3.73 Rear - Interchange for '47 Plymouth?

[Robin (UK)]

Hi all,
I'd like to swap out the 3.9 rear end in my '47 Plymouth and replace with a 3.73.
Searching this Forum, I've found some discussion about the swap, including mention of the spline count.
Does anyone have a list of which models would have run a 3.73 crown wheel and pinion that would be a straight swap?
Any information that could help me in the search would be appreciated.
Thanks,
Robin

[nonstop]

I have heard they are 10 spline, but it would be best to pull a shaft and count the splines.  That way you are certain.  From what I had read, my truck had 16 splines.  When I did the swap, it had 10 splines.  Not sure if something was changed or if there was simply a fluke somewhere.

 

On a side note, why the drop to 3.73?  For the cost, it is not much trade off - 0.17 difference.  You might want to consider going to 3.55 gears.  You would certainly lower RPM’s, but the other question is, can your car handle it?

Edited September 12, 2023 by nonstop

[Robin (UK)]

7 hours ago, nonstop said:

On a side note, why the drop to 3.73?  For the cost, it is not much trade off - 0.17 difference.  You might want to consider going to 3.55 gears.  You would certainly lower RPM’s, but the other question is, can your car handle it?

 

Thanks for the info on the splines.
The rear end swap is all about marginal gains. I have a hotter motor that'll be fitted soon. It'll definitely pull the higher gear, so I'm aiming to move that sweet spot a little bit and gain some more top end speed. 17% is enough as I already run an R10 OD.

[Loren]

I have a 1949 P17 ( 3 passenger business coupe ) it came stock with a 3.73. I found a 3rd member from a 1956 Dodge V8 with 3.54 gears that I plan to install right after I do some rust prevention. Previously I installed an R 10 Overdrive and that really made a difference! 
The 56 Spider gears fit the axles I have so I don’t think you’ll have any problems there. 

[Sniper]

I think what Loren is telling us is that you can swap the side gears over if you need to. My suggestion would be if you go that route to consider replacing the bearings.

[dpollo]

Desoto often used 3.73 ratios, so also Chrysler Royal and Windsor.  The 111 inch wheelbase Plymouth and Export Dodge did as well.

Spline count and fit will match 46 to 52.  53 up axle gears must be changed.  Axles are the same DPCD 37 to 48,  Unique for 47, same length 50 to 56 spline count changes in 53.   The oil filler is on the rear of the housing on earlier models and on the side of the carrier on later models. It is OK to have two filler locations, but I wound up with none in one set up.  Annoying.