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Author Topic: cop R-body diff size? 11" brakes on 8 1/4"?  (Read 810 times)

attkrlufy

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cop R-body diff size? 11" brakes on 8 1/4"?
« on: May 21, 2008, 01:04:18 PM »

I've finally located a guy who has TWO (WHOOO-HOOO!!!) Police spec R-bodies in my area.  But I have two questions about differentail size and brake systems:
 
1) The owner of these cars seems to think the differentials are 9.25" units.  He says no R-body (1979-1981) came with an 8.75" rear.  I always thought the rear on an R-body cop car was usually 8.75".  It was too dark to look at the fender tags - I was wondering if anyone here could clarify this since I can't get back 'till next week. 
 
2) Will 11" rear drums mount on an 8.25" rear?  The reason I ask is because I'm REALLY tempted to swap out the HD brake system from the cop cars into Wanda.  Wanda has a 8.25" rear and I don't want to have to change differentials if I don't have to.  I'm moving to an area that's CRAZY hilly (it's worse than Pittsburgh and as bad as San Fran ), and I like the idea of beefier front and rear brakes on my 2 ton car, you know?  So, if I'm going to make this work, I need to know if I can make the swap on a stock 8.25" differential.
 
I ask because if 11" drums won't mount on an 8.25" rear, and if the cop cars have 9.25" rears, I don't think I can do this.  I don't want 3.23 gears - which the cop cars probably have.  I like my 2.71 setup.   I think you can find 2.71 gears on an 8.75" rear - but I don't think you can find that on a 9.25".  Am I right about that?
 
Thanks.
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1979 New Yorker - 360 4v, 2.71:1 rear, factory moonroof, factory road wheels


Snotty

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cop R-body diff size? 11" brakes on 8 1/4"?
« Reply #1 on: May 21, 2008, 01:30:54 PM »

I would doubt they're 9.25 units - those are usually in trucks and vans.  An 8.25 rear can come in a HD set-up.  Heavy-Duty does not always mean bigger.
 
As for the brakes - ????
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Brian

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cop R-body diff size? 11" brakes on 8 1/4"?
« Reply #2 on: May 21, 2008, 02:42:39 PM »

As for the brakes....Yup you can swap 11 inch brakes onto the 8.25 rear end.   As long as you swap the entire brake drum system...just disconnect the brake and e-brake lines and then unbolt the backing plates and swap the entire unit over to your 8.25.
 
You can use brakes off any 8.25/ 8.75/ 9.25 or Dana, they are all interchangeable.
 
Cop car MIGHT have gotten an 9.25...they did use those rear ends in some cars.
 
 
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Brian

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Leaburn Patey

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cop R-body diff size? 11" brakes on 8 1/4"?
« Reply #3 on: May 21, 2008, 02:52:21 PM »

R body cop cars came with 8.25 rear axles.
The gear choices were 2.7 and 3.2 with Sure-Grip as an option.
11X2.5" drums were standard fare as well.
You can install the cop car backing plates and use the cop brake set up without swapping out Wanda's diff..
For the larger front brakes,all you need to change is the front caliper brackets to clear the bigger rotors.You can use your existing calipers but will need the bigger brake pads. to match up with the rotors.
The 8.75 was last manufactured and used by the factory in 1974.
The 9.25 was used in the C body cop cars 1974 to 1978.
Trucks used the 9.25 rear for many years.( I think they still do) .
If you are going to get parts off the cop car,grab the bigger sway bars front and rear and the full instumentation guage cluster.
Attached is a link to the 1979 Plymouth cop car sales brochure that has all the info you may be searching for. Just click on the pic to go to the next page:
http://public.fotki.com/copcardotcom/miscellaneous_photo/vehicle_manufacture/plymouth/1979_plymouth_01.html
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attkrlufy

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cop R-body diff size? 11" brakes on 8 1/4"?
« Reply #4 on: May 23, 2008, 05:04:46 AM »

Thanks for the info guys.  It's nice to know I don't have to change any of the drivetrain parts to get the bigger brakes.


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Leaburn Patey

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cop R-body diff size? 11" brakes on 8 1/4"?
« Reply #5 on: May 23, 2008, 08:34:04 AM »

You will have to open up the diff cover and remove the C clips,slide the axles out in order to swap the backing plates.
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Tom Dawson

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cop R-body diff size? 11" brakes on 8 1/4"?
« Reply #6 on: May 23, 2008, 10:17:13 AM »

If memory serves me correctly all R-body cop cars
came with the 9.25 rear ratios ranging from 2.71 up to 3.23
all E-85 360,s had dual exhaust.not that hard
of a swap if you ask me.
    Tom
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