I bought an a-body 8 1/4 rear(74) and then bought an 8 1/4 Suregrip unit to install in it. The shop was going to provide the 3.55 gears. The driveline shop called and left a message today that the Suregrip unit that I supplied won't work because its the wrong series. I was not aware the 8 1/4's had different series. I thought it was just the 8 3/4s. Is it true that there is more than one series for the 8 1/4?
Now that I think of it I 've heard of an 8 1/4 with a very tall highway gear, something like 2.5:1. I thought these were very rare though. Ofcourse I plan to call them and get the low down, but I just wanted to run it by ya'all.
Have I provided enough info to be able to give any kind of good feed back?

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67Cuda,FAST EZEFI,340cu,CR=10.25,RollerCam&Rocker (XR268HR,#20-810-9)(#1622-16)(EddyRPM#60779,#7576), (MSD6AL,#6425) A904, GearVendorsOD, 8 1/4,3.55:1, ClassicAir

I know the size of the carrier bearings changed between the A Body and F body 8 1/4 rears. I had problems with installing a 4.10 ring set in a factory sure grip rear out of a 74 Valiant. I could not get the rear to set up no matter how many shims or things I tried. I changed the carrier bearings over to the larger later style, dropped the suregrip in the later F body rear and set it up with no problems....go figure. I do believe the newer suregrip units are smaller than the older ones. That could possibly be your problem. I'm sure an expert will chime it

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There is an 81/4 rear with a 2:45 gear ratio. because of the large ring gear needed for this ratio the housing is somehow differently shaped. As far as I know this is the only ratio available in this housing.

The suregrip unit that I started with had a 2.71 ratio, so I know that a 2.45 carrier has nothing to do with it.

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The only differences I know of on an 8-1/4, is that the 97 up units have 2 splines different from the older ones, I believe that the axles are 27 count on the up to version, and 29 on the after version. ( could have the numbers and years used flip flopped) I had never heard of different size carrier bearings on an A body rear vs an F body one.
Yes, if you originally had a 2.26 or a 2.45 ratio, they had their own special carrier, but from what I remember if you have a 2.71 or higher(number) ratio you should be good to go. Now if yours is an 89-96 SG that is set up with an exciter ring to be used with ABS itll still work as long as the spline count for the axles jives. That exciter ring does come off and I think (Im not looking at mine right now) mounts on the opposite side of the carrier flange from the ring gear.Mine was set up for the exciter ring but had no prob putting it into a '78 housing. The exciter ring looks like a smaller version of a flywheel ring gear that the starter engages into. You might call a place like Randys Ring&pinion or National Drivetrain, both are very helpful on compatibility issues like this.

There telling me that both the Suregrip and my stock open diff have 2.76 gears and the 3.21 and lower (3.55) gears will not bolt to the carrier and therefor won't work.

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67Cuda,FAST EZEFI,340cu,CR=10.25,RollerCam&Rocker (XR268HR,#20-810-9)(#1622-16)(EddyRPM#60779,#7576), (MSD6AL,#6425) A904, GearVendorsOD, 8 1/4,3.55:1, ClassicAir

either you don't have the ratio they claim or they dont know what theyre talking about

or they dont know you don't have the ratio they claim theyre talking about.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

If it was a chevy then a 2.76 carrier would not fit 3.08, 3.42 or 3.73 gears. Maybe thats what they are thinking of. Ain't they all the same??? I would get some numbers and see what you really have.

OK- hears the deal. The orignal open diff that came with the A-body 8 1/4 rear was a 2.76 gear and will take the lower gears.(3.21-3.55 ect) The Suregrip that I bought is a 2.71 gear and will only hold highway gears. Why would they make a Suregrip highway carrier?
Any way I had at least two options. Buy a clutch type carrier or buy the Auburn Gear Limited-Slip Diff which is a cone type. It maybe a little over kill but I went with the cone type.

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67Cuda,FAST EZEFI,340cu,CR=10.25,RollerCam&Rocker (XR268HR,#20-810-9)(#1622-16)(EddyRPM#60779,#7576), (MSD6AL,#6425) A904, GearVendorsOD, 8 1/4,3.55:1, ClassicAir

Why would they make two gear sets, that are that close to each other (2.76 and 2.71)? How many teeth were on the ring and pinion of each set?

Actually, in my opinion the clutch type is a better unit.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

good question as to why they would have a 2.71 and a 2.76, have no idea but I have had vehicles with both. (not on the same vehicle though ha ha)
My 76 FSM does show both available that year.

good question as to why they would have a 2.71 and a 2.76, have no idea but I have had vehicles with both. (not on the same vehicle though ha ha)
My 76 FSM does show both available that year.

Are they both for the same type rear or is one for one type (8 1/4) and the other for a different type (7 1/4)?

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

the ratio is simply the number of ring gear teeth divided by the number of pinion teeth,,,,there are a variety of ways to get close to the same #

10 pinion 41 ring = 4.11
9 pinion 37 ring = 4.11

8 pinion 22 ring = 2.76
7 pinion 19 ring = 2.71

.....and that might mean different carriers.

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Yeah....Im the one who destroyed this rare, vintage automobile.....