Author Topic: Somewhat Urgent Question about Interchangability  (Read 400 times)

Offline JohnnyPopper

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Somewhat Urgent Question about Interchangability
« on: November 17, 2020, 02:04:46 PM »
I'm having a 3.42 installed to replace a 3.73 diff. There is a stock posi-traction on the 3.73.

With help from this forum, I discovered it will drop my highway RPM from 3000 to 24-2500. That's where the torque peaks as well.

I don't off road, it's my daily.

Just got a call from the shop, they said the posi will not work now, and that they need to install a standard diff.

Yesterday when I was there, there was no mention of a problem.

Q: are these interchangeable? Or are they trying to sell my posi from under me? In any case, I told them to box up my posi in case I want to restore it.

Any help would be appreciated.  :D
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Online bd

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Re: Somewhat Urgent Question about Interchangability
« Reply #1 on: November 17, 2020, 02:13:16 PM »
Which year/model truck and model axle?

The differential deck height for an 8.5" 10-bolt allows 2.73:1 and greater gear ratios to employ the same case.  So ask for a specific reason that your LSD won't work.  Without more information, I agree.  It sounds suspect.
« Last Edit: November 17, 2020, 02:32:04 PM by bd »
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline JohnnyPopper

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Re: Somewhat Urgent Question about Interchangability
« Reply #2 on: November 17, 2020, 02:30:00 PM »
'78 12 bolt
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Online bd

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Re: Somewhat Urgent Question about Interchangability
« Reply #3 on: November 17, 2020, 02:43:17 PM »
The truck 12-bolt uses three (3) different cases, dependent on gear ratio range:  2.73:1 down, 2.76:1 - 3.42:1, and 3.73:1 up.  The deck heights between the cases for the gears you have are different.  The 3.42:1 gear requires a differential case that moves the ring gear further from the pinion gear centerline.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline JohnnyPopper

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Re: Somewhat Urgent Question about Interchangability
« Reply #4 on: November 17, 2020, 03:20:18 PM »
So it is legit. Thanks bd!
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline VileZambonie

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Re: Somewhat Urgent Question about Interchangability
« Reply #5 on: November 17, 2020, 05:11:15 PM »
You are changing the rear ratio to 3.42:1 - from 3.73:1 posi? What about the front? Curious why you downgraded yourself?
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Offline JohnnyPopper

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Re: Somewhat Urgent Question about Interchangability
« Reply #6 on: November 17, 2020, 07:56:31 PM »
Originally the shop didn't indicate that I would lose the posi... not real happy about that.

I don't off road this truck, daily driver. This will put highway speed right at 2500 RPM.

Front is disconnected for now.
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline VileZambonie

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Re: Somewhat Urgent Question about Interchangability
« Reply #7 on: November 18, 2020, 04:50:45 PM »
 :o
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              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

Offline JohnnyPopper

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Re: Somewhat Urgent Question about Interchangability
« Reply #8 on: November 19, 2020, 07:52:28 PM »
I'm keeping the stock parts, will probably have the front converted or restore the rear end at some point in time.

Ultimately OD will be the solution.
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline ehjorten

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Re: Somewhat Urgent Question about Interchangability
« Reply #9 on: November 20, 2020, 01:56:44 PM »
If you had been going from 3.42 to 3.73, you could have kept your Posi and used a "thick" 3.73 ring gear set, but you're doing the opposite.
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6