D25 Cl coupe ( aka plodge) with a 25” long block. 
   It replaced the OEM 3 speed standard transmission without any modifications.

   I’m sorry , I cannot comment on the fluid drive bell housing …..👍

 

[Art Bailey]

9 hours ago, Saskwatch said:

I do not have a lot of experience with the R10 overdrive but I have installed one in our Canadian ,1948 Dodge D25 Cl coupe ( aka plodge) with a 25” long block. 
   It replaced the OEM 3 speed standard transmission without any modifications.

   I’m sorry , I cannot comment on the fluid drive bell housing …..👍

 

Great, that's helpful info...thanks!  

[Art Bailey]

 Young Ed says in a thread I hijacked:

Quote

Not sure what trans you have currently but an R10 is the same length as the common 3spd. If you have one of the shorty transmissions without the extension housing then it's longer. It's also very close in length to the gyromatic. We swapped one of those for an overdrive in a 51 dodge with no driveshaft mods. This is all from the mounting surface back. You'll need to swap the input shaft if you're going non FD to fluid drive 

 

Cool...I really appreciate the info. I have the "Tip-Toe" which is another trade name for the M6, same as the Gyromatic (I think that's right). I haven't dug too deep into this yet, but like I said, my trans measures a good 4-5" shorter than the published length of the R10. I looked up the specs for a 51 Wayfarer 2 door sedan (don't know if that's what you had or not), it's a 115" wheelbase. Mine is 121.5, so that could explain why it looks like I'll have to do surgery on the driveshaft, and if you happened to have a sedan, yours dropped right in. Not a world ending problem, and the DeSoto also came with a 3 spd, so maybe there's a different length driveshaft or bell housing out there already. Last sentence: did you mean the OD is set up for non FD? I've got fluid drive. 

[Loren]

The member who has DeSotos of that vintage is James Douglas.

I know for a fact he has Overdrive in them and has had to rebuild them from time to time.

I'd reach out to him. He's recovering from a serious medical issue so be very patient with him.

 

[Young Ed]

6 hours ago, Art Bailey said:

 Young Ed says in a thread I hijacked:

 

Cool...I really appreciate the info. I have the "Tip-Toe" which is another trade name for the M6, same as the Gyromatic (I think that's right). I haven't dug too deep into this yet, but like I said, my trans measures a good 4-5" shorter than the published length of the R10. I looked up the specs for a 51 Wayfarer 2 door sedan (don't know if that's what you had or not), it's a 115" wheelbase. Mine is 121.5, so that could explain why it looks like I'll have to do surgery on the driveshaft, and if you happened to have a sedan, yours dropped right in. Not a world ending problem, and the DeSoto also came with a 3 spd, so maybe there's a different length driveshaft or bell housing out there already. Last sentence: did you mean the OD is set up for non FD? I've got fluid drive. 

The one we swapped was a club coupe. I wonder if part of the discrepancy is measuring with or without the input shaft. The Gyromatic was behind the fluid coupling so the input shaft was longer than the no FD sourced overdrive. 

[andyd]

Unless you have the expertise then get someone who does these for a living to shorten the driveshaft.........I have had 2 done on my 1940 Dodge, 1962 318 poly & cast iron Torqueflite........original 1962 diff with 1962 shaft shortened to suit by having it cut at the front where the splined end is, machined to slip into the original shaft the welded......and balanced accordingly..worked fine.......25 yrs later then replaced the 1962 diff with a 1990 Ford diff so had to have the 1962 shaft front cut off and machined to suit the 1990 Ford shaft........again balanced and works fine........there are ways of doing the job yourself but if you haven't done anything like this then go to the MAN.........uni joints have to be attached and welded in "phase", welding has to be PERFECT and the shaft should be BALANCED..........its not a difficult job for someone who's done it before........lol..........any truck repair shop should be able to tell you where to go to have it properly done........Andyd    

[Art Bailey]

6 hours ago, andyd said:

Unless you have the expertise then get someone who does these for a living to shorten the driveshaft.........I have had 2 done on my 1940 Dodge, 1962 318 poly & cast iron Torqueflite........original 1962 diff with 1962 shaft shortened to suit by having it cut at the front where the splined end is, machined to slip into the original shaft the welded......and balanced accordingly..worked fine.......25 yrs later then replaced the 1962 diff with a 1990 Ford diff so had to have the 1962 shaft front cut off and machined to suit the 1990 Ford shaft........again balanced and works fine........there are ways of doing the job yourself but if you haven't done anything like this then go to the MAN.........uni joints have to be attached and welded in "phase", welding has to be PERFECT and the shaft should be BALANCED..........its not a difficult job for someone who's done it before........lol..........any truck repair shop should be able to tell you where to go to have it properly done........Andyd    

Oh hell yes. Not trusting my sketchy welding skills to anything spinning that fast 3 1/2 feet below and slightly to the right of my butt 

Edited September 22, 2025 by Art Bailey

[Art Bailey]

18 hours ago, Loren said:

The member who has DeSotos of that vintage is James Douglas.

I know for a fact he has Overdrive in them and has had to rebuild them from time to time.

I'd reach out to him. He's recovering from a serious medical issue so be very patient with him.

 

Good advice, I'll do that. From reading his posts, I'm 99% positive he kept the fluid drive, and since they're his daily drivers, he'll have plenty to say about anything that could possibly go wrong. 

[Art Bailey]

17 hours ago, Young Ed said:

The one we swapped was a club coupe. I wonder if part of the discrepancy is measuring with or without the input shaft. The Gyromatic was behind the fluid coupling so the input shaft was longer than the no FD sourced overdrive. 

The Club Coupe is a 118.5 wheelbase, the difference between that and my car's wheelbase is just about the amount of driveshaft I'd need to chop. I didn't take into account the input shaft, I measured exactly the way Tim Kingsbury measured in his photo in the "Rough Spotter's Guide", from the edge of the casting that mates to the bell housing, to the edge of the brake band. From the pics, the shaft looks to be the same length as the one on the spare M6 I have, though it's hard to tell exactly.

[Young Ed]

You really need to measure from the front mount to the rear drive shaft mount. There are different offsets to the parking brake drum and flange that could change the length. I would think all the M6s were the same length and they changed the driveshafts as needed. However I know that isn't the case always as the short wheel base early 50s Plymouths had a shorter transmission and all models used the same driveshaft.

[Art Bailey]

1 hour ago, Young Ed said:

You really need to measure from the front mount to the rear drive shaft mount. There are different offsets to the parking brake drum and flange that could change the length. I would think all the M6s were the same length and they changed the driveshafts as needed. However I know that isn't the case always as the short wheel base early 50s Plymouths had a shorter transmission and all models used the same driveshaft.

Oh wow...this is the stuff I'd have to pay to learn. Thanks.

[Loren]

I have Plymouths ( I am used to living in the low rent district. lol ) so I know about them.

The R10 for my 49 Bus Coupe got a 1940 gear set ( second gear and the cluster gear ).

In that project I found out how many parts interchange between the MoPar 3 speed and the Overdrive.

I am guessing the fluid drive input shaft from your old 3 speed will go into the Overdrive so as to reach the clutch.

BTW when you get around to removing the transmission, all you remove is the transmission. The bell housing, clutch, fluid drive and flywheel stay with the engine. Thus changing to Overdrive is not a huge project at all. Wiring up the solenoid, relay and switches are not a big deal either.

I have changed over to the Rambler/AMC cable type throttle switch because it is really clean and if you change manifolds and carburetors you don't have to re-engineer the throttle switch.

[Young Ed]

1 hour ago, Art Bailey said:

Oh wow...this is the stuff I'd have to pay to learn. Thanks.

I'll PM you my PayPal 😁

[Art Bailey]

31 minutes ago, Young Ed said:

I'll PM you my PayPal 😁

Next time I'm in MN, beer's on me.

[Art Bailey]

On 9/22/2025 at 12:31 PM, Loren said:

I have Plymouths ( I am used to living in the low rent district. lol ) so I know about them.

The R10 for my 49 Bus Coupe got a 1940 gear set ( second gear and the cluster gear ).

In that project I found out how many parts interchange between the MoPar 3 speed and the Overdrive.

I am guessing the fluid drive input shaft from your old 3 speed will go into the Overdrive so as to reach the clutch.

BTW when you get around to removing the transmission, all you remove is the transmission. The bell housing, clutch, fluid drive and flywheel stay with the engine. Thus changing to Overdrive is not a huge project at all. Wiring up the solenoid, relay and switches are not a big deal either.

I have changed over to the Rambler/AMC cable type throttle switch because it is really clean and if you change manifolds and carburetors you don't have to re-engineer the throttle switch.

Thanks so much, Loren. I think I'm gonna grab the one I have my eye on...seems like any obstacles can be overcome without too much sweat or money. I think the shaft on my M6 is considered a long one, and the one on the overdrive is pretty similar, just have to get my hands on some actual measurements. Sounds like shaft length isn't a dealbreaker either, just swap it out. I had a dress rehearsal at the junkyard yanking a trans out of a D11, and yeah, no big deal. I read your post about the '40 gear set, I might give that a go! Did you completely rebuild yours? Any guides you can recommend, or just get my hands on a BW shop manual?

 

Dodgeb4ya wrote out this test for a non-installed OD to make sure it's at least free and functional :

Any other pitfalls to watch out for with a sight unseen OD unit? Thanks!

Edited September 24, 2025 by Art Bailey

[Booger]

dude go for it but whence you go there there no turning back. Ed and Loren know there stuff bravo to them. The BW10 is a booklet and its a must have. Search out Randle Rudle

this is a serious venture good luck. If its highway speed youre looking at consider a new custom rear axle just an option

[Art Bailey]

34 minutes ago, Booger said:

dude go for it but whence you go there there no turning back. Ed and Loren know there stuff bravo to them. The BW10 is a booklet and its a must have. Search out Randle Rudle

this is a serious venture good luck. If its highway speed youre looking at consider a new custom rear axle just an option

Cool, thanks for the tips...the Randle posts look good.

Man, I just want to be able to keep up with local traffic! Forget the highway, this thing is going to be for back roads and relaxing.

Edited September 24, 2025 by Art Bailey

[Loren]

On 9/23/2025 at 4:32 PM, Art Bailey said:

Thanks so much, Loren. I think I'm gonna grab the one I have my eye on...seems like any obstacles can be overcome without too much sweat or money. I think the shaft on my M6 is considered a long one, and the one on the overdrive is pretty similar, just have to get my hands on some actual measurements. Sounds like shaft length isn't a dealbreaker either, just swap it out. I had a dress rehearsal at the junkyard yanking a trans out of a D11, and yeah, no big deal. I read your post about the '40 gear set, I might give that a go! Did you completely rebuild yours? Any guides you can recommend, or just get my hands on a BW shop manual?

 

Dodgeb4ya wrote out this test for a non-installed OD to make sure it's at least free and functional :

Any other pitfalls to watch out for with a sight unseen OD unit? Thanks!

 

On mine I worked really hard on getting everything surgery style clean. I put every part I could in my ultrasonic cleaner with ZEP or other degreasing detergent. So it was super clean then followed the manual and testing for function at each milestone. You can put some things in backwards so careful assembly pays off. It's not like Ikea furniture where you can live with a mistake. It really isn't hard to get one right, if you check your work. I use vaseline as assembly lube because it's the only approved assembly lube for automatic transmissions ( at least the ones I was trained on ).

In the 1941 on gear set second gear is too close to first. When you add an Overdrive you are creating a 4 speed transmission where forth gear is automatic ( upshift and downshift ) so why not make them as evenly spaced as possible. The 1940 gear set helps do that. Next thing about the Plymouth Overdrive is the rear axle gear ratio was around 4.1 to 4.3 so with a 30% Overdrive the over all gear ratio ( in high gear ) was the same as the 3 speed! The idea was to give you quicker acceleration. But that was then this is now. What most people want is to go modern highway speeds of 70 mph. So with the 49-52 ( that I have ) the rear axle ratio is 3.43 so the car at 70 has the same sounds and rpm as 55 used to be. My P17 was the lightest Mopar of the era so I put a 3.54 3rd member in from a 56 Dodge automatic and bigger tires. 

 

[Cpt.Fred]

this is great info! 

 

do you mean you put in the 3.54 rear gear PLUS the overdrive? that would mean it has something like a 2.50 4th gear then in OD?

i tried that once and found my engine wasn't too happy about it.

but i do not want to highjack this conversation, guess i'll start a new thread about my stuff here soon... 

good luck with the swap, Art Bailey!

 

Edited September 30, 2025 by Cpt.Fred