1939 pt-81 rear end gear swap from 4.11 to 3.73

[Pat McCabe]

Hi guys,

Ive read a bunch on this subject, but wondering if I can remove  411 gears and do a swap to 3.73

maybe using a Plymouth car, but what year and model would fit.any help please

[Merle Coggins]

Assuming the '39's used a similar axle housing a car differential should fit. I have a 3.73 diff out of a '50 Plymouth in my truck. I kind of stumbled into it, so I can't tell you how exactly how to search for one. 

[Pat McCabe]

Can I ask you what model car from 1950

 

[Merle Coggins]

I don't know. I never saw the car. A former forum member out in California was searching for a 4.10 diff to use in his Suburban with an overdrive trans. He liked to tow a boat or camper with it and wanted the lower gear ratio. He bought the axle and pulled the diff to find out it had 3.73 gears. He happened to post his disappointing tail on the forum and I sent him a message about a possible swap. I had a spare 4.10 diff from a parts truck . We swapped pictures. Then I pulled off the truck input flange and shipped it to him. In return he shipped his 3.73 diff back to me. Cost us shipping only and we both got what we wanted. 

Merle 

[Pat McCabe]

I had the engine rebuilt and would like a little more speed out of her

Wanted to find someone that went through all this and found the easy swap

[Merle Coggins]

It's not a difficult swap. Finding the diff will probably be the hardest part. Scour the bone yards and craigslist and I'm sure you'll find one. 

[Don Coatney]

Once again this might or might not help.

[Young Ed]

57 minutes ago, jeff ross said:

Can I ask you what model car from 1950

 

For plymouths most 1950 models would have a 3.90 standard. If you really want to increase the odds of finding a 3.73 look at 53-54 models. However that leads to another issue as the side gear tooth count changes. So if you find one you might need to swap some side gears around. 

[59bisquik]

I used a 54 Plymouth center section with a 3.73 in place of my 55 Dodge 4.10 center section. It bolted right in with no modifications besides the input flange swap as previously mentioned. The car axle housing was narrower than the truck and had smaller brakes, so I just used the center section instead of just swapping the whole axle.

Edited August 26, 2017 by 59bisquik

[Los_Control]

I have a 1952 plymouth suburban, when I looked up gear ratios, all 1952 plymouth suburbans had 3:73 gears.

So this is the info I am going off of for my swap, something you can keep in mind for your search.

[Pat McCabe]

Ok then would a 52 Plymouth suburban ring and pinion fit  in a 1939 Plymouth truck

[59bisquik]

First would be to check if the have the same amount of bolts. I think the other issue is axle spline differences. Somebody correct me if I am wrong, but if you have both carriers, could you just swap spider gears to correct spline count?

[Young Ed]

9 hours ago, 59bisquik said:

First would be to check if the have the same amount of bolts. I think the other issue is axle spline differences. Somebody correct me if I am wrong, but if you have both carriers, could you just swap spider gears to correct spline count?

I'm not sure if there are variances for trucks but I did that for my car. Reused my spider gears and swapped them to the 3.73

[Pat McCabe]

I was trying to keep it original from outside,Do you think I should  take pictures and bolt measurements and go to junkyards to try to find same size in a 

3.73.Maybe start with a Plymouth suburban?

[59bisquik]

If I remember correctly, the pattern was either 10 or 11 mounting bolts being the difference. 

You guys actually have wrecking yards that have old Plymouths? Anything around me here in SoCal is gnerally1990's and up  

[ggdad1951]

On 8/26/2017 at 10:34 AM, 59bisquik said:

If I remember correctly, the pattern was either 10 or 11 mounting bolts being the difference. 

You guys actually have wrecking yards that have old Plymouths? Anything around me here in SoCal is gnerally1990's and up  

yep French Lake is a cool yard, even if a PH truck is rare to see there.

[PT81Jan]

Hey Jeff

I was asking a similar question regarding a PT 81 diff swap here some time ago.

Not to destroy your expectations: although I am almost daily searching for one in the www. I didn´t find one up to now.

I also was asking several mopar parts providers. No 3.73 ...
If there is an information that helps you, this was the topic/content:

http://p15-d24.com/topic/40346-plymouth-truck-ring-pinion-gear-for-changing-gear-ratio/?tab=comments#comment-428083

An alternative for me is still the Jeep Cherokee rear axle, but my first choice is the Plymouth 3.73.

Would be nice, if we can benefit from our both information that we got / get to find a 3.73 diff.

Enclosed some information that I have (klick to enlarge):

 

If you have more luck to find one, please find two

 

[Young Ed]

Just so you guys know my 46 1/2 ton has a 3.90 and slightly oversized 16s and runs down the road great. I think you'd be quite happy with a 3.90 unless you have 15" tires

 

[Pat McCabe]

So Jan,  you have not found gears that will fit from any mopar?

 

And Young Ed my truck is 3 speed on the floor,  It has 4.11 gears stamped on housing,15 inch tires.

Wish I could drop rpm a little

I think I seen your dodge at back to the fifties,was it there?

[Young Ed]

7 minutes ago, jeff ross said:

So Jan,  you have not found gears that will fit from any mopar?

 

And Young Ed my truck is 3 speed on the floor,  It has 4.11 gears stamped on housing,15 inch tires.

Wish I could drop rpm a little

I think I seen your dodge at back to the fifties,was it there?

Hey Jeff what part of MN you in?? Yes my truck along with my 48 coupe was there all 3 days. We parked on Randall by the big open lot. 

[59bisquik]

Another option if you cant find the Plymouth rear end and are set against and axle swap might be a R10 overdrive unit. That would give you a .70 overdrive against the 4.10 which would drop your RPM's quite a bit.

[Pat McCabe]

se mn, 
I took pictures of your truck

[Pat McCabe]

9 minutes ago, 59bisquik said:

Another option if you cant find the Plymouth rear end and are set against and axle swap might be a R10 overdrive unit. That would give you a .70 overdrive against the 4.10 which would drop your RPM's quite a bit.

Do you know what kind of overdrive I would need ?A picture,or model number,          Im not familiar with these ,thanks

[Pat McCabe]

13 minutes ago, Young Ed said:

Hey Jeff what part of MN you in?? Yes my truck along with my 48 coupe was there all 3 days. We parked on Randall by the big open lot. 

First time I have been to that show,unbelieable,You gotta beautiful truck

[Young Ed]

1 minute ago, jeff ross said:

First time I have been to that show,unbelieable,You gotta beautiful truck

you sure you were looking at the right one........