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identify this differential
#1350368 Tue Mar 17 2020 03:25 PM
Joined: Aug 2019
Posts: 20
R
New Guy
I have a 1950 3100 chevy truck with this differential. It seems to have 3.55 gearing as deduced from the counting the revolutions of the tire to the yoke turning,

It might be a eaton Napco as I have found pictures where the cover looks the same. But hopefully someone can help me identify?

Rick

Attached Files
differential.jpg (396.05 KB, 260 downloads)
Pic 2.jpg (529.79 KB, 267 downloads)
pic 3.jpg (425.11 KB, 253 downloads)
Last edited by Rick Brown; Tue Mar 17 2020 04:45 PM.
Re: identify this differential
Rick Brown #1350439 Wed Mar 18 2020 12:42 AM
Joined: Aug 2019
Posts: 20
R
New Guy
answering my own question as I looked on a recent stovebolt search and it appears to be a eaton. Now I need to confirm gear ratio as I have a 261 engine with a 1986 s10 T5 transmission,I hope it was changed to 3.55 otherwise it is a 3.90

Last edited by Rick Brown; Wed Mar 18 2020 12:43 AM. Reason: correction
Re: identify this differential
Rick Brown #1350484 Wed Mar 18 2020 05:29 AM
Joined: Apr 2005
Posts: 2,527
C
Shop Shark
If your trans has an 0.82 od then the 3.55 is good (2.91 final drive) but if it has the 0.67 od the 3.90 would be better (2.61 final drive). The 3.55 with a 0.67 is a very shallow 2.28 which might require 4th gear with hills or strong head winds.


Evan
Re: identify this differential
coilover #1350518 Wed Mar 18 2020 04:11 PM
Joined: Aug 2019
Posts: 20
R
New Guy
Thanks Evan for your response. My s10 fifth gear is 0 .76 so accordingly if it is

A 3.90 x 0.76 = 2.96

B 3.55 x 0.76 = 2.69

c 3.73 x 0.76 = 2.83

According to Tom Langdon web site the gear ratio should be between 2.50 and 3.0 so looks like whatever I have is within range. I will take the differential cover off to determine exactly what I have then the question is what is the preferred ratio or should I not worry about it?

Rick

Re: identify this differential
Rick Brown #1350526 Wed Mar 18 2020 04:57 PM
Joined: Sep 2011
Posts: 702
B
Shop Shark
IMHO 3.55 is a little high geared to run overdrive IF you're not in the flat lands (unless you run small diameter tires).


BC
1960 Chevy C10 driver 235 T5 3.73 dana 44
1949 GMC 250 project in waiting
1960 C60 pasture art
G.M dealer tech since 1980 & counting
Re: identify this differential
Rick Brown #1350562 Wed Mar 18 2020 08:22 PM
Joined: May 2005
Posts: 7,165
B
General Purpose
I'm sure you will be taking off the cover. Then you can read the stamp on the ring gear to confirm ratio. That is the only way to be sure.
There was no NAPCO rear end. Same as 2WD.
That is the all time worlds record sketchy rear end swap I have ever seen. A coil that seems to do nothing, welded to the tube, on top of leaf springs with welded Ubolts to the tube. I can't look at it.


I'm away on an ego trip. Will be back on Feb 30.
I'm not an Auto Mechanic, but I play one on TV.
I charge $0.02 for every opinion and I take Paypal.
Plan B is always better than plan A, by definition.


Re: identify this differential
Rick Brown #1350686 Thu Mar 19 2020 03:51 PM
Joined: Aug 2019
Posts: 20
R
New Guy
Good observation bartamos I too thought this was the strangest rear end I have seen. Now the question is it worth trying to make work or start looking for something else? Or could I simply cut off what appears to be coils going nowhere and look for a proper spring bracket kit?

Rick

Re: identify this differential
Rick Brown #1350687 Thu Mar 19 2020 04:04 PM
Joined: Jan 2013
Posts: 1,642
S
Shop Shark
Coil overloads Bart,lol
Somebody needs to hunt the guy that did that down and give him the what fer!

Last edited by sstock; Thu Mar 19 2020 04:07 PM.

1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
https://www.youtube.com/watch?v=Bli...n05i04t1aokgm4p04jiwgffwhyyih5xbk0h00410
1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration
Re: identify this differential
Rick Brown #1350697 Thu Mar 19 2020 05:32 PM
Joined: Mar 2007
Posts: 690
T
Shop Shark
I cringed when I saw the pic. Looks like a lot of short cuts over the years, I hope Rick has better plans for this old truck. wink


Craig

Come,Bleed or Blister somethings got to give!!!
Q:"What yah runnin"
A:"3/8 drive Black and Decker aquarium pump motor" (the movie HOTROD)

59' Apache 31, long term project (30 years and counting)
Re: identify this differential
Rick Brown #1350700 Thu Mar 19 2020 05:41 PM
Joined: May 2015
Posts: 2,367
K
Housekeeping (Moderator) Making a Stovebolt Bed & Paint and Body Shop Forums
I'm afraid that something like those coils were on my truck when I got it. Some big round "sockets" welded below the frame arch. Those were the first things to go. I whacked them off as soon as I had the bed removed. I'm sure they reduced the spring travel and served as "axle bumpers" providing a nice steel-on-steel contact. eek


Kevin
First car '29 Ford Special Coupe
#2 - '29 Ford pickup restored from the ground up.
Newest Project - 51 Chevy 3100 work truck. Photos [flickr.com]
Busting rust since the mid-60's
Re: identify this differential
Rick Brown #1350713 Thu Mar 19 2020 08:18 PM
Joined: May 2005
Posts: 7,165
B
General Purpose
I would take bed off, as Kevin says, cut the coil off, cut the welds on the leaf off. Clean up the tube. Keep the leaf intact and find the proper ubolts and plates. Move shocks and mount per original, if they are not. Somebody was carrying real heavy loads back in the day and wanted to "beef" it up.


I'm away on an ego trip. Will be back on Feb 30.
I'm not an Auto Mechanic, but I play one on TV.
I charge $0.02 for every opinion and I take Paypal.
Plan B is always better than plan A, by definition.


Re: identify this differential
Rick Brown #1350716 Thu Mar 19 2020 08:53 PM
Joined: May 2015
Posts: 2,367
K
Housekeeping (Moderator) Making a Stovebolt Bed & Paint and Body Shop Forums
Fortunately it appears they didn't weld on the spring leaves, just the axle housing. If they had welded on the spring leaves, I would consider them scrap. Useful only for knifemaking stock. Everything else can be salvaged. If they were carrying heavy loads, I would check the axle housing for straightness.
I had a '72 Chevy 1/2 ton that had been used to carry a monster slide-in camper and had a sagged axle. I had it straightened, but they got the center section out of line with the axle tubes, even though it was straight at the wheel ends. It had a limited slip in it and the binding of the side gears shucked the petals off of the friction plates and threw the bits into the pinion bearing. I wound up replacing the entire rear axle. Of course the guy I bought it from neglected to mention the damage. nono I happened to see him at a campsite a year or so later, and he looked incredulous that the truck was still on the road.


Kevin
First car '29 Ford Special Coupe
#2 - '29 Ford pickup restored from the ground up.
Newest Project - 51 Chevy 3100 work truck. Photos [flickr.com]
Busting rust since the mid-60's
Re: identify this differential
klhansen #1350729 Thu Mar 19 2020 10:22 PM
Joined: Aug 2019
Posts: 20
R
New Guy
Thanks Bartamos and Kevin your feedback is appreciated.

I will do as per your recommendations and as a preventative plan, I will take the entire differential into a local place that only does differentials and make sure everything is straight and in good order /rebuild before re- installing properly with new shocks / proper Ubolts and plates

Re: identify this differential
Rick Brown #1350730 Thu Mar 19 2020 10:33 PM
Joined: May 2005
Posts: 7,165
B
General Purpose
thumbs_up


I'm away on an ego trip. Will be back on Feb 30.
I'm not an Auto Mechanic, but I play one on TV.
I charge $0.02 for every opinion and I take Paypal.
Plan B is always better than plan A, by definition.


Re: identify this differential
Rick Brown #1350855 Fri Mar 20 2020 07:16 PM
Joined: Aug 2019
Posts: 20
R
New Guy
Finally was able to pull differential; cover last night.

Confirmed as a 3.90 gear ratio so with my T5 od transmission fifth gear at 0.76 my final is 2.96. I hope this will work as a daily driver but I will inquire with differential shop the cost of putting in a 3.73 and see if it would be worth it.

Attached Files
gear numbers.jpg (37.89 KB, 82 downloads)
Gears.jpg (48.93 KB, 82 downloads)
Last edited by Rick Brown; Fri Mar 20 2020 07:55 PM.
Re: identify this differential
Rick Brown #1350916 Sat Mar 21 2020 03:16 AM
Joined: Mar 2010
Posts: 9,223
5
Master Gabster
I would advise against changing from 3.90 to 3.73 with the .76 overdrive. I am assuming that you are running an inline six.
Your RPM will be too low in 5th gear until you hit 65-70 MPH.
It may sound quieter, but your gas mileage will actually suffer since your engine RPM will be below the optimal power band of this engine between 50-60 MPH. A vacuum gauge will verify this.

Re: identify this differential
52Carl #1351791 Thu Mar 26 2020 09:09 PM
Joined: Aug 2019
Posts: 20
R
New Guy
Thanks 52Carl. You are confirming what the fellow at the differential shop indicated that I am best to stay with the 3.90 which makes it easier anyways. Thanks for your input

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